Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SILVERGUN on July 16, 2013, 03:43:20 pm
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As a result of another thread where we had discussed the use of a post phase inverter cathode follower circuit, I have put together this incarnation of the AB763 circuit on my breadboard in hopes of experimenting with some ideas that had been tossed around.
See old thread here: http://www.el34world.com/Forum/index.php?topic=15679.0 (http://www.el34world.com/Forum/index.php?topic=15679.0)
There is a very popular amp design that utilizes this feature, but there is not a reliable schematic available for it, so, with the help of the collective brain trust, it is my goal to come up with a safe, effective design that will allow us to utilize this circuit without the apparent reliability/safety concerns that could plague the original design (or at least the "unofficial" rendering of that original design)
The goal would be to create a great clean sound that will stay crystal clean as you crank the amp, and provide a greater bass freq. response than what we are used to in the standard AB763 design......so instead of bass notes that tend to "fart out",or get flabby, we will attempt to accentuate a "slap bass" kind of response, and see if there is a noticeable direct correlation between the PPICF and that type of ultimate clean tone...I am aware that some of the magic that I am looking for will be located in the preamp,,,BUT for now I am only concerned with getting the basic design and component values in order for the PPICF,,,,so I will leave the preamp settings the same throughout this process in order to try to get solid proof of my findings.
Here's the schematic, as it sits now, after a few rounds of tweaking the preamp and reverb to my liking....
I'm comfortable with the way it sounds now and I'm ready to experiment for the benefit of the greater good :icon_biggrin:
Big thanks to Sluckey for the use of his schematic :thumbsup:
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Here's a quick cut and paste mockup of the insertion point for the PPICF, just to get the ball rolling....and for anyone who couldn't envision what I was talking about.......we can use this as the template
I'm pretty sure we will be using a 12AT7 in this location, but I'll leave out any markings for now
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I will withhold all but the most prelim drive-by comments for now, but it seems to me you are kind of fighting yourself with this. While I am sure there is way to do it, I'm not convinced of the value.... but I am certainly willing to be.
Obviously, you are going to have to insert caps between the CF cathodes and the G1s of the 6L6's, otherwise output stage bias will be gummed up. But I think that over and above that, you may have trouble achieving the kind of voltage swings you nominally need to drive 6L6 outputs at anything near full pump. The CF twin-triode cathodes are going to have sit atop MUCH larger value resistors than the typical 1.5K-2.2K cathode res.
Eager to see others' contributions.
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11 my friend,,,,,let's get this out of the way early....I'm no genius :icon_biggrin: I almost mis-spelled it
This is just an attempt to figure out what is right and or wrong about the PPICF in the sss....and determine it's value
I'm in no rush and I haven't taken a down payment to complete this amp for someone else....it's for me and my curiosity
If it fails or just plain ole' doesn't make sense,,,then I'll build it like the guy from Japan wrote it out....I doubt at this point that I could just look away
But, I'm looking forward to hearing the idea that HBP had about possibly topping the design, and resolving any issues that stood out in the first hand drawn schematic we were looking at........
This is much bigger than me, so I'm just a good set of ears on some good cheap labor
Hopefully I'll learn something along the way.....I spent my entire last build focusing on the preamp, and this just stands out to me as something different and interesting, even if I don't fully understand the Z's of it.......I will understand the sound difference, if there is any :wink:
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I really like your enthusiasm. Don't stop.
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Simply putting something on the table for discussion purposes only.
With respect, Tubenit
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Some mutation of this may also be worth trying if you have problems getting the valve going in that spot.
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One other pair of PPICF examples
(http://www.chambonino.com/construct/constwire1d.jpg)
(http://www.chambonino.com/construct/constwire2c.jpg)
http://www.chambonino.com/construct/constwire1.html (http://www.chambonino.com/construct/constwire1.html)
http://www.chambonino.com/construct/const8.html (http://www.chambonino.com/construct/const8.html)
http://www.chambonino.com/construct/constwire2.html (http://www.chambonino.com/construct/constwire2.html)
http://www.chambonino.com/construct/const9.html (http://www.chambonino.com/construct/const9.html)
Source
http://www.chambonino.com/construct.html (http://www.chambonino.com/construct.html)
http://www.chambonino.com/index.html (http://www.chambonino.com/index.html)
K
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Also, from the previous thread, Studio Bass. Using known working systems as a starting point for developing a project can be quite successful.
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Thanks for all the support guys......I haven't heard from HBP yet, so I'm a little hesitant to move forward without his input
Also, from the previous thread, Studio Bass. Using known working systems as a starting point for developing a project can be quite successful.
I had thought about that.....one question that stands out to me is the connections to the OT....I'm completely unfamiliar with the application of using the UL taps in conjuction with the non UL taps (which is what I think I'm seeing here)
Again, a lack of experience in general can be a huge stumbling block.....
I'm imagining that this circuit, if strickly intended to be an impedance matching device, is intended to be used with the increased load of multiple output tubes per side,,,as pictured in this schematic and present in the sss
So if I have to add a 6L6 per side, to make it make sense, then that's what I'll have to do
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SILVERGUN, Please bear in mind that I'm a newbie to this valve audio stuff, BUT,
What happens after the grid of the power tubes should not affect (effect ?) the cathode followers. The studio bass is ultra linear but splicing a standard power output section should not effect (affect?) the outcome. The studio bass APPEARS to have a good basic topology for getting bias in order between PI and PA. Dear me, don't add another pair of 6L6's, way too loud for the 21st century. :laugh:
On another note, the TAG thread specified in the previous thread on PPICF's, appears to contain a sufficiently detailed layout picture, of a working amp, to transcribe a circuit from. It comes with some very impressive sound clips. It will be rather close to tubenit's discussion schematic posted above.
Before I proceed with that transcription, AM I MISSING SOMETHING.
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As Glennjeff say, you aren't obligated to use an UL transformer as to use a PPICF in your architecture
Those are two different things that can be used together (one does not prevent the use of other)
but nothing prevents you from using just one of these
K
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:huh:
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From previous (somewhat hijacked) thread is a point of concern from HotBluesPlates
"The problem is switch-on... The SSS has 379vdc at the plate end, and -328vdc at the cathode end (because there is no current at the moment of startup and no voltage drop across the 220kΩ resistor). That's 379v+328v = 707v across the 12AX7 (!!). That's bad design, no matter who did it or how famous they are."
Would not an inductor and capacitor at that B+ node, or even better on the bias supply node, act as a type of "soft on" preventing the sudden but transient application of 700V A/K.
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SG,
Looks like you are ready for your new build, congrats. I checked with the guys over at ampgarage that have built the SSS with the 12AT7 CF and they said they had no issue with the high voltage across the tube, but of course, most of them use NOS tube not the new production from the East. So you may want to start with the breadboard "as is" and see if any issue crops up. Looking forward to see more on your R&D results :icon_biggrin:
Jaz
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As Glennjeff say, you aren't obligated to use an UL transformer as to use a PPICF in your architecture
Those are two different things that can be used together (one does not prevent the use of other)
Thanks K and Glenjeff,,,,I do understand,,,I was just pointing out one question that I had about that specific design and what the effect of the UL xfmr would have....
Looks like you are ready for your new build, congrats. I checked with the guys over at ampgarage that have built the SSS with the 12AT7 CF and they said they had no issue with the high voltage across the tube, but of course, most of them use NOS tube not the new production from the East. So you may want to start with the breadboard "as is" and see if any issue crops up. Looking forward to see more on your R&D results :icon_biggrin:
Thanks jaz, I probably haven't spent enough time reading over there.....the info about the amp in question seems to be fairly scattered
But, I'll take a closer look and start migrating puzzle pieces into the template....
Luckily, this is a no rush puzzle...........I'm just hoping to narrow down the PPICF area that was in question....
It could be a really short thread if someone wants to share a known working schematic from an sss :icon_biggrin: :wink:
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It could be a really short thread if someone wants to share a known working schematic from an sss
They are printed on unobtanium. Very rare. :icon_biggrin:
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On another note, the TAG thread specified in the previous thread on PPICF's, appears to contain a sufficiently detailed layout picture, of a working amp, to transcribe a circuit from. It comes with some very impressive sound clips. It will be rather close to tubenit's discussion schematic posted above.
Before I proceed with that transcription, AM I MISSING SOMETHING.
Yeah Gj, we can derive a schematic from those pics,,,and it will closely resemble Tubenit's posted pic
I think it was the "devil in the details" that raised some questions about the reliability of that design,,,,and fueled the discussion at the end of that last thread, which brought me to this point.
I'd hate to have to rely on 'only NOS 12AT7s', because of a voltage handling issue that possibly can be avoided
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That TAG thread does seem incomplete and scattered but the mhartman sss seems to be a working model that has stood up well to months of tinkering and testing.
I searched pretty thoroughly over there for an updated schematic with no success, mhartman's layout is the closest we'll get I think, you only need to adapt and transplant the PPICF with associated bias circuitry.
For what it's worth, I'll have a closer look at your working schematic and compare it to the Hartman sss to see if any further worries arise tomorrow.
With regard to voltage tolerance of the modern 12AT7's in this application, sometimes you just gotta taste the soup to see if it needs a bit more pepper. The Electro Harmonix are long plates, maybe compare their data sheet with JJ's for example to see if they are a bit tougher.
All the best.
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mhartman's layout is the closest we'll get I think, you only need to adapt and transplant the PPICF with associated bias circuitry.
Based on that layout snippet you posted, and the original hand drawn schematic,,,it would appear that there are no coupling caps betwee the cath. of the PPICF and the grid of the 6L6s......
This is my first issue that we need to overcome.........how does DC coupling to the output stage work? (I'll be reading this afternoon)
There are caps present on the StudioBass,,,,and that dictates the difference between the stages being AC or DC coupled
Does anyone think that this design was meant to counteract the bass freq. deterioration that occurs with "standard" capacitive coupling?
Could that be a contributing factor in the "magic" that i'm hearing in the bass response?
....And is this the "known good" application that we should be looking closer at? (THANKS K!)
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This is just an attempt to figure out what is right and or wrong about the PPICF in the sss....and determine it's value
If you are drawing stuff on the chalkboard:
(http://i.imgur.com/i7kWIOq.jpg)
if you tried this, you'd probably use one 12at7 for the 'top two', and one for the bottom two for ease of wiring...
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If you are drawing stuff on the chalkboard:
Thanks TL, I think we are gonna need coupling caps before the grids of the CF tube
On the Hiwatt schematic I posted, this appears to be a mistake:
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Hi SG!
chech out the mighty ampeg svt schems:
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_Model_6146B.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_Model_6146B.pdf)
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PowerAmp_RevD.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PowerAmp_RevD.pdf)
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PowerAmp_RevF.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_PowerAmp_RevF.pdf)
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_RevA.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_SVT_RevA.pdf)
There are no caps coupling between CF driver and power tubes G1. However check how much negative bias voltage is needed in such design. Also note CF driver is 12BH7.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12BH7A.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12BH7A.pdf)
I will start building a guitar power amp based on the SVT output section, however I will employ LTPI instead of using concertina. I built to a friend a stand alone distortion preamp and now he wants me to build him a PA. He's into metal/stoner and quoting what he asked: "I NEED something that sounds like thunders." So here we go. The very first version of the schem is attached.
I really like your enthusiasm. Don't stop.
+1
Hope this helps.
Best Regards.
R.
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chech out the mighty ampeg svt schems:
There are no caps coupling between CF driver and power tubes G1. However check how much negative bias voltage is needed in such design. Also note CF driver is 12BH7.
Thank you Sir,....I remember you asking me this question in the old thread, and I'm officially ready to respond :icon_biggrin:
Will you try this dc or ac coupled? Fender did it AC, whilst D, Ampeg and hiwatt went DC coupled.
DC
I guess I'll start laying it out as-is, and wait for some help with the component values
The beauty of working with a bench power supply wil be that I can make the negative supply whatever we need it to be :thumbsup:
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Remember the discussion on the AC vs DC coupled difference on the old thread? I think the words "night & day" were used to describe the difference, so if you are going to do it, stick with the DC-coupled version...
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:happy1:
I punched out at 5pm...made up my mind that it was happening...and it happened....
I'll get back to you guys a little later after I get some dinner,,,but I'm so excited that I just realized that I'm sitting here typing with my safety glasses still on :l2:
I wound up hooking it up per mhartmans layout, and just using a lower input voltage to test, and try to stay safe
I started by taking some V readings, and I'll post 'em all later
I was watching the AC and DC voltages on the grids of the 6L6s as I was playing and on DC I was fluxuating between -17v to -24v by playing harder with the vol. on 3.......on AC less than 1v up to 13v if I played harder, and I could get it to peak at 30ish volts if I really laid into a note....
Is there a problem with those #s?......at higher vol. settings I was getting some distortion, similar to what I was getting before i installed it :dontknow:
THE SLAPPY BASS RESPONSE WAS THERE THE SECOND I PULLED UP THE VOL. CONTROL.......WOW!!
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COOL! I'll try it out in my current build in a few days.
Saturation may be coming from 12AX7 / V1b (incorrectly labelled on your original diagram). Try a 12AU7,(or AV / AY) in there to see if the saturation reduces, not that you want to get rid of it permanently.
Are you using a digital or analog meter to get those V readings?
QUESTIONS ???
Can you switch your rig between WITH PPICF's and WITHOUT PPICF's?
Is there any extra gain associated with the PPICF's?
Do they have any effect upon tonality other than bass tightening?
In particular, all the SSS's I've heard sound really transparent, loud and smooth in the highs , do the PPICF's contribute to this?
Enquiring minds want to know.
All the best. :worthy1:
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So, yes,,,that obviously meant an aweful lot to me :icon_biggrin:......so I did something I rarely do anymore,,,and celebrated...stopped at the store, picked up a couple steaks,,had a couple beers..........my wife surely thinks I'm nuts :grin:
The reason this was huge for me is because I pushed past my fear of the unknown, used a little deductive reasoning, and succeeded....the first time
I hooked it all up,,,quadruple checked my connections and component values, and it worked, with NO wasted time
I trusted that mhartman was giving me good info, and that he was smarter than me :icon_biggrin:
Once I pushed past my fear of doing something wrong and just sat and thought logically about what I was doing,, the whole procedure made more sense to me.....and I figured that as long as I can get the 6L6s biased correctly, that I should be able to hear a difference....and I did :thumbsup:
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QUESTIONS ???
1) Can you switch your rig between WITH PPICF's and WITHOUT PPICF's?
2) Is there any extra gain associated with the PPICF's?
3) Do they have any effect upon tonality other than bass tightening?
4) In particular, all the SSS's I've heard sound really transparent, loud and smooth in the highs , do the PPICF's contribute to this?
I numbered your questions so I can answer them easily....
1) No, I guess you could, but I wouldn't want or need to....it just plain ol' sounds better this way....I guess it would be cool to do on the breadboard, to compare heads up,,,but I'm positive it sounds better now
2) I couldn't tell, but there shouldn't be, due to the nature of the cathode follower...the amp sounded as loud on 3 as it did before on 3 (approximately)
the distortion I mentioned seems to be coming from the preamp, because it was there already,,,,and I'm using high output pickups...when I turn them down, it cleans up nicely
3) Yeah, I hear an overall tightening (maybe compression) that I really like for a funky /bluesy/ even Stevie style.....I also think I'm hearing better note separation on chords........
4) I have to believe so,,as soon as I pulled the volume up on the guitar, it had a quality to the overall freq. response that just wasn't there before....I can best describe it as "glassy", tight, and defined....the highs do seem less ice pickey and slightly more rounded and compressed
These are obviously just my opinions, and i'll try to get some kind of demo clip posted as soon as possible.........
I'm still smiling 14 hours later :grin:
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Thanks for the responses, for some reason I expected this outcome.
To wax metaphorical,
Your observations on the "person within" part of the exercise are well worth remembering dragon slayer. Given your name, vampires should present no problem. :icon_biggrin:
I'm still smiling 14 hours later :grin:
Smiling with you.
All the best.
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OK, so there were a couple things wrong...and maybe you guys could help me figure out some better components values
When I first applied power, I had 115ma cathode current per 6L6 :huh:.....
so I immediately shut it down and cranked my available bias supply to max -87vdc,,,,but because I set up the voltage divider incorrectly, only -17vdc was making it to the grids of the power tubes, which brought the cath. currents down to 92ma,, which I thought was OK to test the amp on, because they weren't red plating, and seemed pretty balanced.....so that's where my test result came from....a very HOT bias condition
IS THERE ANY OTHER POSSIBLE PURPOSE FOR THAT VOLTAGE DIVIDER, OTHER THAN KNOCKING DOWN THE BIAS VOLTAGE?.....it does put -18vdc on the grids of the driver tube :dontknow:
Here's the adjusted schematic for how I hooked it up:
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I took some quick voltage readings before I had to run out the door,,,so they may not be perfect, but they're close
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OK, so I had a chance to go out and try to fix something on lunch, so I changed out the 250K pot in the tail of the V divider,,and dialed that to 370K to get the bias voltage up , and it did,,,so here are the adjusted #s
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silvergun, you might read thru this: http://lenardaudio.com/education/13_guitar_amps_4.html (http://lenardaudio.com/education/13_guitar_amps_4.html)
scroll down to the section on emitter followers......you could use an AT7 in place of the SS devices
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Ouch! and Thank you Katie :icon_biggrin: :wink: :l2:
That's a very cool website that I had never seen before and possibly never would have if you hadn't pointed it out :thumbsup:
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I would change the bias voltage divider by a factor of ten. IE, 870K down to 82K and 1M pot down to 100K.
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Correct me if I'm wrong but would'nt
this added tube be nothing more than a buffer
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I would change the bias voltage divider by a factor of ten. IE, 870K down to 82K and 1M pot down to 100K.
Consider it done Sir....I'll let you know what changes occur
Correct me if I'm wrong but would'nt
this added tube be nothing more than a buffer
Hey Jack,,,I'm still reading "Tube Amps For Dummies" :icon_biggrin:
But here's Aiken's explanation:
Add a DC-coupled cathode follower between the phase inverter and the grid of the output tubes, with the cathode follower cathode resistor returned to a high negative voltage, and the grid bias applied to the grid of the cathode follower. This effectively isolates the output tube grid circuit from the phase inverter and its associated AC coupling, and provides a very low impedance source for the output stage. This will prevent the output stage from going into grid clamp, and will eliminate the long time constant of the AC coupling. This method has the unfortunate side effect of requiring an extra tube and completely ruining the value of your vintage amp, so it is best used only on new designs, but is highly recommended. You will also get more power out of the output stage because it is now running in class AB2 or class A2 (the "2" suffix indicates grid current flows for a portion of the cycle).
Thank you Mr. Aiken
excerpt from this page: http://www.aikenamps.com/BlockingDistortion.html (http://www.aikenamps.com/BlockingDistortion.html)
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so the bias on the CF is -1. don't you want a bias of at least -5 (or maybe -10) to avoid clipping at max volumes?
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IS THERE ANY OTHER POSSIBLE PURPOSE FOR THAT VOLTAGE DIVIDER, OTHER THAN KNOCKING DOWN THE BIAS VOLTAGE?.....it does put -18vdc on the grids of the driver tube :dontknow:
Mr. Aiken helped answer my question while I was trying to answer Jack's
Add a DC-coupled cathode follower between the phase inverter and the grid of the output tubes, with the cathode follower cathode resistor returned to a high negative voltage, and the grid bias applied to the grid of the cathode follower.
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so the bias on the CF is -1. don't you want a bias of at least -5 (or maybe -10) to avoid clipping at max volumes?
Good question....it got me reading more and going back to compare to the original hand drawn schematics, where you can see there was -39.5v on the grids, and -40.4v on the cathodes (so it appears that a 1v difference worked for the D man)
The fixed bias method used here is different than your normal cathode biased gain stage....
I don't believe that it matters which component is more/less negative in this fixed bias condition,, as long as there is a voltage difference between the two,,,and you wind up with a high negative voltage on the grids of the driver tube
Unfortunately for me, we lost power here last night so I went home without getting any further
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The fixed bias method used here is different than your normal cathode biased gain stage....
IF you are referring to the CF driver, then I think it's na AC coupled, cathode biased CF which employs large negative voltages to properly bias PA and driver.
I don't believe that it matters which component is more/less negative in this fixed bias condition
Well, I believe it does matter. Cathode should be sitting on less negative voltage then grid - actually more positive. For instance, SVT goes -77VDC on 12BH7 grids and -47VDC on 12BH7 cathodes, which are DC coupled to the G1 grids of 6550/6146 power tubes and unless G1 starts to draw current, then it should see -47VDC on it's G1 gridsd as well.
With Respect.
Best Regards.
R.
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Thanks Rz,
I'm learning as we go here,,,so it's the question and answer education approach :icon_biggrin:
Thanks for staying tuned in.......
I just figured that if I said enough dumb stuff, PRR would straighten me out,,,,and I tend to learn the most by Googleing 90% of what he posts :l2:
The fixed bias method used here is different than your normal cathode biased gain stage....
IF you are referring to the CF driver, then I think it's na AC coupled, cathode biased CF which employs large negative voltages to properly bias PA and
If I read Merlins site correctly, it refers to the bias on "my" CF as being either fixed or grid leak.... :dontknow:
And I was telling terminalgs, to think about the bias differently than you would in a standard cathode biased stage
Maybe I'm wrong........wouldn't be the first time
Your words are appreciated, as the root goal here would be to actually learn what I have done here and not just copy it :thumbsup:
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OK, so I joined the 21st century and started a YouTube channel and posted a short video demo of how the amp sounds right now
Just improvising a little percussive type stuff to try to accentuate the bass response of the amp, and threw in a Stevie type riff because I think that's where it shines :icon_biggrin:
Warmoth hardtail strat with Seymour Duncan Hot Strat Stack neck pickup, amp on "3" reverb on "2"
Video was taken with my smartphone sitting right in front of me so please excuse the poor sound quality...i'm still learning
Video clip here: https://www.youtube.com/watch?v=W_ndQEl28OY (https://www.youtube.com/watch?v=W_ndQEl28OY)
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I am looking at your Visio schematic, just noticed that the voltage on the 6L6s are only ~310V instead of ~380V of the SSS, is that on purpose or just for testing purpose? Also what is the grid bias set at now?
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I am looking at your Visio schematic, just noticed that the voltage on the 6L6s are only ~310V instead of ~380V of the SSS, is that on purpose or just for testing purpose? Also what is the grid bias set at now?
Yeah.....on purpose....not necessarily trying to clone the sss,,,just trying to get the circuit right and I wanted to go easy on the voltage across the PPICF
6L6 bias voltage is now around -24 vdc and gives me about 64ma cathode current with plates at 316v
All seems well :icon_biggrin:..........amp sounds pretty cool to me, and definitely picked up something positive from the inclusion of this circuit
Thanks for staying tuned in jaz,,,I really appreciate the help :thumbsup:
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Sounds great, like your playing too.
Could you just clarify for me what valves you are using as PI and CF?
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Pretty clean sounding with "amp on 3", what happens when you crank it up? The SSS type of design is suppose to provide a lot headroom, but with your low plate voltage and relatively low bias setting, you have taken quite a bit of the headroom away, so it would interesting to hear a clip with the amp cranked up :guitar1 BTW, the recording sounds just fine the way it is.
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Wow SG! So impressed. I just now saw the thread and started reading tonight. So in three days it goes from concepts > to building it > to you AND a sound demo!??? :icon_biggrin:
This is just plain awesome my friend!!! Keep it up (which I know you will) and god speed as they say. Is this something suspected or actually documented from the D-man & his stringer?
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:huh: :thumbsup:
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Thanks guys,
Gj, I really do appreciate ALL of your help....I also appreciate the fact that you would care enough to come back and edit your post if you thought there was a problem with it or you changed your mind about something :thumbsup:
I have done that on numerous occasions in other threads
I had read some of your earlier post,,,and the one thing I remember is that I wanted to share this.....
I always use the weber bias calculator as a guide for setting my output tube bias....found here:
http://www.webervst.com/tubes1/calcbias.htm (http://www.webervst.com/tubes1/calcbias.htm)
Pretty clean sounding with "amp on 3", what happens when you crank it up?
I start to bleed from my ears and my component storage bins fall off the wall... :icon_biggrin:
40 some clean watts is definitely LOUD!!!.....but I'll try to tough it out and give you a clip, because I agree,,,,the amp should retain the headroom when you crank it.......and we will have to confirm that to call this a true victory
I think the biggest problem with cranking it up is the placement of the volume control,,, tends to increase preamp distortion as I turn it up, (even without the PPICF) and I was considering putting a master volume right before the PI, similar to Sluckey's Tweed Deluxe Reverb
Before I changed some component values and "cleaned up" the front end, I couldn't believe how easily this amp would distort, even at low volume...........It definitely felt like my pickups were too high output, and I was just crushing the front end.......sounded best with my guitar vol. on 8 instead of 10.....not really the "cleans" I expected from the AB763
Almost bought some low output strat pickups,,,,but I waited....for now....I have a feeling that is going to be a real winner in this amp
The amp does not sound just like an sss....obviously, BUT, the reason I am so excited about it is because only a month ago we were questioning the design of the PPICF,,and now I can confirm that it does make a difference and now this amp sounds like a Deluxe/Super Reverb with a PPICF
I'm counting on guys like you to help me tweak it to it's potential,,,,and then we can experiment with changing out the preamp.....of course I'm thinking about how it will sound with OD and what kind of difference I can expect if I add this to the TurboWreck, TOS, etc., etc :icon_biggrin:
TDR master shown below
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Pretty clean sounding with "amp on 3", what happens when you crank it up?
I start to bleed from my ears and my component storage bins fall off the wall... :icon_biggrin:
40 some clean watts is definitely LOUD!!!.....but I'll try to tough it out and give you a clip, because I agree,,,,the amp should retain the headroom when you crank it.......and we will have to confirm that to call this a true victory
Just wondering if you have to use 6L6s instead of 6V6s? You could save your ears until things are better dialed in then sub the 6L's, re-bias, and test those in the end? :dontknow:
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Wow SG! So impressed. I just now saw the thread and started reading tonight. So in three days it goes from concepts > to building it > to you AND a sound demo!??? :icon_biggrin:
This is just plain awesome my friend!!! Keep it up (which I know you will) and god speed as they say. Is this something suspected or actually documented from the D-man & his stringer?
Thanks jojo AND Timbo,
It's 3:45 AM here in Philly so that's how you know I'm excited about this :wink:
This "research" is for all of us,,,,and you can trust me when I tell you that I won't stop until it's right
I am working from mhartman's sss layout from TAG,,,,and a hand drawn (scribbled) schematic from some guy in Japan that "had one on his bench" (see above posts)
When we originally started talking about this on another thread,,,I was pushing hard for it,,,,,and HBP inspired me to build an amp that I could try to incorporate it in first,,,,because he thought (and I agree) that it would be easier to install it in a known working amp.
That was a great idea, and since I was looking for an excuse to do an AB763, it was a no brainer for me to try to enhance those cleans with this addition............there had to be something special about that sss circuit for it to sound so unique.....THERE IS :icon_biggrin:
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Just wondering if you have to use 6L6s instead of 6V6s? You could save your ears until things are better dialed in then sub the 6L's, re-bias, and test those in the end? :dontknow:
As a semi-delusional guitar player,,,,I like to imagine that bigger bottles = bigger tone :icon_biggrin:
I had some 6L6s here and an OT that was perfect for the job...I could go back to 6V6, but I would have to buy them,,,and if I want to buy tubes, there's better places to spend my money right now.............I still have to explain this stuff to the old lady, ya know :laugh:
This thing is all about "balancing" the bias between the voltage divider for the CF and the output tubes,,,,,so I guess it's easier to tweak with the actual setup....
Thanks for showing up my friend,,,,,this is pretty cool stuff,,,,,,please take a look back over the thread when you get a chance and make sure I didn't say anything too stupid :icon_biggrin:
Off to bed.......zzzzzzzzzzzzzzzzzzzzzzz
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I thought you went back and forth w/ 6V6s & EL34s when working on the 'wreck? But that's okay, don't let a couple of little ol' bottles get in your way :)
Yeah I saw the mhartman reference (didn't know who or what?) and entire thread but didn't know you were discussing this on another thread before this one. So when I saw "d-man" and "sss" I immediately thought about Dumble and his Steel String Singer.
I'm not trying to distract here but if looking for ultra cleans I'm surprised ultra linear OT is not being considered also?
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SILVERGUN,
Yeah, I had a real bad hangover this morning and the brain just was not working right. :d2:
Could you tell us what valves you are using for PI and CF, you said in the thread somewhere that you were going to use 12AT7 for CF but the circuit diagram with voltages has a 12AX7.
Still wondering why the CF is biased so low on the valves transfer curve.
Seems d-man had a preference for 12AU7 PI and 12BH7 CF in the sss's, from hours spent at TAG today. Tempted to make my next build a Singer, but your AB763 sounds real nice. :dontknow:
Take care of the ears, I use foam earplugs when practicing drums and recording real loud guitar parts.
All the best.
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I thought you went back and forth w/ 6V6s & EL34s when working on the 'wreck? But that's okay, don't let a couple of little ol' bottles get in your way :)
When I built that little custom Champ I cooked one (for research sake :wink: @18 watt dissipation) and then used the other to finish the amp at a much more reasonable output.....EL34s all the way in the Wreck,,,the 6V6s couldn't keep up
I'm not trying to distract here but if looking for ultra cleans I'm surprised ultra linear OT is not being considered also?
No distraction taken....that's the beauty of the breadboard....we CAN try it, if you think it'll make enough of a difference...
Supposedly the sss was built on Twin iron,,,so I wasn't real concerned about it, and I had this Hammond 1760L sitting here..
I mentioned it briefly while we were looking at the Fender Studio Bass schematic, earlier
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1)Could you tell us what valves you are using for PI and CF, you said in the thread somewhere that you were going to use 12AT7 for CF but the circuit diagram with voltages has a 12AX7.
2)Still wondering why the CF is biased so low on the valves transfer curve.
3)Seems d-man had a preference for 12AU7 PI and 12BH7 CF in the sss's, from hours spent at TAG today. Tempted to make my next build a Singer, but your AB763 sounds real nice. :dontknow:
4)Take care of the ears, I use foam earplugs when practicing drums and recording real loud guitar parts
I numbered your questions/ comments again so I could address each:
1) PI is AT7 and CF is AX7.....schematic is correct,,,,and is a result of some reading over at TAG
But all of these are options that we can try
2)Me too, but I just feel lucky that I was able to get this far, this quickly,,,and I don't question it too much because it seems to be in line with the scribbled schematic.........I was hoping that one of the real mathmeticians would weigh in on that one for us :icon_biggrin:
3) I missed that,,,and probably because I promised myself that I was gonna try to not do hours of reading over there....the more I read, sometimes, the less I get done
4)Yeah,,,,thats a big problem for me right now, because I already have some hearing damage from years on stage with a drummer who broke huge sticks regularly,,,,and I have to walk a fine line between hurting myself and hearing results...........when I do the cranked demo for jazbo, I wont be in the picture :icon_biggrin:
Please stay tuned....this isn't anywhere near over....it's just installed and working.....step one complete...I'm sure i'm missing a lot of the magic of the sss in the preamp,,,but you're right, this one sounds pretty cool too, and I wouldn't build another AB763 without it
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There is one other thing that I would like to mention ,,,for the record....
If you look back at Tubenit's original post on this subject and compare it to my adjusted schematic, you'll notice that mine is an exact copy of his.....
So, once I compared his schematic drawing to the Japanese scribbled version and mhartmans layout, I actually used his schematic whilst I was "cleaning up mine" as a template......if I was any good with express sch, I would edit it there, but it's still much easier for me to cut and paste into sluckey's original print
As always, thanks for all of your help T :thumbsup:
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Once you get a feel for using the .sch program you'll be happy you took the time. It's a short learning curve but you just have to dive in at some point...
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Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with... Congrats again on such speedy and successful build (even for bread-boarding it is impressive). :worthy1:
Purple: Class A Load Line, Turquoise: Class B Load Line:
(https://dl.dropboxusercontent.com/u/1326040/6L6_SS_Comp.gif)
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Here is a quote from HotBluesPlate, from the previous thread, that mirrors what I was thinking about yesterday (In the deleted comment):
I'd probably drop the 220kΩ cathode load to a smaller value (since the negative supply is less and we won't need to drop as much voltage), which also works better with a lower-mu tube with less resulting output impedance (12AT7 or 12AU7), allows the cathode follower to run at a little higher idle current and probably sink/source grid current much better. In fact, thinking about it now we could probably fine-tune this circuit element to be superior in every way to the Dumble original, at least for the purpose I believe it serves.
Guess I'll have to build it myself. Don't want to blow up someone else's amp. I'm convinced the biasing (and impedance) is not optimal around the CF region. Don't have the academic skills to back up my assertion however. Have put some industrial strength P channel MOSFETS on order to build a VVR style negative supply rail. If in doubt, use the empirical method, suck it and see.
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Apparently different versions were built differently in the PPICF department.
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I'm pretty sure #1 is a drawing error.
Tubenit
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Looked dysfunctional to me too tunenit.
So this might be worth experimenting with (for me at least).
Think I'll have to take the plunge and start learning ExpressSCH and get some sim software.
Does anyone know of some open source / freeware / shareware simulation software?
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Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with...
Thanks for all of that jaz.....I like the idea of the lower variant sss, and wasn't concerned with trying to get the most out of the 6L6s, other than good tone....It's cool to see it graphed out like that and I look forward to the day that I will figure out how to acquire/use that type software.......
I can't imagine I'll ever be on a stage again where I'll need this to go past 5...max :icon_biggrin:
What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?
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Apparently different versions were built differently in the PPICF department.
Thanks for sharing all of your research Gj
This aint over,,,,,and i'm looking forward to getting it to a point where it makes sense to everyone...
The first hurdle was just getting it hooked up without blowing anything up, and confirming that it actually would have a noticeable improvement......check :icon_biggrin:
DL and PRR seemed to like the potential of the 12AT7 for this application, so that will be step 1.....try it,,,and then try it with lower Rk
From HBP's post you mentioned:
(12AT7 or 12AU7), allows the cathode follower to run at a little higher idle current and probably sink/source grid current much better.
In Merlin's description of using a high negative voltage applied to the grid of the CF tube, he claims that it eliminates grid current,,,
how high of a negative voltage is "required"?
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Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with...
Thanks for all of that jaz.....I like the idea of the lower variant sss, and wasn't concerned with trying to get the most out of the 6L6s, other than good tone....It's cool to see it graphed out like that and I look forward to the day that I will figure out how to acquire/use that type software.......
I can't imagine I'll ever be on a stage again where I'll need this to go past 5...max :icon_biggrin:
What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?
12AT7 and 12AU7 make good CF candidates. they handle the higher current better than 12AX7. 12AX7 can do it, but it might lead to a shorter tube life. see transconductance ratings on the data sheets. a normal gain stage might a total of 1-2ma, a CF might be 7-10ma. same reason you see both triodes in a 12AT7 or 12AU7 driving a reverb tank or transformer. They are up for the harder work load. for your application, 12BH7 is really no different than a 12AU7, and they go for a bit more $$$, whereas AU7s are given away practically.
If I were you, I would try to keep both of the tubes the same (The LTPI and the CFs), 12AT7 seems like the one to make work. This suggestion comes from a practical point of view: When it comes time to convert from breadboard to real amp, having a 1/2 LPTI and CF in the same bottle will let you keep the coupling cap and plate resistor on the tube and save eyelet board space. (couple the plate to grid with a cap from pin 1 to 7, B+ direct to 6, LP's plate resistor from 6 to 1,, etc...).
Even if you operate the the amp at 3 or 5, you want it to stable with any combination of knob turnings. If troubleshooting is difficult at max volumes, maybe build a dummyload for the purpose of initial stability testing, or build a 'dead box' with a real speaker in it.. Also, as far as 'how loud it is in the next room' or for the neighbors,, I think there is little difference in a 12W amp "dimed" and a 40W amp "dimed". If they can hear it,, its annoying them just the same :-)
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Another tube to try: ECC88.
If you get brave enough, you may even try a pentode CF - I have planning it for some time :think1:. It would unleash some possibilities regarding compression effects.
Hope this helps
Best Regards
R.
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What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?
As I suggested on the old thread (before reading the TAG threads), that the AU, AT and BH were superior than the AX. But those have built the SSS had zero issue with the AX, so may be my concern were over-blown... especially since HPB suggested that the key was to deal with the onset of the grid current and not purposely driving the output tubes into A2 operation. The remaining concern (if it should be there at all) is the high plate-to-cathode voltage that the tube sees with the SSS arrangement, so below is a safer version for you to consider, the downside is more parts count, since you need to create an extra voltage node off the B+ and properly bypass it, ditto for the bias supply, you need to add a resistor divider to get a lower bias voltage. In the SSS design, the supplies were just tap off the existing ones - less parts.
(https://dl.dropboxusercontent.com/u/1326040/CF%20Bias%20Comparison.jpg)
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As I suggested on the old thread (before reading the TAG threads), that the AU, AT and BH were superior than the AX. But those have built the SSS had zero issue with the AX, so may be my concern were over-blown... especially since HPB suggested that the key was to deal with the onset of the grid current and not purposely driving the output tubes into A2 operation. The remaining concern (if it should be there at all) is the high plate-to-cathode voltage that the tube sees with the SSS arrangement, so below is a safer version for you to consider, the downside is more parts count, since you need to create an extra voltage node off the B+ and properly bypass it, ditto for the bias supply, you need to add a resistor divider to get a lower bias voltage. In the SSS design, the supplies were just tap off the existing ones - less parts.
Thanks AGAIN for taking the time to do that jaz, I really appreciate it..... :thumbsup:
I was actually considering taking the negative supply right off of the 120vac input power (in the actual amp build), so this will be easy for people to duplicate, and it will be just a little different than what i'm doing now (running a separate bias supply on the breadboard).....so your suggestion for -100vdc is perfect for that .......
Also, yes,,,,I remember HBP's concern about the high voltage going across the tube at startup, and that's why I choose to use lowered voltages to begin experimenting....
When I first fired the amp up I think I had approx. 250vdc B+,,,,,,and I had plenty of bias supply voltage until I started to crank up the B+ and ran out of bias supply....and that's when adjusting the voltage divider became mandatory,,,,so now I see what he was talking about when he wanted me to give him the specs of my PT ahead of time
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Also, yes,,,,I remember HBP's concern about the high voltage going across the tube at startup, and that's why I choose to use lowered voltages to begin experimenting....
When I first fired the amp up I think I had approx. 250vdc B+,,,,,,and I had plenty of bias supply voltage until I started to crank up the B+ and ran out of bias supply....and that's when adjusting the voltage divider became mandatory,,,,so now I see what he was talking about when he wanted me to give him the specs of my PT ahead of time
You are quite welcome, I am learning something new each time running a sim as well, so it is time well spent. In this particular setup, there is more inter-play between the bias supply and the B+, thus HPB's question on the PT that you plan to use, he is usually a few steps ahead of us anyway.
Notice that the design still has the bias at ~-40V just in case you need the headroom, if not, you can always dial it down. Anyway, let us know if the lower supply voltages work or not, perhaps some adjustments are still needed.
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Posted another short clip, just to emphasize the punchiness of the current configuration......... same settings as before,,, same session, different take
SSdemo2 (http://www.youtube.com/watch?v=NXpxpFViD4M#ws)
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I like that! Very percussive without being too compressed. That is a very cool tone. I think that's getting towards the SRV Dumblish tone.
Thanks for sharing your findings and success on this amp build!
With respect, Tubenit
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Yeah you sound real good SG.
Brad :m8
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Thanks a lot guys....the biggest thing that stands out to me now, is how it stays clean even when I really dig in and I kinda get a rubber band feel off of the strings (they are 11's).....even though, like T said--without being too compressed
Whereas with the original build, it was constantly breaking up, even with the amp on 3,,,and I was surprised with how much "dirt" was in the tone of the AB763.......and because of the SS rect and large filter caps, it was VERY stiff
I honestly expected it to be "more clean" when I originally wired up the stock circuit,,,,so this is a very pleasant surprise
I did make a couple tweaks to the pre-amp and PI,,,, and then with the addition of this CF it just all came together :icon_biggrin:
Now that I see how easy it is to record on my smartphone and that it sound OK, I'll be taking requests :grin:
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Very nice tone!
I did make a couple tweaks to the pre-amp and PI
Mind sharing a schem?
Best Regards
R.
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Very nice tone!
Mind sharing a schem?
Thanks.... I think it's a cool middle ground between an sss and a blackface DR with 6L6
The schematic for the preamp and PI still match the first schem. on post #1
I also slightly tweaked the reverb by removing the cath. cap from the driver tube (as the result of my other thread)...and thats on there
What I was saying was that, when I built the stock circuit, it needed a little tweaking to get it to the point where I was ready to try the PPICF
That's where the month went between the original thread and this one.....
That schem. (in post #1) is my "cut and pasted" adaptation of sluckeys original "AB763 single channel, reverb, no trem" print...look at the cathodes on V1 and the 1K in the PI, thats about it, besides the reverb driver
.....pretty good for a counterfeit print :wink:
The one disclaimer that I should make is that I never changed anything on the schematic to reflect the fact that I'm running a separate bias supply for testing.........once we get it all narrowed down I'll fix the schem. to reflect all changes and how/where I tapped the negative supply...still up in the air
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Now that I see how easy it is to record on my smartphone and that it sound OK, I'll be taking requests :grin:
Ooh, ooh, Louie Louie, no wait, Wild Thing, no, no, In a Godda Devida, no, Whippin Post, no, no, Free Bird!
No wait, I've got it! Little Green Bag, but you need an electric 12 string and a good pair of maracas. Cool bass part too!
http://youtu.be/5ixrvc-YDI0 (http://youtu.be/5ixrvc-YDI0)
Sorry, couldn't help myself, here allow me. :m13
Brad :l2:
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Sorry, couldn't help myself, here allow me. :m13
Yeah,,,thanks for taking care of that for me :violent1:
Well,,,, if you didn't post I never would have known that song existed,,,so it isn't all bad.......but Louie Louie?????
And,,, I wouldn't know that you're actually paying attention to my thread...soooo.....I'm honored by your mere presence Sir :worthy1:
Now don't you have a new workbench to go straighten up? :l2:
I'm gonna expect to see a PPICF in that GA-77,,,,,so ya better get busy,,,,none of us are getting any younger around here :wink:
:icon_biggrin:
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I spent this evening trying a couple of different tube types and messing around with the voltage divider,,,trying to figure out how to increase the bias on that CF tube...
Here's the results:
1) Tried a 12AT7 and it actually introduced more distortion at a lower volume setting, so I can only imagine it's pushing the grids of the 6L6s too hard........forgot to take V measurements while in there, so maybe it was just my haste that ruined the results...
2) Tried a 12AU7 and hated it,,it seemed to have less drive, but also distorted and kinda killed the freq. response across the board.....bleh....couldn't pull it out fast enough
So when I get back in there i'll take V measurements and see if it was something that I was/wasn't doing wrong.....
I couldn't wait to get the 12AX7 back in there and put the amp back to where I love it......it makes me wanna play :thumbsup:
But after that, I gave my ears a break, and messed around with trying to increase the bias on the CF tube and then balance it with the power tube bias......interesting
It's truly a "balancing act" and I can see why a higher negative V supply would be helpful,,,because then your not limited by that one puzzle piece.....as it is now, I'm maxxed out at -85vdc
I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power, and just using that as a separate "highly adjustable" fixed bias supply.....sound good?.....I can't see needing more than that :huh:
EDITED- CROSSED OUT A REALLY BAD IDEA!
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Yeah,,,thanks for taking care of that for me :violent1:
You said any requests? :laugh:
Well,,,, if you didn't post I never would have known that song existed,,,so it isn't all bad.......but Louie Louie?????
That song, Little green bag, is cool, so is Louie Louie, your too young to get it. AM top 20 stuff.
And yeah I read your posts, there good.
Brad :icon_biggrin:
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And yeah I read your posts, there good
Well alrighty then :icon_biggrin:,,,,,I was afraid I was bringing down the collective IQ of the group
Man if I could turn back the clock and pay attention to this stuff when my brain was still pliable,,,I'd be much better off
It is infinitely interesting to me.....
I remember seeing the TUT books for sale (at reasonable prices) and thinking "man that stuff is cool,,,but there's no way I'm smart enough to get it"
Now I have to read and re-read everything, and my reading prompts more reading,,,because I forget what I read a month ago :think1:
I had to purge my brain of every last Top 40 hit I ever memorized just to make room for this stuff.......some days it actually physically hurts....that's how I know i'm getting old :icon_biggrin:
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Well alrighty then :icon_biggrin:,,,,,I was afraid I was bringing down the collective IQ of the group
Not at all, if anything your helping bring it up.
Brad :icon_biggrin:
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I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power
Now why would you do that? We go to extra trouble to keep the AC line separated from the chassis. Just steal some AC from the PT HT winding.
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sluckey
Could you explain that a little more please.
Sounds like SILVERGUN is going to make (-)ve DC straight from the wall mains. Which sounds wrong and / or dangerous at first glance. I just can't explain why.
Why not develop a (-)ve rail from one of the HT taps (340V or whatever) on the PT ?. That tap is only drawing a couple of milliamps at most and could be varied with a VVR as necessary. Can a bridge rectifier be used instead of a single diode, this will enable much better filtering of the (-)ve bias, especially if an inductor / capacitor combination is used. This will stop 50/60/100/120 Hz hum being injected into the CF and 6L6 grids, as always, I don't know nothing.
All the best.
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Why not develop a (-)ve rail from one of the HT taps (340V or whatever) on the PT ?.
Just steal some AC from the PT HT winding.
Brad :icon_biggrin:
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Willabe,
I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power,
That scared me, I felt it was explained too casually,so I wanted it reinforced.
I'm neurotic, MmmmK.
Just steal some AC from the PT HT winding.
All the best.
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SILVERGUN
Got hold of an evaluation version of a SIM.
Built your CF in it and it came up reasonably close to your DC conditions. The SIM says each side of the CF will handle about 5 V RMS signal before clipping. It also said the CF will not drive an impedance below about 330k, which is probably not a problem if we stay in Class AB1. That is with 315V B+ and 85V (-)ve bias.
With 380 B+ the SIM seems to think you'll need 120 V (-)ve bias, fiddling with resistor values didn't get me too far with less negative bias. SIM suggests that CF will handle 7 - 10 V RMS input on each side with the extra volts. Some of the experienced amp builders may know the signal level here offhand.
Tube datasheets for 12AX7 do give plate characteristics for up to 460V at low current. Which means they've been tested at those voltages. With higher volts the AX7 will have about 420V across it.
TheValveWizard (you've probably got his books) here, http://www.valvewizard.co.uk/dccf.html (http://www.valvewizard.co.uk/dccf.html), has a diode-resistor protection circuit for startup grid overvoltage in preamp cathode followers. Could something similar be devised to protect from cathode over-negative-voltage at start up?
Like a 50 Volt zener in series with resistor, zener anode towards valve cathode, zener cathode towards ground.
Don't know if that is of any use or not.
Unfortunately, to get any valves other than 12AX7 you gotta buy the Pro version of the SIM - $1500. Not anywhere near within my budget.
All the best.
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That scared me, I felt it was explained too casually,so I wanted it reinforced.
It is scary. Here's a little more info...
We all like to use a proper 3 wire power cord so we can connect the amp chassis (and guitar strings, microphone body, etc.) to earth ground for safety reasons. And most amps (all of mine) also connect the circuit common to chassis ground.
Looking at the whole picture, you see that Neutral and Ground are connected together in the main power panel. Now look at the bridge and two things should be noted. First, D2 is totally short circuited by the neutral to ground bond in the power panel. Second, D1 has 120VAC directly across it due to the neutral/ground bond. D1 will pop as soon as the power is applied. Hopefully it will just blow apart, leaving an open circuit. As drawn, the circuit cannot work.
There are some easy/safe ways to get safe AC to power the bias supply. The easier/cheaper way is to steal AC from the PT HT winding and use a common half wave rectifier. Another way would use a small 6.3VAC filament transformer, connecting the 6.3 secondary to the filament string. This will produce an isolated 120VAC on the primary winding. Now you can safely use a full wave bridge.
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I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power
Now why would you do that? We go to extra trouble to keep the AC line separated from the chassis. Just steal some AC from the PT HT winding.
Just keeping you sharp sluckey....I gotta make sure you're still with me :icon_biggrin:
But this is a good topic to cover, because I have had this thought for different applications
Now,,,, what about the center tap of the HT winding that we connect to the chassis?......that's where my confusion comes from
Coming from welding machines I tend to like to keep things separated, and overbuild for current handling,,,etc.
I have no problem tapping off one of the HT legs, but i'm just curious why this other way is a bad idea? ....since this bias supply is so critical in this application, I want to make sure it's rock solid
EDITED:
I was typing while you were posting,,,so I'll just go ahead and post this so you know where I'm coming from.....but, now I do understand what you're saying
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Looking at the whole picture, you see that Neutral and Ground are connected together in the main power panel. Now look at the bridge and two things should be noted. First, D2 is totally short circuited by the neutral to ground bond in the power panel. Second, D1 has 120VAC directly across it due to the neutral/ground bond. D1 will pop as soon as the power is applied. Hopefully it will just blow apart, leaving an open circuit. As drawn, the circuit cannot work.
Well here's the ugly truth.....I did it already :huh:
My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....and when I first hooked it up, one of the diodes did go,,,,and I thought I had overheated it during soldering
So I replaced it, and it has held steady for the time being,,,but probably because the bridge is sooo overrated for the application (400vac--8amp)
I'll take this conversation to another thread so as not to detract from the original intent of this thread.....thanks for throwing flags guys!
EDITED AGAIN...NO NEED FOR ANOTHER THREAD,,,SLUCKEY STRAIGHTENED ME OUT
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My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....
No, that's not the same. Your little step transformer changes everything. It provides total isolation from the wall.
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My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....
No, that's not the same. Your little step transformer changes everything. It provides total isolation from the wall.
So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN
Now I really get it,,,,because of the isolation that the xfmr provides....of couse I should know that
Thanks so much for taking the time to explain that!!!!!.........talk about having just enough knowledge to be dangerous :wink: :embarrassed:
So what I will do now is:
Create a negative supply that taps off of one leg of the HT, with caps that are sized appropriately (right now my - supply tops out at -85vdc,,,so I have 100v caps in there).........
The main reason I didn't do it that way in the first place was so that each supply would stay independantly adjustable,,,,buit since we are designing for an actual amp build, I'll start down that path :icon_biggrin:
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Got hold of an evaluation version of a SIM.
Hey GVlennjeff, what's the name of the SIM? I'd like to try the eval out.
Thanks
-LLama
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http://www.spectrum-soft.com/demoform.shtm (http://www.spectrum-soft.com/demoform.shtm)
Micro Cap 10 by spectrum Software. Oh and it's $4.5k for the full version, sorry about that.
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So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN
Connecting the positive tab of the bridge to chassis ground was never a problem, either on you proto board or in the amp. The only problem was connecting the AC mains (line and neutral) to the AC tabs on the bridge.
Now, you can safely connect the line and neutral to a bridge in a circuit that is totally insulated (isolated) from the user. The positive tab would have to be floating and not connected to any earth ground. Not a good idea for an amp though.
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Quote from SILVERGUN:
So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN
When I read this back it sounded a little smart-assed if you read it a certain way.....please don't take it that way
Take it like I'm an idiot whose trying to re-explain it to himself to make sure he completely understands :icon_biggrin:
Just for the record, here's what my current bench bias supply looks like in case anyone was wondering:
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OK,,,,back to work....
I am going to start by updating the bench bias supply,,,by feeding it an isolated 120vac (instead of the 64vac currently),,,and upgrading the caps to 350V.
So this will give us up to approx. -170vdc to continue experimenting with.....for now
I figured it would be easier to just change my bench supply to continue experimenting (for ease and range of adjustment), and then when we have a solid design, we can adjust everything for the final HT voltage we come up with....use a smaller value bias adjustment pot,,,etc......sound like a decent plan?
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Head 'em up... Move 'em out...
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Here's my take on what I'm seeing as I experiment with the voltage divider for the bias, and the reason for needing the increased negative supply......posted so that just in case I'm missing something,,,you guys can correct me....that seems to be working out for me so far :icon_biggrin:
Also, in case someone is following along and needs it laid out for them like I do.....
These results seem to be in line with what Gj and jazbo have figured out and posted above :thumbsup:
EDITED 7/24---FIXED CATHODE VOLTAGES TO READ NEGATIVE,,,ORIGINALLY POSTED AS POSTIVE.....
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Head 'em up... Move 'em out...
Do you know wreck of the old 97?
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SG,
Couple of things.
When you swapped valves around, I think different valves settle with quite different voltages between grid and cathode. That could affect output valve bias dramatically. As you did not do any voltage checks during the valve swapping exercise we have no way of coming to a conclusion about what different valves really sound like. You rushed that one a little.
Also, when you put in the new, more negative bias, you can pull out the power tubes and check that you can obtain a suitable negative voltage on the cathodes of the CF's before inserting the power tubes. Pulling the power tubes should not affect the measurement of bias voltage, and you may need up to -40V with 380 B+. Don't want to stress those 6L6's.
This experiment is very educational and entertaining by the way, thank you.
All the best.
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That scared me, I felt it was explained too casually,so I wanted it reinforced.
Good point and your right, it is dangerous. I thought you might have missed Sluckeys reply.
Brad :icon_biggrin:
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Here's my take on what I'm seeing as I experiment with the voltage divider for the bias,
with a single knob, you are trying to balance two bias's,, one for the CF grids, and one for the 6L6 grids/CF-k's.
perhaps this would solve the problem? remove the 82K-R,, and isolate the two neg. bias's:
(http://i.imgur.com/6TDbJ56.jpg)
The bias on the LTPI is about -40v, the bias on the 6L6's is about -40v,, I'd shoot for a bias on the CFs of ... -40v !!! (so that would be -80vdc on the CF grids).
but then, whats your plate voltage on the CF's? 300v? what happens to a 12at7 if you drop 340V across it? (isn't max plate voltage 300V?) and if the plate to grid is 380v ??
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When you swapped valves around, I think different valves settle with quite different voltages between grid and cathode. That could affect output valve bias dramatically. As you did not do any voltage checks during the valve swapping exercise we have no way of coming to a conclusion about what different valves really sound like. You rushed that one a little.
You are 100% correct Sir,
So I stayed tonight and did the right thing.....got your results
I did increase the negative supply and was able to bring -142vdc to the table after all was said and done.....it didn't have the effect that I thought it would have with that 12AX7 in there, and the most difference I was able to dial in was close to 2 volts.....
So I shut it down and did 2 separate tests with each other tube type I had here,,,,because I want to know the real deal too....
These next 2 charts are accurate readings with those 2 different tubes,,,and you're dead right..........the voltages were way different
SERIOUS NOTE:
I noticed that just by swapping from the 12AT7 to the 12AU7 (AND NOT ADJUSTING ANYTHING) it immediately increased my cathode current in my 6L6s to 130ma EACH :huh:
SO BE CAREFUL OUT THERE KIDS.....DON'T TRY THIS STUFF AT HOME! :icon_biggrin:
EDITED 7/24- CHANGED A RESISTOR VALUE IN THE VOLTAGE DIVIDER TO REFLECT ACTUAL VALUE (82K NOT 870K)..IN THE AT7 EXAMPLE
UNFORTUNATELY I DIDN'T MEASURE WHERE THE POT WAS SET
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OK, so thats the scientific stuff,,,,,but my other reason was to put the different tube types in and listen
Here's the facts as I heard them: (with the amp re-biased with each CF tube changed)
1)The original 12AX7 stays pretty clean through 5 on the volume, and gets more overdriven by 7, but still tolerable, and just brings more bite and push to the sound (it already has a lot of bite and push at 3).....above that I don't hear an increase in volume,,,just distortion
2)The 12AT7 was fine around 3, but seemed a little more harsh than the AX,,,and as I turned the vol. up to 5 it was overdriven to the point that I wouldn't want to go any further......but for the sake of experimenting, I went to 7, and lost clarity, and it was just too overdriven and sloppy sounding.....not where I want to head with this build
3)The 12AU7 was more overdriven on 3 than the other 2, and lost the "controlled clean definition" that I loved about the 12AX7, 5 was terrible and 7 was unbearable.....just bad distortion, that lost the crystal clear glassiness completely,,,,and felt wayyy too pushed (maybe that had something to do with how badly balanced the triodes seemed to be) :dontknow:
Hope that helps :icon_biggrin:
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Leaping Electrons Batman !
Those differences in 12AT 12AU tube bias are HUGE aren't they. The results of the listening tests are the exact opposite of what one would expect from those valves and I suspect that each valve needs to be run at much different current values to get them to work well in that particular circuit. Just drew a few lines on plate characteristics graphs and confirmed that (somehow, without knowing what I'm really doing)
Don't mind me SILVERGUN, I'm using this great effort of yours as a learning exercise. It was sounding great so, maybe that was it.
I've got just a touch of Mad Scientist Syndrome and will certainly be trying out any ideas put forward here when my parts arrive.
Having said that, terminalgs's schem looks good to me, you could set it up so that the AX is biased exactly the same as original and there should not be much of a difference in sound. PLUS, that will give me a better platform to experiment with different valves and impedances.
SERIOUS NOTE:
I noticed that just by swapping from the 12AT7 to the 12AU7 (AND NOT ADJUSTING ANYTHING) it immediately increased my cathode current in my 6L6s to 130ma EACH :huh:
SO BE CAREFUL OUT THERE KIDS.....DON'T TRY THIS STUFF AT HOME! :icon_biggrin:
Sorry bout that but I did mention to test (voltages on cathodes of CF's to be more explicit) with 6L6's out of socket first.
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The bias on the LTPI is about -40v, the bias on the 6L6's is about -40v,, I'd shoot for a bias on the CFs of ... -40v !!! (so that would be -80vdc on the CF grids).
I don't know term,,,,,I'm starting to lean back towards the way we originally had it..
that's the best it's sounded....
Actually, I'm leaning more towards jazbos drawing from reply #69
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Sorry bout that but I did mention to test (voltages on cathodes of CF's to be more explicit) with 6L6's out of socket first.
I would never blame someone else for me being a knucklehead :icon_biggrin:
Luckily I've left a second meter hooked up to measure 6L6 cathode current constantly, and it's how I make some quick adjustments....
Just thought I should let people know, that if you build it,,,don't think you can just safely swap tube types :blob8:
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Regarding jazbo's #69, if you're referring to the SS section it looks real good, lots of headroom.
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The bias on the LTPI is about -40v, the bias on the 6L6's is about -40v,, I'd shoot for a bias on the CFs of ... -40v !!! (so that would be -80vdc on the CF grids).
I don't know term,,,,,I'm starting to lean back towards the way we originally had it..
that's the best it's sounded....
Actually, I'm leaning more towards jazbos drawing from reply #69
the original circuit sounds good at 3/10 volume. the bias on the CF was -1, so the AC voltage was going through that stage unclipped because it was a small Vpk signal,, maybe 8V? 10V,, when you turned up to 5/10,, you exceeded the -1 biased CF's ability to pass signal unclipped,, and further up the dial, between 7/10 and 10/10,, there is no volume increase,, and not much distortion increase because the signal is so heavily clipped, it probably looks like a square wave instead of any semblance of a sine wave.
your original goal was "great clean sound that will stay crystal clean as you crank the amp", and it doesn't sound like you there yet...
a hallmark of an AB763 is it continues to sound better and better between 7 and 10!
Go back and look at the bias's on some of the CF's of those referenced known to be solid amps. I'd look no further than the most stable, rock-solid clean amp I've ever loaded in and out of a Dodge van: the SVT: the 5/69 rev.B sch I've got shows plate at 220v, grid at -77v, and cathode at -47v.
BTW: re: tube rolling. each of those tubes has a different plate resistance, so each one would change the DC current flow and thus the cathode voltage, thus changing voltage at the cathode, thus changing the bias of the 6L6's and the CF stages. those changes effected distortion, etc. but you aren't looking for distortion in this part of the circuit... and... IMHO, at this point in the development of the circuit, tube rolling 12xx7s isn't good tuning step (the circuit seems to still need tweaking...)..
press on!
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the original circuit sounds good at 3/10 volume. the bias on the CF was -1, so the AC voltage was going through that stage unclipped because it was a small Vpk signal,, maybe 8V? 10V,, when you turned up to 5/10,, you exceeded the -1 biased CF's ability to pass signal unclipped,, and further up the dial, between 7/10 and 10/10,, there is no volume increase,, and not much distortion increase because the signal is so heavily clipped, it probably looks like a square wave instead of any semblance of a sine wave.
If you are talking about a grounded cathode stage, then yes, hard clipping will occur when the input voltage exceeds the Vgk bias voltage; but that is not the case here with a cathode follower stage, which has another name - the grounded plate amplifier. So to see what is happening, flip the tube upside down, now the real clipping point is actually between the grid and the plate (which is at AC ground) and not the grid and the cathode, to illustrate this point, please see the following chart: the input voltage is set at a ridiculously high 80Vpeak (do not try this on the bench!) and look at the output waveforms: the SSS section with a low ~-3V bias remains un-clipped and the SS with ~-16V bias may appear clipped but only because of its low bias supply (-100V). The SSS waveform also shows possibly why Mr. Dumble designed the CF stage with such high bias supply - it provides a ton of headroom!
X1: 12AX7, X2: 12AU7
(https://dl.dropboxusercontent.com/u/1326040/SS%20CF%20Comparison.jpg)
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Thanks for pushing term,,,,,and thanks for pushing back jaz!
Great stuff with a great explanation.....priceless
If you are talking about a grounded cathode stage, then yes, hard clipping will occur when the input voltage exceeds the Vgk bias voltage; but that is not the case here with a cathode follower stage, which has another name - the grounded plate amplifier. So to see what is happening, flip the tube upside down, now the real clipping point is actually between the grid and the plate (which is at AC ground) and not the grid and the cathode, to illustrate this point, please see the following chart: the input voltage is set at a ridiculously high 80Vpeak (do not try this on the bench!) and look at the output waveforms: the SSS section with a low ~-3V bias remains un-clipped and the SS with ~-16V bias may appear clipped but only because of its low bias supply (-100V). The SSS waveform also shows possibly why Mr. Dumble designed the CF stage with such high bias supply - it provides a ton of headroom!
It sounds like we're onto something here......Where would you like to see me go from here?
I have the ability to take the negative supply up to approx. -300vdc pretty simply and with minimal modifications and no safety concerns....could be done today...
Maybe I can benefit from keeping a lower B+, by not stressing the CF tube as much as the sss design?,,,,but stil use a high - supply V
Does this explanation partially explain why it may be true in the original hand drawing of the sss that the cath. was marked as measuring 1 volt less negative than the grid?(+1v bias?)....or do you think that was an error and it has to be the other way around?
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Here's the quote from Merlin's website......"the triode gain stage" chapter that is available for free download (so I don't think he'll mind me using it)
http://www.valvewizard.co.uk/gainstage.html (http://www.valvewizard.co.uk/gainstage.html)
I had mentioned this concept earlier in the thread, but rz shot me down pretty quick and I wasn't informed enough to fire back.....
Am I possibly taking it too far out of context by imagining that it applies to this application?
This is an excert from page 12---Biasing section
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It sounds like we're onto something here......Where would you like to see me go from here?
I have the ability to take the negative supply up to approx. -300vdc pretty simply and with minimal modifications and no safety concerns....could be done today...
Maybe I can benefit from keeping a lower B+, by not stressing the CF tube as much as the sss design?,,,,but stil use a high - supply V
Does this explanation partially explain why it may be true in the original hand drawing of the sss that the cath. was marked as measuring 1 volt less negative than the grid?(+1v bias?)....or do you think that was an error and it has to be the other way around?
Good catch, I missed that completely, from the simulations and your bench measurements, I do not think those readings are correct - at least not the grid... Also, in one of your earlier posts for the 12AX7, the cathode voltages were mis-labeled, they should be negative not positive 24V, I hope...
As for which way to go, you need to lock down the output stage first, are you sure you want the low power output? You are leaving quite a bit of power on the table sorta speak, sorry to be a nag... Do you already have the PT?
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My first ExpressSCH piece of art. Could you humour me and plug it in jazbo8.
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Also, in one of your earlier posts for the 12AX7, the cathode voltages were mis-labeled, they should be negative not positive 24V
I went back and fixed that pic in that post.....thanks again
As for which way to go, you need to lock down the output stage first, are you sure you want the low power output? You are leaving quite a bit of power on the table sorta speak, sorry to be a nag... Do you already have the PT?
I don't take it as nagging.......your help is priceless
6L6s are staying, and I like the low B+ just for the sake of running cooler across the board....
I'm fine with the lowered power output (can't see ever needing this amp to be louder), and will just make up for the lower voltages by running a little more current.....the bias calculator says 65mA per tube @ 320V plates is right on @ 70%
I don't have the PT yet, so thats still up for discussion.......it might be difficult to find a perfect choice that fits these voltages
Maybe I should look at a more generic 120 to 240vac xfmr?.....and then just run a separate filament xfmr
Maybe this one with it's 260-0-260 secondary taps: :dontknow:.....that'll obviously bump thing up a little
http://www.classictone.net/40-18065.pdf (http://www.classictone.net/40-18065.pdf)
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What about HBP's concern about a high voltage accross that CF tube at initial power up?
If we wind up with 320V on the plates and a -240 supply, does that seem a little more reasonable/ acceptable to expose it to 560V? (instead of 700+)obviously it is a slight improvement....and is my thinking correct?
Is there a 12AX7 that will tolerate this better than others, that you know of?.....and are we still thinking AX7 is the best choice in my application?
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What about HBP's concern about a high voltage accross that CF tube at initial power up?
If we wind up with 320V on the plates and a -240 supply, does that seem a little more reasonable/ acceptable to expose it to 560V? (instead of 700+)obviously it is a slight improvement....and is my thinking correct?
Is there a 12AX7 that will tolerate this better than others, that you know of?.....and are we still thinking AX7 is the best choice in my application?
When I checked with the builders over at TAG, not one said they had an issue with the HV at start-up, so perhaps it is a non-issue? Perhaps HPB could give some guidance on this. Some have suggested that the new JJ 12AX7s have no problem with the high voltage, so that's worth a look. In any case, you now have several options to lower the plate and the negative bias supply if HV across the tube is indeed an issue. Also because you are running at a lower power, the negative bias requirement could be reduced, so that should help as well.
My first ExpressSCH piece of art. Could you humour me and plug it in jazbo8.
Yup, your design works well. With the 50k pot max'd out, the CF's cathode is at ~-52V, which is ample for this design, when set to 20k, the cathode is ~-24V, just about right for the SS. I would also insert a "stopper" resistor in series with the pot, so no one will accidentally turn the pot all the way down and kill the tubes. :sad2:
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My thinking is that the supply rail + and - are fairly close so it should like handle 300V peak to peak input. :l2:
Means setting up special supply rails, easy with a few R's, a couple of FETs and a couple of electrolytics though.
This would solve all overvoltage concerns, especially if there were some caps hanging off the gate resistor of the FETs controlling the rails, 10 second soft on if one was so inclined, like me.
As I'm merrily setting up multiple B+'s and B-'s, had the menacing thought of direct coupling the PI as well upon waking this morning.
From experience, how well does your SIM track reality?
Hows this?
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In simpler circuits, my experience with the simulations were quite good, but since I don't have a test bench at the moment, I am relying on SG to give us feedback on the accuracy of the simulations, I wager they will be pretty close, at least within the tolerance of the tubes used. The stopper looks fine, SG can always set it to where he feels comfortable (or needed).
So if the voltages stay where they are now, then my earlier plate characteristic chart could be used to estimate the output power and distortion, I have not looked at the preamp section yet, it may need a few adjustments to make sure it does not overdrive the PI and output, since this is suppose to be a clean amp with good headroom... more fun ahead :icon_biggrin:
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Here is a little mod that will enable precision calibration of the PI balance, well almost.
82k may need to be tweaked up or down a bit.
A 68k resistor and a 50k pot will give a greater range of adjustment but probably overkill.
B+ is arbitrary, the more you can put on the valve, the greater the headroom.
DC coupling the PI presents a LOT more difficulties with +180V / -150V than I initially imagined. :embarrassed:
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Last night I ran some more tests and just pushed the boundaries around to see if I could hear any differences...
I changed voltages, and went as high as 380v B+ and messed around with bias setting from low to high and back,,,,in search of a noticeable sweetspot...
I couldn't seem to find any reason to go in any other direction than where we have been going, because nothing really made that much of a difference....
I tried the AU7 and AT7s again and just wanted to go back to the AX7,,,,because even though those tubes work fine and I can manipulate the setup to get an acceptable sound,,,,,the AX7 still sounded the best to me
My thought about this is in line with something that PRR had mentioned on the last thread......
> why not use an even more capable driver tube?
How much can you shove into the 6L6 grids before they melt?
It's not clear on the datasheets. With all the many different 6L6es made, and variations in grid-wire processing, it may be best to be conservative.
And maybe thats the point......even though the AU +AT may be better suited to do more of the job (especially if your running more output tubes),,,,,we only need so much work done, in this smaller application,,,,and the AX7 covers it
SO here, the AU+AT might just be too much of a good thing.....?...make sense?
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As I have already stated, YOU MAY HAVE IT! :icon_biggrin: If you want to push the 6L6's up to 380V, will the current configuration handle that? Do you want to design an amplifer that can meet real life professional requirements, and does the current platform satisfy that engineering specification?
The issue of potential over voltage at power up may still be an issue, also, it may not really be an issue in the real everyday world.
Depends! Possibly on how often you like to replace 12AX7's, possibly not.
The schemata I derived for solving that issue, by the manipulation of supply rail feeds to the CF, can be applied to a 12AX7. I see no reason that it would change anything about the small signal performance of the stage. That is if you call 200 volts peak to peak small signal. Leaves the question of switch on stress addressed. I'm going to try it out when the mailman arrives. Some would call it an un-necessary waste of components.
DO NOT TRY MY EXPERIMENTS WITH A 12AX7. Love this emoticon, been waiting to use it. :blob8: AT or AU only. I'll post a 12AX7 version shortly, just for you, even if you choose not to use it. :icon_biggrin:
It all depends on how much fun / fulfilment you get out of playing with your spaceport breadboard setup from my philosophical perspective. (Wish I had one, working on turning my tool-shed into a physics lab at the moment)
On a side note, I think I've cracked the - DC coupled to PA phase inverter - , so CF may be redundant. That depends on weather the phase of PS ripple of (-)ve supply is IN or OUT with phase of (+) supply from a hum perspective. Which I will have to measure in a "Physics Lab" environment. That means nails n screws n fitting woodcuts n sealing spider cracks n applying long term pesticide against ants n termites n running mains power cables n determining lighting requirements n ventilation n fume cupboards n n n ....l. :cussing: :BangHead:
I should point out, I LOVE MY D'MARS ODS, but why stop there. What a fascinating hobby.
All the best and MORE SOUND CLIPS PLEASE SILVERGUN.
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Hit the pause button! Found a discussion on MEF (http://music-electronics-forum.com/t33819-post312076/#post312076) that's relevant to the SS design, the SVT has a similar CF design like the SSS and it seems that some 12AU7 failures could be related to the HV across the tube and/or the cathode and the heater, like HPB described... But since the SVT has a HV timer, it would point to the latter case as the likely candidate. :dontknow: Whatever the reason, it seems putting in new production tubes gets rid of the early failure.
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jazbo8,
Any links on how to elevate (or depress in this case) the 6.3 V AC heaters to a higher or lower DC level.
Guess you could tie either the 6.3 V AC ground tap to a DC supply voltage, but you need it at ground zero for the rest of the circuit.
All the best.
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jazbo8,
Any links on how to elevate (or depress in this case) the 6.3 V AC heaters to a higher or lower DC level.
Guess you could tie either the 6.3 V AC ground tap to a DC supply voltage, but you need it at ground zero for the rest of the circuit.
All the best.
No links offhand, but this CF design is a special case, where a separate filament supply might be useful. As you said, you don't really want/need to lift the heater up for the other tubes, so two independent filament supplies is an option, of course, it would also mean higher cost and more real estate taken up... Alternatively, may be just go with more robust new production tubes and call it a day, since Enzo said in his MEF post that he never had one SVT came back in for service after the tube replacements.
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SG you can get Edcore iron in about any voltage you might want and if you don't see it on there web site call and ask . Only down side 4 to 5 weeks for delivery, I love there pretty blue color .
Bill
:guitar1
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Thanks TJ, I'll check their stuff out..... :thumbsup:
Hit the pause button! Found a discussion on MEF (http://music-electronics-forum.com/t33819-post312076/#post312076) that's relevant to the SS design, the SVT has a similar CF design like the SSS and it seems that some 12AU7 failures could be related to the HV across the tube and/or the cathode and the heater, like HPB described... But since the SVT has a HV timer, it would point to the latter case as the likely candidate. :dontknow: Whatever the reason, it seems putting in new production tubes gets rid of the early failure.
Thanks for continuing to do your homework on this one jaz,,
I'm hoping that the lower voltages in the SS will calm things down a bit too...I haven't heard a reason why they should be higher....the Cf makes the amp plenty punchy @300 B+
I guess, if anything,,,it just appears that it's more difficult to find lower voltage/higher current xfmrs, which is probably partially why it's not very common to see :dontknow:
People don't usually put this much effort into a 25w single channel clean amp, or deviate much from the AB763 stock numbers :wink:
I did modify my bench bias supply and was able to get to -280vdc for testing, but was unable to hear any significant difference.......To me it doesn't make any sense to have a huge negative supply if it's not needed to make an audible difference........
I sent my scope out about about 3 weeks ago, so I'm hoping to get it back any day so maybe I can see whats going on.........I'm just starting to get the hang of it, so I might have to ask for help to know what I'm looking at/for, in this case
GJ, your interest and excitement in this project help keep me going and I appreciate all of your ideas.......can't say I'll use em all,,but it makes for interesting reading :icon_biggrin:
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SILVEGUN,
You have built a breadboard system to try out ideas.
What is the point of that exercise if you are reluctant to try stuff?
Not saying my ideas are guarenteed to work, but trying to put a man on Mars takes a bit of calculated risk.
I will use the ideas generated here, and I have you to thank for being TEAM LEADER, so keep up the pursuit of what it is that you want to achieve.
What is it?
The ultimate clean punchy amp?
Have you defined your engineering objective?
You got a great buzz out of pushing through your uncertainty about direct coupling the CF to the PA. So which mountain shall you climb now. Given that you've got a great sounding amp it's got to be somewhat disconcerting to tackle the next summit (metaphor).
So, I have to ask, where are you, "we" , on this project, what do you think we should put our energies into?
You appear to have control of bias, so why not put 380V on the 6L6's?
Anyhow, it's been a lot of fun and very educational. I'll refrain from further suggestions until specifically requested to do so.
All the best.
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GJ, I hope you didn't take what I said there as anything negative.....(that's the problem with words on a page,,,they are up for misinterpretaion)
You tend to think on more levels than I am capable of (at least with my limited education) ,,,and I was letting you know that I appreciate you throwing stuff out there, and just want to let you know, that I may not be able to keep up with you :grin:
It is frustrating for me to be somewhat intelligent, BUT not be able to grasp some of the concepts that get presented here...I've still got a lot of reading to do...
It can be embarrassing to get an education on a public forum :embarrassed:....
I've numbered your questions again, and consolidated them...
I am listening :icon_biggrin:
SILVEGUN,
You have built a breadboard system to try out ideas.
1) What is the point of that exercise if you are reluctant to try stuff?
2) Not saying my ideas are guarenteed to work, but trying to put a man on Mars takes a bit of calculated risk.
I will use the ideas generated here, and I have you to thank for being TEAM LEADER, so keep up the pursuit of what it is that you want to achieve.
3) What is it?.....The ultimate clean punchy amp?
4) Have you defined your engineering objective?
5) You got a great buzz out of pushing through your uncertainty about direct coupling the CF to the PA. So which mountain shall you climb now. Given that you've got a great sounding amp it's got to be somewhat disconcerting to tackle the next summit (metaphor).
6) So, I have to ask, where are you, "we" , on this project, what do you think we should put our energies into?
7) You appear to have control of bias, so why not put 380V on the 6L6's?
8) Anyhow, it's been a lot of fun and very educational. I'll refrain from further suggestions until specifically requested to do so.
1) I'm not reluctant to try stuff,,,,just have a limited time schedule,,,and an already large list of stuff to try
2) I am learning as I go, so I try to stick to one specific thing at a time,,,,and that alone is difficult,,because I want to try everything, but am not experienced or imaginative enough to want to continue pushing further for the sake of experimentation.......just get this one done, and then on to the next
3) AB763 with PPICF safely and solidly installed, and then migrated into a combo chassis (that I still have to build) with one 15" speaker,,,for me to eventually use on stage as part of an A/B configuration,,,,,,with this amp running the "super clean" part of the program
4) To eliminate the mystery surrounding this PPICF circuit as it was installed in the SSS, for the purpose of reducing my fear (and maybe some fear of others) to actually some day build a SSS or derivative..........I am still considering the idea of laying out the entire SSS circuit on the breadboard while this part is already done.........once we get the SS off of here
5) I don't want to build complicated amps,,,and I hope I'm not addicted to the buzz of discovery.....it was just a side effect of feeling like I accomplished something I have been thinking about for the last 4 months leading up to this......the first time I heard that amp I thought "I'm gonna build that someday".....so to take some of the mystery out of that is HUGE, and cause for celebration
I left my original project hanging to start this one because I was moved by the spirit to do so.....If you read a little bit of that thread, you'll see where I came from and how quickly I jumped in over my head;
http://www.el34world.com/Forum/index.php?topic=14575.0 (http://www.el34world.com/Forum/index.php?topic=14575.0)
6) I was truly hoping that a couple of the really big fish would swim along here and confirm/deny some of our thoughts.....BUT, if they don't and we continue down this path, I'm looking forward to getting my scope back and hooked up, and taking a closer look at what's going on here, and stop speculating by merely hearing and/or not hearing a difference
7) Good question....but I think---"why PUT 380v on the 6L6s?".....if we're worried about high voltages, why continue to try to inject high voltages? ......I don't need 50 clean watts, and I like the idea of a lower voltage, high current design (I like to build "special" amps)
8) Yes it has, and continues to be......you learn a lot about yourself when you "put yourself out there" like I have ......it can be difficult to "juggle personalities" that arise in a thread.......if you notice we seem to have lost a few along the way, possibly somewhat due to jazbo's fact finding :dontknow:
Please don't let one misworded post chage your opinion of your place here.....here's the correct wording:
Gj, your interest and excitement in this project help keep me going and I appreciate all of your ideas.....thanks for all of your continued help!
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This has been a very very interesting thread!
And I like the results I hear from SilverGun's video. It's positively impressive. I think SG has been pretty gutsy and innovative on this.
What are the remaining questions to be answered at this point? Can someone list those? (if there are any?)
It seems like there is a dynamic of something unfinished or unanswered, but I can not tell what that would be? I pick up on that unfinished or unanswered theme because there continues a plethora of more info and perspectives?
Do we have reasonable closure for this project on this specific amp? I can't tell where things are at this point?
With respect, Tubenit
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What are the remaining questions to be answered at this point? Can someone list those? (if there are any?)
1) Should I be concerned about the fact that when using a 12AX7 in the CF I only wind up with the grids being approx. 1 volt more negatine that the cathode?....that seemed to be a concern when you look at this from a standard biasing scheme (as terminalgs brought up earlier),,,,but if jazbo is correct about this being a "plate grounded" design, does that over-ride the way we would normally view those measurements for biasing this stage? (sorry jazbo, it wasn't obvious to me)..
2) Can anyone confirm/deny Merlin's quote above as being relevant to this design?.....
Is it possible that it might be OK to have the grids even slightly more positive than the cathodes in this stage?....not that I need them to be that way, but it'll just help me confirm/deny the validity of the hand-drawn scematic of the sss
3) How can I confirm/deny the actual negative supply voltage needed to satisfy the CF for what is was designed to do?....I don't think I have heard a difference between -85v or -280v and in each case, the voltage divider just knocks it down to -25ish volts.........so what was the point of Dumble putting -328 there?......and, should I be aiming for the -40vdc to be making it through to the CF?
3) Am I missing anything by concluding that the 12AX7 is "THE" tube for this job, based on the smaller scale of this design?........the perception being that we don't "need" anymore current handling in this stage
4) Is HBP OK? :wink:
Do we have reasonable closure for this project on this specific amp? I can't tell where things are at this point?
We're close....once I get my scope back and hooked up, I'm hoping to trace down the point in the circuit that is causing clipping at high volume...
I was getting some "breakup" in the amp before the CF install, so I tweaked some of it out by reducing gain in V1....
Once we nail down the specifcs of the negative supply and cofirm that it's not the CF that is clipping, I'll go back thru the preamp and try to find it and reduce it without killing the tone and response we've created......I didn't want to change anything in the preamp while we were tweaking the CF...
While at a standstill the other night I tried a 5751 in V1, which didn't change the audible OD much,,,and then a 12DW7 that did, but made the amp too quiet, and took away too much ...
With the 12AX7 in the CF spot,,,there is a spot on the volume control (around 7) where the amp seems to begin to clip (audibly distort---which my friend liked) hard, and it is immediately recognizeable by an associated low hum, that gets introduced into the normally quiet output signal, when I hit 7-8
With the AU7+AT7 that spot gets lower on the vol. control......(approx. examples - AU7-3 AT7-5)
I'll probably never have to turn it up past 7.......I'm 44 now
:icon_biggrin:
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OK, had a couple minutes tonight, and since I'm tired of claiming ignorance,,,I decided to use my head and try something different :think1:
So I hooked up 2 meters at the same time to see what happened to the relationship of the grids to the cathodes on the CF as I fed it a signal and turned it up past 7......
The first phenomenon that I noticed was that the grids now appeared 2v more negative than the cath.--------so maybe the impedance of the meter has an effect on the readings when you take one reading, take it off, and take the other reading, when you're only workling with one meter at a time :dontknow:
So at idle we have a 2v spread (-21v grid; -19v cath.) , and as I increased the signal I retained that 2v spread throughout the entire span of the volume control (up to -55v grid; -53v cath.)
Just a note, while it was fresh in my head :thumbsup:
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Something odd with the reading when crank'd up, how is the signal applied and how are you measuring the voltage?
I was writing when you posted...
===
I tend to get into analysis paralysis sometimes, sorry if that slows things down for some... The fact is, there seems to be conflicting info on the PPICF, not so much on how it works but rather its safe operating conditions, unfortunately no amount of simulations by me or the others can give a definite answer, since you have a breadboard with variable supplies, I was hoping that you can observe the waveforms/voltages on startup and during over-drive, i.e., when it's the most stressful for the tube to see what we are dealing with. Where is the scope when you need it? :wink:
I think the reason that you do not hear much difference between the high vs. low bias supplies, is simply that you are not pushing neither the PI, CF nor the PA to their limits. Do you have a dummy load? It would be interesting to see the waveforms when the gain and volume controls are turned up, you don't want to do that with a speaker, it will be too damn loud...
Although, I am still somewhat puzzled by the performance of the AX vs the AT & AU. If the AT & AU are properly setup, they should perform equal or better as a power amp driver than the AX, so much for being an armchair designer :w2: But don't let that stop you from completing your build, just curious, that's all...
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I phrased that a bit clumsily. Sorry if it came across as a dummy spit. :icon_biggrin:
What tubenit said, about closure - that's what I was kinda trying to express.
Too many different ideas lead to confusion, so I'll stop with the experimental suggestions.
I believe that the hard clipping you are hearing at 7 is the CF clipping as per the graph in jasbo8's reply #112.
I can SIM 12AX7's. Can't get them to cope with much more than 5 V RMS with the SS's current bias setup (+315 / - 85). I think the PI is capable of putting out about 50 V RMS. Got notification that my parts have been dispatched from local warehouse so should get them Monday. Then I'll build my AU suggestion reply #120 above and splice it into my D'Mars temporarily (I'll do AX, 5751, AY,AT, AV and AU versions). If it works, I'll ExpressSCH the CF and it's MOSFET controlled power rails and post here.
To address your point form query;
1) and 2) Don't be too concerned about grid bias anomalies at this point. Although the new readings are more in line with expectations.
3) regarding negative supply, it's a complicated juggling act that in practical terms determines at what point the hard clipping occurs. (What your hearing at 7, what you see in the graph of jasbo8's reply# 112.) That is why I'm going for + 170 / - 130 or thereabouts. It's a fun juggle that provides maximum headroom without overvoltaging the valve (in theory).
4) HBP seemed enthusiastic on the previous thread, let's hope it's a well deserved vacation in some tropical paradise.
It is kind of unfortunate that some of the big fish haven't rocked up to lend a hand. They have their reasons no doubt.
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It is kind of unfortunate that some of the big guns haven't rocked up to lend a hand.
I was wondering about that as well, does anyone know what happened to HPB? He was an active participant on the old thread, and many of the work-arounds came from his suggestions... We can sim all day, but nothing like someone with actual building/servicing experience with the amps.
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3) How can I confirm/deny the actual negative supply voltage needed to satisfy the CF for what is was designed to do?....I don't think I have heard a difference between -85v or -280v and in each case, the voltage divider just knocks it down to -25ish volts.........so what was the point of Dumble putting -328 there?......and, should I be aiming for the -40vdc to be making it through to the CF?
Try to determine if changing the negative supply from -85 to -280 changes the point at which hard clipping occurs, Volume at 7 with -85V or volume at 8 with -280 V.
(Foam earplugs or wait till you get the oscilloscope back and use a dummy load.)
May have misunderstood your reference to -40V DC making it through to the CF BUT;
The CF design should end up being a "building block" that can be spliced into any guitar PA IMHO.
Given 6L6's, depending on the B+ voltage used, anywhere between 280 and 400 V DC. This CF design should be able to provide adequate bias. So that means it should have an adjustable range of say -15V to -50 V DC at the cathodes. Maybe an even greater range if you want to include other output valves EL34, KT77, KT88 etc.
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Something odd with the reading when crank'd up, how is the signal applied and how are you measuring the voltage?
Signal is going into main guitar input, and just quickly tried to dial in something that sounded like an "A" 440hz
I cranked the amp up to 8 and then took the level of the s.g. to a point that sounded as loud as the guitar signal when cranked
I had one DMM connected to the grid of one side of the CF, and the other one connected to the cathode of that same triode
At idle I was reading -22vdc on the grids and -19vdc on the cath.,,,,I had to get the signal pretty loud to get those voltages to move but once they did, they stayed evenly spaced at 2 volts apart all the way up to -55 grids ...-53 cath.
I thought that was a good sign.....shows you how much I know :l2:
I was hoping that you can observe the waveforms/voltages on startup and during over-drive, i.e., when it's the most stressful for the tube to see what we are dealing with. Where is the scope when you need it?
Me too.....I will get a scope hooked up as soon as possible, and try to follow your instructions more closely, and just let you guide me through some of the measurements we need
I do have a dummy load but I just haven't wired it up for use yet (8 ohm- 200watt "corncob" resistor),,,,and you read my mind...I kinda felt like I could blow a speaker when I was doing the V measurements earlier....LOUD
I went back and edited my post because I had reported a difference of 3 volts when it was actually 2....math is not my friend :icon_biggrin:
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Got notification that my parts have been dispatched from local warehouse so should get them Monday. Then I'll build my AU suggestion reply #120 above and splice it into my D'Mars temporarily (I'll do AX, 5751, AY,AT, AV and AU versions). If it works, I'll ExpressSCH the CF and it's MOSFET controlled power rails and post here.
Very cool, I'm looking forward to hearing your results...
Try to determine if changing the negative supply from -85 to -280 changes the point at which hard clipping occurs, Volume at 7 with -85V or volume at 8 with -280 V..
Good plan.....it will make sense to do all of these checks with the scope hooked up, so I'll make a list of request that you and jaz have mentioned, and get answers instead of my opinions or guesses.....including confirming/denying some of the previous supplied sims
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I had one DMM connected to the grid of one side of the CF, and the other one connected to the cathode of that same triode
At idle I was reading -22vdc on the grids and -19vdc on the cath.,,,,I had to get the signal pretty loud to get those voltages to move but once they did, they stayed evenly spaced at 2 volts apart all the way up to -55 grids ...-53 cath.
I see... but I do not think that's the correct way to measure it, since the cathode is the output and has large voltage swings, instead you should take the measurements with respect to a fixed reference like the ground. If you look at the plot I posted earlier, you can see the bias voltages remain at ~-40V (center line), and those reading are with respect to ground.
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jazbo8,
I think SILVERGUN is measuring wrt ground. I THINK (don't know) that what is happening is that once clipping sets in the whole DC environment is becoming compromised and pulling down. That may be shutting down the output tubes as well, giving lots of crossover distortion. Be interesting to observe if the DC shift corresponds with the onset of clipping.
All the best.
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Good stuff guys it will be interesting to see what scope shows .
Bill
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jazbo8,
I think SILVERGUN is measuring wrt ground. I THINK (don't know) that what is happening is that once clipping sets in the whole DC environment is becoming compromised and pulling down. That may be shutting down the output tubes as well, giving lots of crossover distortion. Be interesting to observe if the DC shift corresponds with the onset of clipping.
All the best.
I think you are right after re-reading the post, so the simulation is bogus?! :sad2: Must get to the bottom of this - if like you said, the bias is being pull down to -52V, the tubes are in complete cut off :huh:
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I think you are right after re-reading the post, so the simulation is bogus?! :sad2:
Have you actually SIMmed the PI, CF and PA and bias supplies as one unit. If not then the SIM is not necessarily bogus. I've seen this paradigm of behaviour before in all sorts of "out of boundary" situations.
We need to wait until it can be measured before we throw our computers in the fish pond. :l2:
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I think you are right after re-reading the post, so the simulation is bogus?! :sad2:
Have you actually SIMmed the PI, CF and PA and bias supplies as one unit. If not then the SIM is not necessarily bogus. I've seen this paradigm of behaviour before in all sorts of "out of boundary" situations.
We need to wait until it can be measured before we throw our computers in the fish pond. :l2:
Trust but check as they say :laugh:
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and those reading are with respect to ground.
One meter was connected so that I was reading just the grid to ground, and the other meter was measuring cath. to ground....
Just a silly experiment that I thought up, that I was hoping might show me something.......I'm not really working out of a book, so I'm learning by doing
Must get to the bottom of this - if like you said, the bias is being pull down to -52V, the tubes are in complete cut off :huh:
jaz, did you mean to say bias voltage there?
This is one of the questions that I need to clear up in my own head:
Do we still look at biasing these triodes the same way we see bias in a cathode biased stage?.....so what I'm asking is does -21 on the grid and -19 on the platecathode still imply a -2vdc bias?
I'm sorry, I just want to make sure I understand what is being said..... :think1:
I don't always understand what I read from other sources,,, and then try to apply to this (odd) application
Do my results partially explain (or make us question more) why Dumble might have started with -40vdc on the CF at idle ??(as seen on the hand drawn schematic)
Just for the record...my workbence is at my place of full time employment,,,,,so (usually) I only work on it Mon.-Fri. 5pm-?pm
So I won't be able to supply anymore test result until then.......today is golf :icon_biggrin:
Feel free to give me specific test instructions so that I can give you the answers you need.......you don't ever have to say please :wink:
EDITED to reflect the wording error sluckey pointed out (plate=cathode)
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so what I'm asking is does -21 on the grid and -19 on the plate still imply a -2vdc bias?
I'm sure you meant to say "does -21 on the grid and -19 on the plate CATHODE still imply a -2vdc bias". If so, the answer is yes.
Tube bias always refers to the voltage difference between grid and cathode. To say I have zero volts on the grid (typical preamp gain stage) or I have +50v on the grid (think LTP or bootstrapped PI) or I have -50v on the grid (your particular CF or fixed bias power tube) tells me nothing about the bias of that tube... until I also know the voltage level on the cathode.
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2) Can anyone confirm/deny Merlin's quote above as being relevant to this design?.....
Is it possible that it might be OK to have the grids even slightly more positive than the cathodes in this stage?....not that I need them to be that way, but it'll just help me confirm/deny the validity of the hand-drawn scematic of the sss
Merlin's quote make the point that the absolute voltage of either the cathode or grid is not critical, but the difference in those voltages is critical. (sluckey's point). So, the actual bias is important to the stage. in that doc, Merlin gears the examples to the first gain stage of the amp, where a -1 or -2 is his target.
If you read his AC-couple CF page ( http://www.freewebs.com/valvewizard/accf.html (http://www.freewebs.com/valvewizard/accf.html) ), in those exmaples, his input signal voltage is higher, and he chooses a higher bias:
"In this case a bias of about -3.4V is chosen, giving about 50Vpk before clipping, into easy loads" (from linked doc)
regarding, plate voltage , check the datasheet for the tube you are using for this rating. If its 300, then the difference between the plate and cathode shouldn't exceed 300. If you have 350 @ P, and -47 @ K,, then 397 exceeds that number.
The SVT schematic I referenced earlier:
the 5/69 rev.B sch I've got shows plate at 220v, grid at -77v, and cathode at -47v.
has a bias of -30, and a plate voltage of +297. ( the 220v supply comes from it's dedicated 6146B's screen power supply that provides 220v).
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Thanks for the help term and sluckey,,,,I edited my previous post to reflect the correct wording (
plate=cathode)
There was one other note that I wanted to review about that last experiment that I ran...
I had one DMM connected to the grid of one side of the CF, and the other one connected to the cathode of that same triode
At idle I was reading -22vdc on the grids and -19vdc on the cath.,,,,I had to get the signal pretty loud to get those voltages to move but once they did, they stayed evenly spaced at 2 volts apart all the way up to -55 grids ...-53 cath.
Those negative supply voltages didn't move easily, and it took a very "loud" signal to get them to down to -55 and -53
Could that move be a direct reflection of the point in the volume control where the clipping starts to become audible?
My mind wants to believe that we are seeing the voltage drop, as a result of increased current...........am I thinking correctly? :think1:
From what I've read,,, the high negative supply is placed on the grids to inhibit grid current (and I had questioned what value that needs to be).......is there a chance that I put so much signal there that it overtook the negative supply, and "introduced" grid current,,,which might have caused those volt. readings to drop (more negative)? :dontknow:
Maybe I'm way off, but I have to ask......
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If you read his AC-couple CF page ( http://www.freewebs.com/valvewizard/accf.html (http://www.freewebs.com/valvewizard/accf.html) ), in those exmaples, his input signal voltage is higher, and he chooses a higher bias:
"In this case a bias of about -3.4V is chosen, giving about 50Vpk before clipping, into easy loads" (from linked doc)
regarding, plate voltage , check the datasheet for the tube you are using for this rating. If its 300, then the difference between the plate and cathode shouldn't exceed 300. If you have 350 @ P, and -47 @ K,, then 397 exceeds that number.
The SVT schematic I referenced earlier:
the 5/69 rev.B sch I've got shows plate at 220v, grid at -77v, and cathode at -47v.
has a bias of -30, and a plate voltage of +297. ( the 220v supply comes from it's dedicated 6146B's screen power supply that provides 220v).
Thanks term,,,I had missed that ,,,,partially due to the fact that I was looking harder at the stage being DC coupled to the output stage in stead of being focused on it being AC coupled to the PI
So I'm just studying the cathode follower in general, and trying to grasp the entire concept.....
I think that I've been so distracted, and worried about trying to match the design to the original, that I've blocked out some very helpful info. (like that SVT scem.)
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I'm still mulling over the apparent bias shift from your measurements, I still would like to see some scope shots - I don't really trust the DMM readings when you have large AC sitting on top the negative bias. Also, perhaps we are spending too much effort on the CF after all it is just a follower - it simply follows whatever appears on its grid, and if we are not talking about AB2 operation, then the PI is what we should be focusing on. All the effort that we have spent on the CF mainly centers around IMO safe operation, with the reduced B+ and lower cathode voltage, I think you got that sorted already.
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Did you see the TAG thread about the CF on a AB763?
As far the bias you will need -135 to 140 volts on the grids of the PI.
http://ampgarage.com/forum/viewtopic.php?t=21942 (http://ampgarage.com/forum/viewtopic.php?t=21942)
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SILVERGUN.
Don't think grid current is an issue as grid remains more negative than cathode at all times. It is in the nature of a cathode biased valve stage. Once the ability of the bias network to provide negative voltage is exceeded everything is being pulled negative, the signal is being clipped on the negative side and is no longer centred around zero volts DC, but has a negative DC component to it.
jazbo8,
I don't think it's too cool having an amp that goes radically out of bias if you turn it above 7.
What do you think needs adjusting with the PI. Or do you mean testing it for adequate operation.
tubenit,
From that thread you referenced I notice about 15 V AC indicated coming out of the PI, the SS CF can't handle that in this configuration IIRC.
Note also that the output tubes have protection diodes strapped cathode to ground, think this aspect of the design needs to be addressed also.
Also, the bias adjustment pot is configured differently.
All the best
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Wow Gunny! I love that sound. That amp has the BB King vibe and tone gyratin' all thru it. Nice work Mad Professor.
smacko jack :icon_biggrin:
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I don't think it's too cool having an amp that goes radically out of bias if you turn it above 7.
What do you think needs adjusting with the PI. Or do you mean testing it for adequate operation.
Nope, not cool and probably isn't really the case... we'll have to wait for the scope shots...
Since it is just a follower, I believe nothing needs to be changed in Fender's classic design - after all that is part of the sound that I think SG is looking for...
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Excellent observation jasbo8. Doing the worst I can to do the best I can for SG. As I have said many times, "I am a newbie".
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Did you see the TAG thread about the CF on a AB763?
Thanks T, no I hadn't seen that....
It kinda sounds like a pretty cool idea though :wink: :l2:
I learned how to share information from you, openly and without hesitation, so it was only a matter of time.....
(he didn't have to make it sound like it was his idea though) :cussing: :violent1:
Unless maybe it's one of our guys working as a secret agent :undecided:
Maybe I'll learn something from the thread....
The only reason we're still on this is because I don't have the technical knowhow to say:
this is what it is, this is why it is, and here's how you want to do it..........
It's why I'm here.....to learn and share
:thumbsup:
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I'm still mulling over the apparent bias shift from your measurements, I still would like to see some scope shots - I don't really trust the DMM readings when you have large AC sitting on top the negative bias. Also, perhaps we are spending too much effort on the CF after all it is just a follower - it simply follows whatever appears on its grid, and if we are not talking about AB2 operation, then the PI is what we should be focusing on. All the effort that we have spent on the CF mainly centers around IMO safe operation, with the reduced B+ and lower cathode voltage, I think you got that sorted already.
Sounds good jaz,
I'll get the scope hooked up ASAP.....
Just for the record, for all involved parties, this amp sounds great, it just develops some distortion when you crank it up,,,,,sound familiar?......I was aiming for more pure clean all the way up on the dial, and we will get there........I've just slowed things down because I need everyone to dumb-down their explanations :icon_biggrin:
after all it is just a follower - it simply follows whatever appears on its grid
That's the way I need to view this.....instead of treating it like it's some super secret, highly engineered, Dumblefied stroke of genius that I could never possibly understand :icon_biggrin:
Thanks again for all of your help......I'll let you know when the scope's ready, and I can post another clip of the amp cranked so we're all on the same page
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Did you see the TAG thread about the AB763
As far the bias you will need -135 to 140 volts on the grids of the PI.
They need sluckey over there :l2:
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OK, I got the scope hooked up and ran a short, narrated video clip shot.....looks like we've found the culprit.....this shows the clipping as it occurs at the PI...and it starts right around 7 on the vol. control
I looked at the CF and it was just a carbon copy of what we see coming off of the PI, and it didn't change (or clip any more) as it came off of the cathode of the CF....
What was I waiting for? :huh: :dontknow:
http://youtu.be/nRiVU4Z9s5c (http://youtu.be/nRiVU4Z9s5c)
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Was the scope's input AC or DC coupled? Was the center line at -19V when the video starts? And the y-axis was set to 50V per division? Sorry for the all questions, just want to make sure...
As far the bias you will need -135 to 140 volts on the grids of the PI.
I think that's typo, grids of the PI?! :huh:
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Excellent stuff SG.
There is a possibility that the CF could be loading the PI down. Would you be able to repeat that with CF's disconnected from PI coupling cap's and with 470k resistor's from output of coupling caps to ground? I get PI distortion with my D'Mars PI, which is constructed almost identical to yours, at high volumes but just to be thorough.
I ask because the clipping you are getting there is not symmetrical.
Also as jazbo8 mentioned, it would be helpful to know the settings on the scope.
All the best
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Was the scope's input AC or DC coupled? Was the center line at -19V when the video starts? And the y-axis was set to 50V per division? Sorry for the all questions, just want to make sure...
AC coupled....not sure where the center line was set...I honestly just tried to dial in a clear wave in the center of the screen
What does -19v correspond to?....(I hope that isn't a super dumb question :dontknow:) :BangHead:
I'm pretty sure the Y axis V/div was set to .5V (probe x10) when the video started and then I switched it to 2V to look at the signal coming out of the PI,,,,just to get it on the screen
I basically cut to the chase and just zero'd in on where I found the clipping.......so the first wave represents the signal as it goes into the grid of the PI.....and the second wave is where I saw it clipping heavily coming out of the PI (not the CF)
There is a possibility that the CF could be loading the PI down. Would you be able to repeat that with CF's disconnected from PI coupling cap's and with 470k resistor's from output of coupling caps to ground? I get PI distortion with my D'Mars PI, which is constructed almost identical to yours, at high volumes but just to be thorough.
I ask because the clipping you are getting there is not symmetrical.
Yes, I can perform the test you requested.....If the CF is loading down the PI, it's doing the exact opposite of what it's intended to do... :think1:
I found that my PI was horribly out of balance,,,and spent some time trying to get it into balance,,,,and managed to balance all of the gain out of it :l2:
But I'm learning, and it's fun!!
It's probably better this way,,,,that someone didn't just come along and give me all of the answers......... :BangHead:
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A few things to verify: -19V, because your DMM measurements showed the bias shifted from -19V to -52V, we would like to know if that is indeed happening. So you should set the scope input to DC couple and the vertical scale to 20V/div. First set the zero line where you want it (the top line on the scope is usually a good place), then if the bias is -19V, without any signal applied, you should see a horizontal line about 1 division down from the reference line. Now take measurements with the volume settings at 1, 3, 5, 7, etc. and make a note of where the center line of the sine-wave signal is, for each case - that is your bias voltage, in theory that center line should not shift, you may need to let it settle down a bit after you turn the volume pot.
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Here is what I'm getting out of my PI.
The first one is just under saturation.
The second one is driving it into very audible saturation.
The third one is totally hammering the PI.
Notice there is no hard clipping. It's managing 10V RMS (28V Pk to Pk) clean as a whistle and gets pretty grungy by 13 V RMS. These levels are in good agreement with those written on the circuit from TAG forum mentioned by Tubenit in reply #153
You're doing a great job SG, learning is a struggle, even for a spring chicken like yourself, borderline impossible at my age, but I'm not going down without a fight. :laugh:
Now I'm going to build your CF section and convert my PA to fixed bias.
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Here is what I'm getting out of my PI.
So these are the signals at the grid of the CF (AC coupled), right? Does the D'Mar have the same PI arrangement like the Fender, sorry too lazy to look it up ;-)
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jasbo8,
These are at the plates of the PI with no CF, AC coupled into cathode biased PA.
The PI is almost identical, one resistor is 820 ohms instead of 1 k. (I'll change that in the rebuild).
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OOOPS!
Forget that I had put a long plate 12AX7 in the PI as experiment.
Results are the same for lower drive but the 12AT7 can take a bigger beating.
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A few things to verify: -19V, because your DMM measurements showed the bias shifted from -19V to -52V, we would like to know if that is indeed happening. So you should set the scope input to DC couple and the vertical scale to 20V/div. First set the zero line where you want it (the top line on the scope is usually a good place), then if the bias is -19V, without any signal applied, you should see a horizontal line about 1 division down from the reference line. Now take measurements with the volume settings at 1, 3, 5, 7, etc. and make a note of where the center line of the sine-wave signal is, for each case - that is your bias voltage, in theory that center line should not shift, you may need to let it settle down a bit after you turn the volume pot.
I will do exactly that tonight.....thanks for keeping me focused jaz
I kicked right into troubleshooting mode and just tried to nail down the clipping.......
I wound up replacing the NFB resistor with a 5K pot, and tried to dial in a "duplicate" waveform into the grid if the bottom half of the phase inverter,,,,and although it did clean up the signal, it took away all of the gain of the PI......so I'll get that set back to 820R (where we were), and go from there
*So now I'm studying the AC LTP :icon_biggrin:
These are at the plates of the PI with no CF, AC coupled into cathode biased PA.
Thanks Gj, that's a great reference point for me to shoot for....you can see how squared off my signal gets off the plates of the PI :huh:
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SG,
With regards to jazbo8's test procedure. Could you do that for a couple of different values of bias supply voltage, eg -85V and -130V, and see how that effects the outcome, both of onset of clipping and effect on bias drift (if any).
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Gj,,,Yeah...I kinda did that, but not to that extreme with the voltages....what I'm noticing is that the AX7 is more succeptable to the drift than the AT7, and it is more likely to drift when you start at a lower negative supply voltage.......which I hope explains my question ("why the high negative supply?")
At this point I have also raised my B+ so that I now have 380vdc on the plates of the 6L6s, because of the higher neg. supply, I needed to increase the plate voltage to get some current moving (at least until I figure out a better way to balance it all)
V/div is set to 2V (with x10 probe), DC coupled
Here's the first version that jaz asked for, and I'm definitely watching it drop from -19vdc down to -53 very predictably
http://youtu.be/gYCPr7WM79Q (http://youtu.be/gYCPr7WM79Q)
Then I put in a 12AT7 at -25vdc at idle, and it dropped to -32vdc, with the volume maxxed out:
http://youtu.be/Q9JH0-FnBSU (http://youtu.be/Q9JH0-FnBSU)
I left the AT7 in there and increased the negative supply to -50vdc, and it ONLY dropped to -53vdc:
http://youtu.be/sRdLm52xKJY (http://youtu.be/sRdLm52xKJY)
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Gj, here's the result of the test you asked me to do where I disconnected the coupling caps from the PI and CF and connect thru 470K res. to ground.
Like before, I show the signal going into the PI and then coming off the plate clipped :huh:
Looks like we've got some Pi work to do....
There is obviously no NFB at this point because the signal never made it to the OT
http://youtu.be/lisieu6ASB4 (http://youtu.be/lisieu6ASB4)
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Ok, thanks for posting the new clips, it is pretty clear from the AT shots that the bias did NOT shift all that much (the center line of the sine-wave stayed pretty much on the line at idle), but for the AX, the bias appeared to be pulled down - the waveform became asymmetrical about the bias line, the question is why, my quick guess is the tube failed to deliver the current :w2:
I will be traveling for the next few days, when I get back, I will try and re-run the simulations with the output tubes, since my current sims only have the CF stage and that does not tell the whole story.
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SG,
Thanks for your patience. These results are not too bad.
In the original AB763, if our PI's are pretty close to original manufacture, we had an amp that was designed to see 10V AC going to the grids of the PA for a clean sound, and about 13V AC for a saturated sound. If the CF's are replicating those voltage levels OK then we don't have too much to worry about in terms of distortion versus volume.
If that is 20 V per division (2 v/div X 10) then you are getting about 14 V RMS from AX7 and 20V RMS at moderate saturation which is fine.
Where I'm a little confused is that, in earlier listening tests, you said that the AT sounded more distorted and at lower volumes than the AX :w2: :dontknow:
The average DC level is drifting more negative at extreme high settings which actually shuts the 6L6's down a bit so it is not a "she's going to blow if we give here any more captain" situation, an unexpected protective measure in a way.
Also the saturated output from the CF's appears more symettrical than the output from the PI, who says CF's don't make a difference eh :icon_biggrin:
Could you post the current circuit diagram of PI CF PA and (-)ve bias supply with no signal DC voltage readings. If you're still running 250V on CF plates and -85 V on (-)ve bias supply then the tubes should not have a problem with switch on voltage, 10% is nothing when we know those spec ratings are conservative.
All the best.
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I have no idea what I've done, but;
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Where I'm a little confused is that, in earlier listening tests, you said that the AT sounded more distorted and at lower volumes than the AX :w2: :dontknow:
Could you post the current circuit diagram of PI CF PA and (-)ve bias supply with no signal DC voltage readings. If you're still running 250V on CF plates and -85 V on (-)ve bias supply then the tubes should not have a problem with switch on voltage, 10% is nothing when we know those spec ratings are conservative.
Yeah, I'm a little confused about that too,,,but that's OK....if it turns out that the AT7 is the better choice, then that's what it'll be.....looks that way now
Yes, I can post the adjusted voltage chart later,,,, I'm still using the higher negative supply of approx. -220vdc
Here's a great article on "tuning" the PI: (I'll be adjusting the breadboard to accomodate trimpots in critical locations to simplify PI tweaking)
http://www.aikenamps.com/LongTailPair.htm (http://www.aikenamps.com/LongTailPair.htm)
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Very very nice work guys!!! Congrats to you all for pushing the boundaries.
SG, AFAICT, you seem to be getting LTP PI overdriven. I have a suggestion which might help??
I believe you are still using the AB763 preamp posted on first page. If so, I will suggest removing the cathode bypass capacitor on the last stage prior to PI. It will chop down signal and might help with LTP PI overdrive??
Whether you perform this test, let us know how it turns out.
Hope this helps
Best Regards
R.
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SG,
Don't know if there is any clear choice as to what valve may be best in this situation. I certainly have no ideas of what may be best for you.
Just configured my PA as fixed bias in ultralinear mode and connected my 12AT7 PPICF to it. Bias came up real easy and rock solid. 55.6 mA on one valve and 55.2 mA on the other. Got that sound like you hear on the Singers, like on your amp. Real Nice. Still gets overdrivey above 7 though. Haven't done any diagnostics yet.
Only problem, can't get negative feedback to work, it sounds hideous and distorted, tried swapping phase of OT, that was no go at all.
Ultralinear provides a degree of negative feedback anyway. (Negative feedback would not work before the mods either :dontknow:)
I've got my AT7 wired up as in reply #120 except I backed off a little on the positive supply - 170V. I'm running the tube a bit over half a watt, I would not recommend trying this with a 12AX7, 5751 or 12AY.
The tube will never experience greater than 320V and is generally operating at about 200V. That has eliminated the question mark about switch on stress to my satisfaction. The negative rail at this level is also within tolerance for Cathode / Heater max voltages also.
My poor amp has been such an experimental lab rat that I'm going to have to rebuild it from scratch I think, every component has been chopped and resoldered and I never planned on the extra circuitry needed to create the specialised supply rails for the CF's.
All the best.
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http://www.ampbooks.com/home/amplifier-calculators/long-tailed-pair/ (http://www.ampbooks.com/home/amplifier-calculators/long-tailed-pair/)
i use tube-cad - it's fast, easy to use and reasonably accurate, + plus you can add other less popular tubes.
--pete
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Just configured my PA as fixed bias in ultralinear mode and connected my 12AT7 PPICF to it. Bias came up real easy and rock solid. 55.6 mA on one valve and 55.2 mA on the other. Got that sound like you hear on the Singers, like on your amp. Real Nice. Still gets overdrivey above 7 though. Haven't done any diagnostics yet.
Only problem, can't get negative feedback to work, it sounds hideous and distorted, tried swapping phase of OT, that was no go at all.
Ultralinear provides a degree of negative feedback anyway. (Negative feedback would not work before the mods either :dontknow:)
I spent at least 3 hours last night just trying to balance the PI to a point where I got decent gain, but without the clipping,,,and that is quite a challenge.
To get that great tone at 3 on the vol. you have to settle for a little clipping above 7........I was able to dial it back a little, but the PI is very critical in the overall tone of the AB763 with or without the CF
If you try the calculator that DL supplied above (thanks!) and enter common values, you'll notice a pretty big difference in gain between the inverted and non-inverted signal......and it doesn't take into account NFB
I tried a 12AX7 and 12AU7 in the PI and attempted to balance those as well but got too much gain with the AX7 and not enough with the AU7...
The biggest thing I learned was that if you try to balance both phases perfectly, you have to give up a lot of gain,,,,,and NFB had a huge impact on gain thru the PI
I had a lot of fun and did a lot of wave watching last night, while taking every value to the extreme......
Tonight I'll go back and try it the way I left it, and then try it with the SSS values in the PI, which I haven't done yet
SG, AFAICT, you seem to be getting LTP PI overdriven. I have a suggestion which might help??
I believe you are still using the AB763 preamp posted on first page. If so, I will suggest removing the cathode bypass capacitor on the last stage prior to PI. It will chop down signal and might help with LTP PI overdrive?
Tried it and just lost too much of the front end feel (and vol.) of the amp.....lifeless, would be the way to describe it,,,,so while I had that cap off, I decided to try to make up for that lost gain in the PI and see how that sounded, and it just wasn't real good....not sure how else to say it...we tried :icon_biggrin:
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Sometimes I feel like I'm just rambling off into cyberspace,,,and then I see how many times this thread has been read and it makes me feel like this work has a bigger scope than what I am actually capable of realizing,,, because I'm right in the middle of it.....
I do realize that I should know this stuff, and I'm sure that there are guys who could've figured this out in way less time,,,,,but I have grown to truly appreciate the learning process.....
After many hours in front of the scope, I have a new found respect for the phase inverter,,,,as it is the heart of this design, and has much more to do with the sound and drive of an amp than I ever truly recognized before.......probably because the values always look fairly close, and the architecture of the LTP stays constant throughout,,,, it lull's you to sleep with it's simplicity
In reading some more of the available info on Dumble, it now stands out to me that the PI was a large focus of his.....
So,,,in my previous post I had mentioned that I had tried the 12AU7 in the PI, and was unimpressed with the lack of gain,,,,BUT, when I went back at it last night, I used the sss values in combination with th AU7 and it was a pure winner......so I'm questioning what I thought I knew about the tube compliment for the sss.....( someone else had mentioned 12AU7-PI and 12AT7-CF ),,,,and that's where we are now
I was unable to get the AT7 to not clip while keeping an acceptable amount of gain,,,but with the AU7 the clipping was immmediately gone and I was able to increase the gain and keep a fair amount of balance by manipulating the bias and tail resistors in conjunction with NFB........an "ah-ha" experience for sure....
Here's the updated values and voltage chart for now: (with changes highlighted in red)
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Spent the morning doing some "test measuring" on the beastie.
Now I know this is not meant to be hi-fi but,
Conventional PI's seem most inadequate to drive a pair of 6L6's for an ULTRA CLEAN amp design, managing only about 12 volts RMS before getting audibly grungy, and noticeably squared up on the signal analyser.
PPICF's are much more suited to the task managing to generate about 48 V RMS before getting grungy.
Whilst I was doing this "measuring the beastie" thing, I measured the input impedance of the 6L6's grid. Whilst on the fuzzy low end of my measuring equipment it came in at a surprisingly low 33 k ohm @ 220 Hz. This may account for the PI's poor performance when used without CF's. High impedance source being loaded down.
At these levels I measured clean, as in not noticeably grungy, as 20 Watts RMS with PPICF making the power for standard PI's about 5 Watts RMS before grunge.
Those who thought that putting CF's after the PI amounted to a "simple buffer stage" that was "a waste of parts" and "not at all an interesting problem to solve" were plain wrong. Sorry to be brutal about it.
As they said on the TAG forum "Like the difference between night and day".
When I do crank the amp up real loud, the overdriven sound is also greatly improved. (Although my ears are still ringing 3 hours later)
Now need to think and research how to get the PPICF's generating about 100 V RMS, or even if that is necessary.
We do like that distortion after all.
I must conclude that most of that distortion, in the past, has possibly been from PI saturation rather than output tube saturation.
Kudos for you're pioneering and imaginative spirit SILVERGUN. :thumbsup:
All the best.
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I must conclude that most of that distortion, in the past, has possibly been from PI saturation rather than output tube saturation
I agree,,, and that really surprises me....not what I expected at all
Everyone always talks about pre-amp distortion or power tube breakup but you never hear anyone say "that is some great PI clipping"
I'll never take the PI for granted again, and I'm really thankful that we pushed ourselves right into this conclusion....it definitely changes the way I see the whole picture now :huh:
I think I can safely say that the PPICF mystery is solved, and I feel comfortable moving on to tweaking the pre-amp, and seeing where I can go from here.....as it is , I think this amp is a funk players dream, because of it's bouncy response, and mix-cutting punchiness...
It's got me thinking of doing some James Brown covers :icon_biggrin:
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Finally had a chance to run some sims on the CF/PA, unfortunately, I was not able to duplicate the bias shift as shown in SG's video, still wonder if that's due to the scope settling time... Anyway, here is the output waveform - stepping the input from 10Vpk-60Vpk, clearly the grid of the PA clamps the input waveform at ~0V when the input voltage exceeded 40V, also there was very little grid current due to the 10k grid stopper used in the simulation, so what was causing the bias shift in the video? :w2:
(https://dl.dropboxusercontent.com/u/1326040/SS%20CF%2012AX7.gif)
While we are on the subject of the PI, here are some useful info:
An essay (http://www.el34world.com/Forum/index.php?topic=14280) by HPB well worth a read.
Basic LTP design (http://www.aikenamps.com/LongTailPairDesign.htm) tutorial by Aiken.
Balancing the LTP (http://www.ampbooks.com/home/amplifier-calculators/long-tailed-pair/) by Ampbooks.
Happy reading... :icon_biggrin:
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so what was causing the bias shift in the video?
It still does it,,,only to a much lesser degree now with the AT7 in the CF
With -24vdc on the cathodes of the CF at idle, it wil drop to -27 with the volume control on 10........and it doesn't start to pull down right away,,,it waits until we hit about 6 on the vol. control to start dropping
It's gotta be an increase in current causing the voltage to drop.....how can I measure grid current on the 6L6s?.....and could grid current there be pulling the bias voltage down?.....I'll go read more
Let me know if you want to see anymore scope shots
Welcome back....I found your blog while you were away :icon_biggrin:
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The bias shift is not suppose to happen, since it is the primary reason for using a cathode follower to drive the output tubes in the first place. Here is a quote from R.G. Keen's MOSFET Follies:
"When output tubes are overdriven, their grids are often driven positive. If the coupling to the grids is by capacitor, this can lead to grid blocking/ bias shift as the grid current skyrockets when the grids go positive. This current builds up as a negative charge on the coupling capacitors and causes the tube to effectively be biased more negative (towards class B) than the static bias setting. So on overdriven notes, the output tubes are pushed into a harsh-sounding crossover distortion. In addition, the usual 12AX7 or 12AT7 phase inverter plate impedance is so high that it can't pull the output tube grid positive in the face of grid current, so the grid of the output tube is hard-clipped at Vgk=0V. This clipping can be as harsh as any solid state situation.
The solution to the class B bias shift and grid current clipping is to drive the output tube grids direct coupled from a cathode follower. The follower is set up so that its cathode sits at the normal negative bias voltage of the output tube, maybe -10V to -40V depending on the tube type. This means that the cathode resistor is tied to a more negative voltage of something more than twice the bias voltage, so it can pull the output tube grid negative..."
Here is the whole article. (http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm)
It just drives me batty not knowing what's causing the shift... :BangHead: Could you please do me a favor and change the grid load resistors of the 6L6s to 100k and see if the bias shift still happens?
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Great article there...thanks AGAIN
It just drives me batty not knowing what's causing the shift... :BangHead: What size is the grid stopper now? Does changing it alter the amount of bias shift?
How much shift could we expect to see in a standard AC coupled output stage? :think1:
The grid stoppers are still the 1.5K and I havn't tried any other value.....that will be a simple exercise tonight
I've got the scope hooked up for a dual trace now,,,,,, if there's anything you want to see, just let me know
The amp definitely stays cleaner longer now since the PI tweaks,,,,,but now after 8 on the vol. I can really hear the 6L6s getting ugly
***Amp sounds great set up next to a Deluxe Reverb re-issue, and you can really tell the difference that the CF makes in bass response :grin:
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The grid stoppers are still the 1.5K and I havn't tried any other value.....that will be a simple exercise tonight
I've got the scope hooked up for a dual trace now,,,,,, if there's anything you want to see, just let me know
The amp definitely stays cleaner longer now since the PI tweaks,,,,,but now after 8 on the vol. I can really hear the 6L6s getting ugly
***Amp sounds great set up next to a Deluxe Reverb re-issue, and you can really tell the difference that the CF makes in bass response :grin:
Great to learn that the amp is "bettering" the DRRI, at least things are going the right way :laugh: I think the 1.5k stopper is ok, but the the grid load resistor for fixed bias 6L6 should be <100k per the datasheet. Even though the simulations showed no grid current even with large input voltage, on the bench, there is got to be some grid current flowing when the power tubes are over-driven, so the larger the grid load resistor, the more the grid will get pulled down. Also, you can verify if the bias shift increases the crossover distortion, when both tubes are driven to cutoff - this is a good chance to use the dual trace on the scope.
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I think the 1.5k stopper is ok, but the the grid load resistor for fixed bias 6L6 should be <100k per the datasheet. Even though the simulations showed no grid current even with large input voltage, on the bench, there is got to be some grid current flowing when the power tubes are over-driven, so the larger the grid load resistor, the more the grid will get pulled down. Also, you can verify if the bias shift increases the crossover distortion, when both tubes are driven to cutoff - this is a good chance to use the dual trace on the scope.
Here's what I saw...when I increased the grid-stoppers to 100K, the bias drift appeared worse as a result...
So I added in the 470K "grid-leaks" (pictured)...and they introduced some hum....I put them in as 500K pots and tried adjusting different values to "clean up" the signal getting to the grids of the 6L6s (and I could use them to help balance),,,,,and what I noticed was that one side was acting differently than the other....so I switched the position of the 6L6s, and the odd variation stayed with the (left side) socket.....what I saw was that the bias voltage and current were the same at idle, but at max signal, one side was drifting a lot further than the other, and reading much more cathode current......and when I swapped tubes, the variation stayed with the socket, not the tube
I kinda think that the signal coming into the grids was "hot" and imbalanced, and was causing more grid current on one side than the other and that was causing more bias shift on the one side, and in turn, pulling down the whole bias circuit......I should have mentioned this before,,,but the bias shift always appears a little worse on one side......I was just discrediting that as an imbalance in the triodes of the CF and PI....but know that I think more about it,,,it could be the result of more grid current through one 6L6 than the other....
So then I got determined to figure this out and took the output tubes out, and checked and removed and reconnected all connections on the breadboard and sockets, just to be sure....
While I had everthing all apart, I measured the OT and got equal readings on both sides of the center tap...
One other very important note....obviously,,,, grid current is directly linked to the input signal voltage......so I decreased the signal coming from my sig, gen to the input and was able to eliminate the grid current and resulting bias shift that way, by decreasing the signal that was reaching the grids...
---How to you determine what signal voltage to feed the input?
---When I tried to reduce the signal at the PI (in my earlier attempt to reduce clipping),,,I lost a lot of gain there
Is there a "normal" amount of gain that should be acheived coming out of the PI?......voltage wise,,,,,or is it normal to just push it to where it's causing grid current, and then just back it off slightly?
---What is the max. signal voltage that should make it to the grids?
---How do I measure grid current?.......when using ohms law, how do I know where to insert the values
i.e.-----if I measured a 5mV drop across those 100K grid-stoppers at -24vdc bias, at max. signal,,,what does that equal?
I'm assuming it's the same method as figuring cathode current,,,but the extra 5 zeros throws a wrench in it for me
Of course,,,, I got rushed away from it and had to go help my kid move a refridgerator :BangHead:,,,,,so I'll try to confirm some things tonight
I can look for crossover distortion on the scope, and try to get a better grasp on the big picture
Sorry this is like tube amps 101 jaz,,,,thanks again for hangin in there with me :thumbsup:
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Of course,,,, I got rushed away from it and had to go help my kid move a refridgerator :BangHead:,,,,,
Gotta love those father/son bonding times.
Brad :laugh:
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First some clarifications, when I said to change the grid load resistor to 100k, I meant the 470k shown in the SSS schematic and not the grid stop resistor which is 1.5k, you can leave it as is.
---How to you determine what signal voltage to feed the input?
If you are refering to the power tube, then then its input should equal to the bias voltage, so 24Vpeak, this should give you a relatively un-distorted sinewave at the speaker terminal (assuming none of the other stages are over-driven). Again, since I did not simulate the rest of the amplifier, I could not tell you what level the signal generator should be set at the input - but you can use the scope to set the level at the power tubesīgrid - they should be nearly 24Vpeak sinewave.
---When I tried to reduce the signal at the PI (in my earlier attempt to reduce clipping),,,I lost a lot of gain there, Is there a "normal" amount of gain that should be acheived coming out of the PI?......voltage wise,,,,,or is it normal to just push it to where it's causing grid current, and then just back it off slightly?
Loss of gain? Do you mean when you try to balance the PI, the overall gain got reduced? If so, that is a separate issue. For now, you can watch the scope at the power tubes grids while increasing the signal generatorīs output until the waveforms clip (near the 0V line) and back off a bit. I would not worry too much about the PIīs balance for now, since we are trying to identify the source of the bias shift. If the waveform at either one of the grids isnīt clipped, then there should be little or no grid current flowing, so the bias should stay put as you vary the signal level, from zero upto and perhaps even a bit beyond 24Vpeak.
---How do I (can I) measure grid current?
This is tough one, I would like to know as well...
I hope we can get to the bottom of this puzzle soon and thank you for putting up with my pestering :wink:
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---How do I measure grid current?
This is tough one, I would like to know as well...
I went out there at lunch time and tried to do it just like you would try to get a cathode current measurement...
I put a 1R resistors in palce of the 1.5K grid stoppers, and read the V drop accross each one....
At first I wasn't getting any reading, but I was able to get up to 7mV (=7mA) by forcing grid current with a hotter input signal
I might have been forcing grid current all along by not regulating the input signal, and just guesstimating that the volume I heard matched the vol. of my guitar signal (or at least close enough for testing)
THAT'S IT....I just went back out and confirmed 100% that when I force grid current,,,that forces bias drift,,,and one side is definitely more affected then the other,,,,and it doesn't "follow the tube" when I swap them,,,BUT, it does drag the whole bias circuit down with it
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You could have left the 1500 R grid stoppers in there and measured the voltage across them and used ohms law to figure out the current. Ohms Law: Voltage = Resistance * Current.
Have you measured the actual RMS output power of the amp at the point where bias starts going off. With 300 V DC on the 6L6's you can only get about 20 Watts RMS, so if the max output power gets there at or before bias drift point then the bias is not an issue.
If you run 380 V on the output tubes, you will have to run greater negative bias voltage and thereby change the onset of grid current to a higher volume.
If you want to fix the bias problem with the PI/CF, increase the voltage on the anodes of the output tubes.
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You could have left the 1500 R grid stoppers in there and measured the voltage across them.
V-IR
I just figured the math would be easier with the 1Rs in there......no math
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Fair enough. A bit of math comes in handy sometimes and nearly all "do it with a calculator" level stuff.
Will measure my amp a bit more in the next couple of days to see if my thoughts were valid.
From what we've discovered so far the PI / CF can put out about 50 volts peak so it is not surprising that it is exceeding the6L6's -24 V bias on positive swings and inducing grid current.
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Have you measured the actual RMS output power of the amp at the point where bias starts going off.
no
If you run 380 V on the output tubes, you will have to run greater negative bias voltage and thereby change the onset of grid current to a higher volume.
If you want to fix the bias problem with the PI/CF, increase the voltage on the anodes of the output tubes.
I went out and tried that quickly,,, and it appears to be correct.....it seems much more difficult to induce grid current with the higher plate/bias voltage
I'll try to confirm 100% and report back later
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If you run 380 V on the output tubes, you will have to run greater negative bias voltage and thereby change the onset of grid current to a higher volume.
If you want to fix the bias problem with the PI/CF, increase the voltage on the anodes of the output tubes.
I went out and tried that quickly,,, and it appears to be correct.....it seems much more difficult to induce grid current with the higher plate/bias voltage
Nope....I must have rushed the results earlier, and turned down the amplitude on the sig. gen....
It is just as likely to conduct grid current if I have 300, 380, or as high as 420 on the plates (maybe not in theory,,,but in reality ,yes)
If I overload the grid with a hot signal, it doesn't help with grid current if I have a higher - bias voltage.....it still drifts
I reverted back to getting the input signal correct first and used the scope while I played, and tried to match the amplitude of that sine to the one that I was feeding the amp from the sig. gen.....ok check,,, close enough for rock and roll
I noticed that my signal looked great coming through the PI and CF.......but as I cranked the vol. control on the amp,,,,still grid current, and bias drift
So I used those two 500K pots that I had put in place of the 470K grid leaks to dump some signal just before the grid stoppers,,,, I kept my eye on the bias voltage and monitored the drift from -25v to -30vdc,,,,with the vol. cranked and the bias reading @ -30vdc, I dialed the grid-leak pot down to get the bias voltage back down to -25, and wound up at 180K on the pot....I did the same thing on the other side and wound up with the same 180K over there...
So that got me NO bias shift,,,but resulted in MORE grid current
EDITED: Couldn't go to sleep until I figured out what happened....
I made a mistake...I wasn't compensating for putting the 1.5K grid stoppers back in,,,,so when I thought I was reading MORE grid current, I was actually reading a larger voltage drop across the 1.5K (compared to the 1R I was using for my calculations earlier today)
So now that I have done the math, it appears I'm down to 2.6mA grid current,,,at full signal voltage
4 volt drop across 1.5K resistor = 2.6mA grid current....right?
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I went out there at lunch time and tried to do it just like you would try to get a cathode current measurement...
I put a 1R resistors in palce of the 1.5K grid stoppers, and read the V drop accross each one....
Duh, that wasn't as hard as I thought... so you have verified that grid current was the culprit, so we can check that off. Now if you recall HPB's point on the CF's function to handle the onset of grid current as oppose to AB2 operation, then you need to control the amount of drive at the power tubes' grids, with little or no grid current when the volume is fully dimed - you can do that by adjusting the overall gain in the preamp stages. Love to hear some more clips when you get all that under control. :m17
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SG just for S&Gs, what's the DCV measurement across the Rk (2.7K) of the LTPI?
--Pete
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SG just for S&Gs, what's the DCV measurement across the Rk (2.7K) of the LTPI?
6.5v @ idle drops to 5.9v with full signal.....with 22K in the tail
Thanks for looking DL
On my other subject...
Is a small amount (a few volts) of bias drift normal/acceptable as current increases in the PA?.....is it "the right idea" to use those grid leaks to prevent it?.....or is it OK to just let it happen,,,so that as cathode current increases, bias voltage increases to keep it in check?
How much grid current is too much? (I've been thinking that zero is ideal,,,but where's the boundary?)
If I've got 2mA-5mA grid current at max signal,,,can I stop obsessing about it and just get on with my life, knowing that the amp will probably never see 9 or 10 on the volume control?
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Now if you recall HPB's point on the CF's function to handle the onset of grid current as oppose to AB2 operation, then you need to control the amount of drive at the power tubes' grids, with little or no grid current when the volume is fully dimed - you can do that by adjusting the overall gain in the preamp stages.
I did notice one other thing last night....in my effort to reduce/eliminate g current, I measured the 12AT7 (CF) as pushing about twice the current that an AX7 did
The AT7 was causing about 7mA on one side and 5mA on the other,,,,while the AX7 brought it down to an even 2.6mA per side
I had a matched triode Sovtex 12AX7-LPS that I usually use for a PI (in a different amp),,,and I put that in there, and not only did it drop the g.c., but it evened it out side to side......so I think the imbalance in g.c. was being caused by an imbalanced signal coming through the CF.
One side was just getting hit harder than the other.....
Maybe this explains why AX7s are an OK choice for the CF....
Like PRR said in the other thread:
> why not use an even more capable driver tube?
How much can you shove into the 6L6 grids before they melt?
It's not clear on the datasheets. With all the many different 6L6es made, and variations in grid-wire processing, it may be best to be conservative.
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SG did you try it out with your guitar and see how it sounded with AX7.
Bill
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SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?
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SG did you try it out with your guitar and see how it sounded with AX7.
Yeah,,,It sounds about the same to me as having the AT7 in there....it's just that the AX7 dropped the grid current readings I am getting on the 6L6s...
It's really hard to tell if there is a difference in the point where OD kicks in because my ears are beat, and at that vol. I can't stand next to it for very long..
It does sound like a really bad-ass Deluxe/Super Reverb that wants to blow some speakers :icon_biggrin:......
I'm thinking some more efficient speakers might actually help it stay clean....I think I'm hearing power tube and speaker breakup combined (plus a little blocking distortion)....I've tried dropping some signal but it looses fullness and way too much volume if I lower the signal to the point where I get ZERO grid current
SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?
Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up
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Yeah,,,It sounds about the same to me as having the AT7 in there....it's just that the AX7 dropped the grid current readings I am getting on the 6L6s...
It's really hard to tell if there is a difference in the point where OD kicks in because my ears are beat, and at that vol. I can't stand next to it for very long..
It does sound like a really bad-ass Deluxe/Super Reverb that wants to blow some speakers :icon_biggrin:......
I'm thinking some more efficient speakers might actually help it stay clean....I think I'm hearing power tube and speaker breakup combined (plus a little blocking distortion)....I've tried dropping some signal but it looses fullness and way too much volume if I lower the signal to the point where I get ZERO grid current
SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?
Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up
[/quote]
Sounds like a good combination right were you have it, now does your last schematic have your latest values in it . I wouldnt mind throwing it out on the breadboard and hearing it for myself. Gotta love a fairly clean poweramp with good bass responce. Sounds like heaven to me .
Bill
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Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up
Thanks for the response, and I congratulate you on forging ahead! :smiley:
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Thanks for the response, and I congratulate you on forging ahead!
Thank you John.....some nights it feels like I'm out here on my own, and it's good to hear that someone (besides the regulars) is actually interested :icon_biggrin:
I'm hoping to complete the thread with a full schematic, possible layout, and bias supply solution,,,,,and last but not least a couple good sound clips :thumbsup:.....I gotta give some more thought to speakers
Sounds like a good combination right were you have it, now does your last schematic have your latest values in it . I wouldnt mind throwing it out on the breadboard and hearing it for myself. Gotta love a fairly clean poweramp with good bass responce. Sounds like heaven to me .
I agree,,,it does sound good right where it is now,,,and I'd love to hear your thoughts if you do lay it out
I got a little too obsessed with trying to keep it clean all the way up to ten,,,,and completely eliminating grid current
On 5 I think you'll agree it's a very strong clean sound............single coils sound much better to me
The preamp schematic in the 1st post of the thread hasn't changed (much),,,and the power amp portion in reply #182 is accurate
The 12AU7 in the PI has really helped me clean things up,,,and the only tube that is still up for debate is the AX7/AT7 CF
Keep in mind that I have a separate bench bias supply, so that does not match the schematic (currently about -210vdc)
Have fun with it and please let me know what you think.....it still needs some tweaking to complete, and I'd love to get your input
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6.5v @ idle drops to 5.9v with full signal.....with 22K in the tail
thanks, tube cad sim indicates with 290V B+ and Vgk of -6.5V 12AU7 can deliver an output of around 55Vpk before clip. with a gain of roughly 8.5 that's about 6.3Vpk of input threshold.
does that compare to you're measurements?
Thanks for looking DL
you're welcome. :-)
On my other subject...
...deferring to the experts.
:icon_biggrin:
--pete
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I think notwithstanding my obsession with the grid current and bias shift :BangHead:, it's time for you to wrap it up and build the darn thing, especially if you are happy with the sound and the response of the amp. And thank you for letting me be a part of the "debugging" processing :icon_biggrin:
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thanks, tube cad sim indicates with 290V B+ and Vgk of -6.5V 12AU7 can deliver an output of around 55Vpk before clip. with a gain of roughly 8.5 that's about 6.3Vpk of input threshold.
does that compare to you're measurements?
I'm gonna take a couple days off and then take a fresh run at it on Mon....I'll let you know
I think notwithstanding my obsession with the grid current and bias shift :BangHead:, it's time for you to wrap it up and build the darn thing, especially if you are happy with the sound and the response of the amp. And thank you for letting me be a part of the "debugging" processing :icon_biggrin:
Jaz, I'm gonna stay at it and tweak it down a little further before I start cutting wood :icon_biggrin:
It has been my pleasure to have you here,,,and I must thank you again for asking some great questions that have prompted me to take a crash course in tubes 101....I have learned a ton in the last couple weeks, and I'm gonna give my ears a little break, and get in some golf, and go back at it as a more educated experimenter.....
I'm not happy with some of the dead end threads over at TAG, so I'm very tempted to go after the SSS full steam ahead :icon_biggrin:
I found this interesting thread here: (while googling AB2).....happy reading
http://music-electronics-forum.com/t197/ (http://music-electronics-forum.com/t197/)
Oh yeah,,,,and then there's this: :wink:
https://www.youtube.com/watch?v=TB465GMTsjQ (https://www.youtube.com/watch?v=TB465GMTsjQ)
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Nice sound on that Hartman amp. Anything going on SG or are you taking a little break.
Thanks Bill
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Yeah TJ, taking a break,,,,,marinating in information a bit :icon_biggrin:
I'm a little stuck at the moment and not sure which way to go....all of my free time is being spent helping my son get his house ready to live in :rolleyes:
There was one thing that I wanted to add as a follow up to something I had mentioned earlier.....
I used those two 500K pots that I had put in place of the 470K grid leaks to dump some signal just before the grid stoppers,,,, I kept my eye on the bias voltage and monitored the drift from -25v to -30vdc,,,,with the vol. cranked and the bias reading @ -30vdc, I dialed the grid-leak pot down to get the bias voltage back down to -25, and wound up at 180K on the pot....I did the same thing on the other side and wound up with the same 180K over there...So that got me NO bias shift
Well that was a bad idea....although it did help with the bias drift,,,,it also cut off the bottom of the signal on both sides :lipsrsealed:
I didn't see it until I looked at the waves going into the grids of the 6L6s and saw a completely squared off bottom half on each side :huh:
I dialed the values back up and watched the wave "reform" into it's original shape,,,and that happened right around 420K, so the original 470K value was good....so I'll leave em there
I'll work on a completed schematic, so we have a reference....
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Here's an updated full schematic as of this point......it's nothing more than what we have been talking about, all cut and pasted together
The negative supply values have been left blank to protect the innocent.....if you can't figure it out,,,you probably shouldn't build it :wink:
I'm gonna veer off the path a little and try to incorporate some Dumble type mods to the preamp, similar to what I have seen called Ultra-phonic modded AB763....since I'm not so far away from there now, it seems like a fun direction to take,,,,,and I'm thinking this could morph into a channel switchable amp, with the CF power stage staying in place.........we'll see
The inclusion of an OD channel will also force me to figure out how to tame the grid current on those 6L6s,,,,or melt them trying :icon_biggrin:
I'll take that project to a new thread and call it Ultra AB763 with PPICF.......coming soon
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HI SG!
Couple of questions to you Sir:
1) I would like to ask if you tried boost/OD/Dist stomb boxes in front of this amp?
2) If so, how did it sound?
3) Did you try a high gain preamp?
I'm asking these questions because a fellow studio owner got interested in building this amp. But he also asked whether it was possible to build a high gain channel inside it too.
Many thanks in advance!
Best Regards
R.
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1) I would like to ask if you tried boost/OD/Dist stomb boxes in front of this amp?
2) If so, how did it sound?
3) Did you try a high gain preamp?
I'm asking these questions because a fellow studio owner got interested in building this amp. But he also asked whether it was possible to build a high gain channel inside it too.
Hey R,
I have some pretty strong opinions about your questions
1+2) I tried pedals out front and Boost/OD distortion was only good at lower dist/OD settings, and more just plain clean boosted
The thing that makes this amp so special is how clean it stays, and 'for me' any clipping of the wave created a 'not so desireable' result...
I believe that this is partially a result of the driver tube being too capable of creating grid current in the output tubes. (grid current does not sound good :icon_biggrin:)
3) I did and hated it immediately, and just wound up doubling back and taking a time out
If I was going to build this amp,,,it would be because I wanted the best possible clean sound I could get...it really is awesome.
If I was doing a gig where I need some high gain, I would use a separate/different head.
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Then again R,,,I've learned a few things since then...
One of those, just recently,,,while contributing to the Major thread...
Based on this info posted by Willabe on that thread, maybe I shoulda used a UL OT?...and some 6550s
From TUT5, chapter 7, page 3;
"Since the UL connection reduces the effective gain or sensitivity of the tube, much more drive voltage is needed to get full output. Referring back to Mullard's own spec; in tetrode, -34V of bias on g1 with 67Vpp drive, versus -75V and 140Vpp in UL.
So don't let my strong opinions fool you into thinking I know what I'm talking about :wink:
There 'may be' a way to make it work, by taming the grid current , and using the UL OT hookup to allow a bigger signal voltage. :dontknow:
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SG,
Looks like you are ready for your new build, congrats. I checked with the guys over at ampgarage that have built the SSS with the 12AT7 CF and they said they had no issue with the high voltage across the tube, but of course, most of them use NOS tube not the new production from the East. So you may want to start with the breadboard "as is" and see if any issue crops up. Looking forward to see more on your R&D results :icon_biggrin:
Jaz
use a 12AZ7 instead of 12AT7 if that's worrisome.
use the cap coupled CF driver - it's much less trouble than dicking around with a high volt neg PS to direct couple. GEC figured that out in the 50's.
http://www.triodeel.com/gec400w.gif (http://www.triodeel.com/gec400w.gif)
referring to the GEC400 scheme; replace 6SN7 driver/cathodyne with 12AX7 or 5751 or 12AY7, use a 12AT7 or 12AZ7 for the CF driver. the cathodyne and pre-cathodyne gain stage can be any tube. maybe copy a fender deluxe or super 5F4 type plan? bet it sounds cool. if you want to copy the GEC 400W plan but use novals, then use 12BH7 or 6CG7/6FQ7.
--pete
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Thanks guys!!
SG: I was asked to build a clean sounding amp and immediately remembered this project. But I was also asked whether it would be nice with pedals in front of it...Let me talk to the guy - studio owner - and see what happens. I would like to thank you for you kind attention!!
Pete: I already own a 1968, Linden made, Ampeg SVT...man this thing is heavy..It hurts just thinking to carry it outside.
However, I also have handy a kit of iron which may be used with a 6x6L6/KT66/EL34. Specs:
P.T.
B+(1): 340 - 0 340 @ 1A.
B+(2): 250 - 0 250 @ 250mA.
Bias: 60 - 0 @ 100mA.
Fil. 3,15 - 0 3,15 @ 10A.
O.T.
U.L. @ 40% winding turns
150WRMS @ 30Hz
1750 ohms plate to plate Z
0-4-8-16 speaker out
Choke 10H @ 100 ohms @ 250mA.
I got interested in building a power amp only with the GE schem you provided. If going KT66, you can call it Kattie66six :dontknow: :l2:
Once again, thanks for your kind attention!
Best Regards
R.
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cool. that's some nice iron. are they custom winds? post your progress.
respectfully,
--pete
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Here is info I had from a while back. You may have seen this, but it is still worth kicking around on Fender tone stacks.
There may be a couple of things in here to help with the Fender/Dumble flavor.
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Hey Ed did he (Steve Ahola) also draw up a schemo for those? Way to hard to figure out the circuit by looking at a layout drawing IMO.
Brad :icon_biggrin:
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are they custom winds?
Yes.
I got them sometime ago to a project, but customer changed his mind and instead of 150Wrms, he decided to make a 50Wrms. So we ordered new iron and I kept it for myself.
I've been looking forward to give this set of iron a "special" home.
To tell you the truth, I'm very interested in marrying this output section to a Standel type of preamp.
Any inputs on that??
Thanks guys!!!!
Best Regards,
R.
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Hey Ed did he (Steve Ahola) also draw up a schemo for those? Way to hard to figure out the circuit by looking at a layout drawing IMO.
Brad :icon_biggrin:
No Schematic. I will try to find more info on this. What this is part of is doing the modes Dave Funk did on Fenders when he was working in Nashville.
It is hard to tell. Let me check. I have done many of what was called the "Funk" mods.
Here is a link at TAG fore more info on Dumblizing a fender. Look at the first post where it verbally explains a lot of the mods.
http://www.ampgarage.com/forum/viewtopic.php?t=5762&postdays=0&postorder=asc&start=0 (http://www.ampgarage.com/forum/viewtopic.php?t=5762&postdays=0&postorder=asc&start=0)
I guess Dave found out it was more profitable to writ the book "How to Hot Rod Your Fender amp.
I actually have the book in PDF form. I will check to see if there are any schematics in it.
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I looked up the book and it says it's written by Jeffrey Falla, who builds Tonetronic amps?
Brad :dontknow:
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I looked up the book and it says it's written by Jeffrey Falla, who builds Tonetronic amps?
Brad :dontknow:
Sorry, wrong book.
Here:
http://www.amazon.com/Dave-Funks-Tube-Amp-Workbook/dp/1500115185/ref=cm_cr_dp_asin_lnk (http://www.amazon.com/Dave-Funks-Tube-Workbook-Volume/dp/0965084108)
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:laugh: Ok.
I have that book.
Brad :icon_biggrin:
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Let's take this over here KIDs:
http://el34world.com/Forum/index.php?topic=17586.new#new (http://el34world.com/Forum/index.php?topic=17586.new#new)