The SS Project.....AB763 with PPICF

[SILVERGUN]

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

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  
Big thanks to Sluckey for the use of his schematic  

[SILVERGUN]

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

[eleventeen]

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.

[SILVERGUN]

11 my friend,,,,,let's get this out of the way early....I'm no genius  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 

[sluckey]

I really like your enthusiasm. Don't stop.

[tubenit]

Simply putting something on the table for discussion purposes only. 

With respect, Tubenit

[Glennjeff]

Some mutation of this may also be worth trying if you have problems getting the valve going in that spot.

[kagliostro]

One other pair of PPICF examples






http://www.chambonino.com/construct/constwire1.html
http://www.chambonino.com/construct/const8.html

http://www.chambonino.com/construct/constwire2.html
http://www.chambonino.com/construct/const9.html

Source

http://www.chambonino.com/construct.html
http://www.chambonino.com/index.html

K

[Glennjeff]

Also, from the previous thread, Studio Bass. Using known working systems as a starting point for developing a project can be quite successful.

[SILVERGUN]

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

[Glennjeff]

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.

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.

[kagliostro]

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

[TIMBO]

 

[Glennjeff]

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.

[jazbo8]

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 

Jaz

[SILVERGUN]

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 

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 

[John]

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. 

[SILVERGUN]

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

[Glennjeff]

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.

[SILVERGUN]

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!)

[terminalgs]

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:



if you tried this, you'd probably use one 12at7 for the 'top two', and one for the bottom two for ease of wiring...

[SILVERGUN]

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:

[rzenc]

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_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_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

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.

[SILVERGUN]

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  
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  

[jazbo8]

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...

[SILVERGUN]

 
 
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  
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  

THE SLAPPY BASS RESPONSE WAS THERE THE SECOND I PULLED UP THE VOL. CONTROL.......WOW!!

[Glennjeff]

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.

[SILVERGUN]

So, yes,,,that obviously meant an aweful lot to me  ......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 

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 

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 

[SILVERGUN]

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

[Glennjeff]

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.

I'm still smiling 14 hours later

Smiling with you.

All the best.

[SILVERGUN]

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  .....
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  
Here's the adjusted schematic for how I hooked it up:

[SILVERGUN]

I took some quick voltage readings before I had to run out the door,,,so they may not be perfect, but they're close

[SILVERGUN]

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

[Katie 77]

silvergun, you might read thru this: 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

[SILVERGUN]

Ouch! and Thank you Katie  

That's a very cool website that I had never seen before and possibly never would have if you hadn't pointed it out 

[sluckey]

I would change the bias voltage divider by a factor of ten. IE, 870K down to 82K and 1M pot down to 100K.

[Jack1962]

Correct me if I'm wrong but would'nt
 this added tube be nothing more than a buffer

[SILVERGUN]

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" 
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

[terminalgs]

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?  

[SILVERGUN]

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  

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.

[SILVERGUN]

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

[rzenc]

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.

[SILVERGUN]

Thanks Rz,
I'm learning as we go here,,,so it's the question and answer education approach  

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  

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....
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  

[SILVERGUN]

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  

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

[jazbo8]

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?

[SILVERGUN]

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  ..........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 

[Glennjeff]

Sounds great, like your playing too.

Could you just clarify for me what valves you are using as PI and CF?

[jazbo8]

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 BTW, the recording sounds just fine the way it is.

[jojokeo]

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!??? 
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?

[TIMBO]