Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on October 13, 2011, 07:18:09 am
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I am wondering if anyone has tried a push/pull 6AU6 amp? I know people use a 6BM8 pentode and even use 12AU7 triodes for small watt amps. I've got some Tung-Sol 6AU6's laying around.
I'm thinking something like a 12AX7 triode preamp, 12AX7 triode with mosfet CF into a 12X7 LTPI into a p/p 6AU6 power amp?
http://www.nj7p.org/Tube4.php?tube=6AU6 (http://www.nj7p.org/Tube4.php?tube=6AU6)
Found this: http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html (http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html)
And a U-toob of a p/p 6Au6: 6au6 parallel push pull amp running on a 9 volt walwart (http://www.youtube.com/watch?v=cKKMd11c3kc#)
Q1) Any ideas whether that would work w/LTPI?
Q2) And how many ma's of current a p/p would draw?
Q3) Whether it would work with an 8 ohm speaker?
Q4) And what the tone might be like?
With respect, Tubenit
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Maybe something like this???
Not sure about the shared cathode resistor value? I am guessing this 47R based on PRR's 1/3 watt SE 6Au6 amp.
Not sure what OT secondary ohm to use with 8 ohm speaker?
With respect, Tubenit
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Hi Tubenit
a new project ............... you are a gold mine ...............
only one consideration
don't you think that a PPIMV is a bit excessive for the power of that amp ?
Kagliostro
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one control...
one input jack...
one speaker...
one guitar...
one watt...
many happy neighbors...
sometimes, i do love simplicity.
peace.
--DL
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Not sure about the shared cathode resistor value? I am guessing this 47R based on PRR's 1/3 watt SE 6Au6 amp.
Not sure what OT secondary ohm to use with 8 ohm speaker?
The OT z isn't super important like on higher powered amps, not as much voltage, sensitivity, cone movement, sound differences, etc. on smaller stuff like this. The MV isn't necessary nor is all of that preamp circuitry. You'll see that you've got the vol at 50% min to 100% and be getting power tube saturation/distortion which is what you want at bedroom volume. Figure out your optimum bias when the amp is done like you would on other pp amps and you'll be fine - but same as the other things, you'll find it's not too critical and you won't hear a whole lot of differences w/in a certain range. HTH, Keo
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some googling with a feedback loop to this forum!: http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html (http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html)
http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html (http://www.tdpri.com/forum/shock-brothers-diy-amps/145252-push-pull-6au6-build-thread-warning-verbose.html)
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Tubenit --
As long as you are in a 7-pin mood, please consider that there are a multitude of very cheap 7-pin single triodes that would provide
hours of tube rolling fun in the V1 and V2 spots: Here are the major ones. First column is the tube base number, 2nd is amplification factor, then filament current, Max Plate voltage, plate impedence, and some notes:
Base mu Fmnt Plt Notes
6ER5 7FP 80 .18A 250V 10.5K
6GK5 7FP 78 .18A 200V 15K
6ES5 7FP 75 .20A 250V 9K
6FQ5A 7FP 74 .18A 200V 12K
6FY5 7FP 70 .20A 200V 13K
6FQ5 7FP 60 .18A 200V 11K
6FH5 7FP 50 .2A 150V 9.0K
6AN4 7DK 70 .225A 300V 10K (Output? 13mA)
6DX4 7DK 30 .20A 150V 11K (Output? 10mA)
6DY4A 7DK 28 .125A 135V 11K (Output? 10.4mA)
6AF4A 7DK 15 .225A 150V 6.6K (Output? 15mA)
6DZ4 7DK 14 .225A 200V 6.4K (Output? 15mA)
6T4 7DK 13 .225A 200V 7.0K (Output? 18mA)
6AV6 7BT 100 .3A 330V 1.6K
6BK6 7BT 100 .3A 300V 1.6K
6AT6 7BT 70 .3A 300V 1.2K
6BT6 7BT 70 .3A 300V 1.2K
6AQ6 7BT 70 .3A 300V 1.2K
6BU6 7BT 16.5 .3A 300V 1.5K (Load 10K 9mA)
6BF6 7BT 16 .3A 300V 1.9K (Load 10K 9mA)
Some of these little dudes push a goodly amount of current themselves (see the notes column) and could be output tubes in another topology perhaps. They are pennies each on fleabay.
The 6J6 which is a dual triode (35 mu each) shared-cathode that can make a decent (if a bit tame) phase inverter.
Been hoping a heavyweight designer type like yourself might take a shine to them someday. :worthy1: :icon_biggrin:
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some googling with a feedback loop to this forum!: http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html (http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html)
That is almost just like my "Tiny Giant" amp!? I wish I'd known about it earlier but then I wouldn't have learned what I did if I'd simply copied it. After reading his thread, much of his thinking and planning was very close to mine. It's a pretty bizzare world sometimes and is another example to the recurring theme that most everything has been done before at one time or another in this tubie world. Here's my Tiny Giant link I posted a little while back - http://www.el34world.com/Forum/index.php?topic=11226.0 (http://www.el34world.com/Forum/index.php?topic=11226.0)
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doodling... just my 2cents... :icon_biggrin:
--DL
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For what is is worth (not much) - I would be more interested in the original 6AU6 output design. thx.
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So let's figure what a real 6AU6 ouput stage should look like, then work out the details of the preamp.
Can we assume that we're tied to the Hammond 269EX as out PT? If so, it's 380v CT, or 190-0-190. I don't think we'll come close to taxing it, so 190*1.414 = 268v for the B+.
The 6AU6 is a 3.5w tube, and we'd like to run it with the screen somewhere near the plate. Let's also note that the screen dissipation rating is 0.75w. We'd like to know what ratio the cathode current is split between plate and screen. If we look at the pentode typical conditions on page 2, we see numbers for plate and screen current that vary from about 2.3:1 to 2.5:1. The ratio of max plate dissipation to max screen dissipation is 3.5/.75 = ~4.7.
What does this mean for us? If the plate and screen are nearly the same voltage, the ratio of screen current will lead to excessive screen dissipation. We will need to run the screen at a significantly lower voltage than the plate, to keep dissipation in check.
Maximum output power will probably happen near max plate dissipation, if we're designing a class A output stage. Plate current will be 3.5w/270v = 12.9mA. Given the ratio of screen current (let's play it safe and assume 2.3:1 for figuring), we will get 12.9mA/2.3 = 5.6mA of screen current. Max screen dissipation is 0.75w, so 0.75w/.0056A = 134v. We should keep the screen voltage down this low.
Look at the 6AU6 data sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf). If you look at the "characteristics and typical operation" on page 2, we're landing awfully close to the 250v plate, 150v screen option. If you add the plate and screen currents, then multiply by the cathode resistor value shown, the bias is a hair over 1v. That means this output stage would only need 1v peak input to drive this tube to full power! Good for a voltage amplifier, but that means the preamp (and phase inverter especially) needs to be seriously re-thought to make sense with this tube. I say that because typical big-amp phase inverters assume something like 1v of input to drive the phase inverter to make the big output swings needed by the output tube grids. That's not what we have here, and may suggest why some other folks built their push-pull 6AU6 amps with a self-split design.
We hadn't figured output power or good OT load, but I want to give you a chance at this point to evaluate whether to proceed. The condition we're pulled towards copying idles at 2.65w plate dissipation, and a push-pull class A pair will wind up making around that much power. Is this where you want to go?
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That works great for me -- I hope Tubeknit chimes in too. It is his thread after all.
Given what was said above about OT flexibility -- might this be a good opportunity to do some output type switching? I was thinking about switching between the self split setup you seem to be leading to and a parallel SE output perhaps?? I am in over my head even bringing it up so -- don't hesitate to set me straight (usually not a problem around here -- thank goodness LOL).
Thought I might also mention - Weber has this cool little transformer that takes either 9v or 12v input and pumps it up to around 240V I think. Maybe that is how the guy in the video in the first pane made his amp.
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Wrong again -- :help:
Went out to the original YouTube. He is using a SMPS to boost the voltage ... here is his description:
"6au6 mini pentode push pull guitar amp at around 2 watts or so. A sweet practice amp. 12au7 preamp tube using triode 1 as a gainstage and triode 2 as a concertina phase inverter. no eq or tone knob. power supply is 9v walwart into smps 1363 with 200v dc output and 3 22uf filter caps with 100ohm dropping r between each one. 1k5 on the screens and 94r cathode bypassed."
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Look at the 6AU6 data sheet. If you look at the "characteristics and typical operation" on page 2, we're landing awfully close to the 250v plate, 150v screen option. If you add the plate and screen currents, then multiply by the cathode resistor value shown, the bias is a hair over 1v. That means this output stage would only need 1v peak input to drive this tube to full power! Good for a voltage amplifier, but that means the preamp (and phase inverter especially) needs to be seriously re-thought to make sense with this tube. I say that because typical big-amp phase inverters assume something like 1v of input to drive the phase inverter to make the big output swings needed by the output tube grids. That's not what we have here, and may suggest why some other folks built their push-pull 6AU6 amps with a self-split design.
I'm not knowledgeable enough to fully understand this. However, I think (?) you're saying that the 6AU6 power tubes would be possibly too easily overdriven?
IF I had a master volume going into the phase invertor AND a PPIMV after the phase invertor, I think I should be able to control that? Correct? Also I could use a lower gain tube like a 12AT7?
Anyway, here are a couple of ideas. I kind of like the idea of the HoSo56 with 6AU6 tubes because of the simplicity.
Would both of these ideas work OK? IF not, would values would need to be changed?
Comments, thoughts?
With respect, Tubenit
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I've never even thought about a 6AU6 power amp so take this idea at face value. I'd build up just the power amp with the split load phase inverter and start patching in some of your other preamps via the loop jacks. That would give you a feel for what to do next. You might could even get one of the Texas proto boarders to build a mockup for you. :icon_biggrin:
Also, look at the Firefly (http://ax84.com/media/ax84_m276.gif). It may give you some ideas.
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However, I think (?) you're saying that the 6AU6 power tubes would be possibly too easily overdriven?
I'm really saying the 6AU6 is a preamp pentode, so the available voltage is much more than it needs as an output tube, and more than the phase inverter and other circuits need to drive it.
When you have a big, bad output stage, it might need a large driving signal. That implies you need a large supply voltage, to allow the phase inverter to develop one (or two) big output voltages, and still have voltage left across the tube.
We have the opposite case here. It's worth noting the original 6AU6 1/3w amp used an isolation transformer (120:120) to deliver the B+.
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Hot Blue Plates --
Could you suggest operating values for the 6au6 p/p along the lines you are suggesting? The guy in the YouTube vid was using 240v from what he says. What are you suggesting??
I am still interested in trying a p/p with the 6au6 - I have some old black plate ones that I would like to find a home for. I have a nice Iso xformer, filament PTs and some PA class OTs that I could throw at this design. 12au7 (ish) tubes for a preamp do not bother me. I like how they sound. There are other 7 pinner triodes to try as listed above (I have most I think) I also have a sub-mini tube that is supposidly like 1/2 a 12au7 (5977) as well.
Perhaps this could be an interesting preamp tube: 12FQ8. There was the attached schematic on the web a while back and a YouTube. Search YouTube for "ValveLitzer". Cute name - the 12FQ8 are most known for being in Wur"Litzer" organs. The one in the YouTube has WurLitzer printed on it. This one is starved plate so - we could actually "feed" it more juice I suppose (LOL). Sounds pretty cool as is IMHO).
I also included the version of PRR's Third Watt amp for reference (not sure if it is the latest).
If you point the way I will try to build it. Thanks!!
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....Texas proto boarders to build a mockup for you.
work'in on it... :icon_biggrin:
:think1: :BangHead:
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So let's flesh out the details for the output section first, and find out what it implies we need from the phase inverter and preamp.
We'll work again from the assumption that the Hammond 269EX is our power transformer, and will not be substituted. We're also using solid-state diodes for the rectifier, so the B+ available will be 268v. This will be rounded up to 270v for calculation.
We've already found that the screen voltage will have to be lower than plate voltage to manage plate and screen dissipation. Since there are handy plate curves for G2 = 150v, we'll use that as our planned screen voltage. Right now we know plate = 270v, G2 = 150v.
If we look at the 6AU6 plate curves (see attachment below), we can draw a line from the knee of the 0v gridline, which occurs at Vplate = 40v and 15.5mA of plate current. That will be one end of the line, and the other end will be at 270v and 0mA plate current. From this line, we can estimate power output and the OT load. Note that we selected our screen voltage to be 150v, and that is the stipulated G2 voltage (Ec2) of these curves.
The equation for power output is Po = 1/2 * Imax * (Eb - Emin).
Imax = peak plate current, which is 15.5mA in our case.
Eb = supply voltage, 270v
Emin = plate voltage at peak plate current (voltage at knee), which is 40v.
The 1/2 factor converts from peak power to RMS power.
Po = 1/2 * 0.0155A * (270v-40v) = 1.78w.
The line itself implies a resistance of (Eb - Emin)/I = (270v - 40v) / 0.0155A = ~14.8k ohms. Ordinarily, we think of the load for one tube as being plate-to-plate primary impedance divided by 4. If the output stage runs class A (never cuts off either tube), each tube sees half the primary impedance. Given that, our estimated primary impedance required is 14.8k * 2, or about 30k plate-to-plate.
A suitable transformer is the Hammond 125A, which handles 3w of output, and has many possible primary impedances. Assuming no problems with this approach, we can get a 30k primary impedance by placing an 8 ohm load on the 4 ohm secondary tap, with the transformer wired to provide 15k when a 4 ohm load is placed on the 4 ohm tap.
Great, but how do we bias this thing? There are two places to find the answer, but the "Characteristics and Typical Operation" on page 2 of the 6AU6 data sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf) tells us for G2 = 150v (shown in the column with Vp=250, Vg2 = 150) that the approximate G1 voltage for plate current cutoff occurs at -6.5v. We want to bias halfway between cutoff and 0v on G1, or -3.25v.
Now we need to know the plate and screen current at idle, and figure biasing. We see from the plate curves that plate current at -3.25v is ~2.6mA. Looking at the bottom graph on page 5, screen current when G2 = 150v and G1 = -3.25v is 1.05mA. Cathode current is then 3.65mA for a single tube, or 7.3mA for the pair, so the cathode resistor should be 3.25v/0.0073A = 445 ohms. 470 ohms is the closest standard value. 3.25v * 0.0073A = ~1/4 watt, so a 1w resistor will run cool. I'd suggest a bypass cap for the shared cathode resistor, and even a 50-100uF 16v cap is small and cheap.
The output stage at this point only needs a 3.25v peak input signal to drive it to full power, so we don't need anything bigger than a split-load inverter.
We know we have ~270v B+ and 150v for the screen. Since we don't need much signal from the phase inverter (or the rest of the preamp), there's no reason we can't drop the screen node of the power supply to 150v. We'll have the screen current for both output tubes running through this resistor, so we'll have at least 1.05mA * 2 = 2.1mA through this resistor. (270v-150v)/0.0021A = 57k ohms, and 56k is the closest standard value. We'll have to revisit this value as we determine the current requirements of the phase inverter and preamp. It is probably advisable to add a 100 ohm or so screen stopper in series from the screen supply node to each 6AU6 screen.
At this point, we need to breadboard just the output stage and feed it test signals to verify that the estimates so far agree with reality. There's no point proceeding with a preamp design until we know this initial draft works.
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I am still interested in trying a p/p with the 6au6 - I have some old black plate ones that I would like to find a home for. I have a nice Iso xformer, filament PTs and some PA class OTs that I could throw at this design. 12au7 (ish) tubes for a preamp do not bother me. I like how they sound.
I'm thinking Tubenit has moved away from using the push-pull 6AU6, based on his other thread to use the Hammond PT.
So that said, do you want to look at options for push-pull 6AU6's with your isolation transformer? I'm assuming the voltage is 120:120. What is the current rating for it?
It is probably worthwhile to build the circuit from such a transformer, as the high supply voltage isn't needed to get all the usable power from a 6AU6. I had already envisioned a split-load 12AU7 phase inverter and low supply voltage, based on the small drive requirements for the output stage.
So if you're game, we'll proceed along that detour.
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I'm thinking Tubenit has moved away from using the push-pull 6AU6, based on his other thread to use the Hammond PT.
i take it this project is dead then?
i'd hate to be the only guy that shows up to the party... again... ;)
--DL
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Moonbird may still be interested, and working without the constraint of needing to fully utilize the Hammond PT we assumed earlier.
It might be silly to do a push-pull 6AU6 amp, other than to do it "just because" but I also have a bunch of them hanging around.
In the search to help Tubenit with a different tube option, I found a bunch of beam power tubes that are candidates for future exploitation. I mean, experimentation... :laugh:
They are generally for car radio use, run on low plate and G2 voltage, and seem to be available NOS for $2-5 bucks each.
Anyway, if Moonbird still wants a push-pull 6AU6 amp, we might let this thread fade and start anew in a new thread.
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> We want to bias halfway between cutoff and 0v on G1, or -3.25v.
No, for Class A we want to bias for plate current halfway between peak and zero current.
Maybe a hair low.
Assuming 15mA peak, pencil 7mA/tube idle.
-1.8V bias.
This will "center" the output.
If you use your -3.5V 2.5mA bias, the cathode current is 5mA at idle and 10.5mA at full power. With a cathode resistor, the bias voltage changes dramatically from soft to loud.
In any case: the 6AU6 is a "skinny" tube, made for gain not power. Speakers need power. Also transformers suck-off power, especially when working at high voltage and low current. The high gain is a dubious bonus; the poor power handling makes 6AU6 a weak OT-speaker driver. There are (as you say) better choices.
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If you use your -3.5V 2.5mA bias, the cathode current is 5mA at idle and 10.5mA at full power. With a cathode resistor, the bias voltage changes dramatically from soft to loud.
Well, this just points out the problem with the 6AU6, and perhaps working about 150v B+.
I considered the issue of biasing to half-current. But as you showed, the bias winds up at -1.8v. It can swing up 1.8v, and has to swing down almost 5v to get the other half of the current swing.
I figured we'd have to bias for equal voltage swing. I'm guessing that means this should really be fixed bias for this condition, even though 3.25v of fixed bias feels silly.
Which just seems to underscore the poor working conditions. We'll rethink if Moonbird wants for ~140v (and probably better results).
It probably makes more sense in the end to do push-pull dual-triode tubes anyway, a la the Firefly. Maybe we should save the 6AU6's for pentode preamps instead of the more expensive 5879 and EF86.
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It probably makes more sense in the end to do push-pull dual-triode tubes anyway, a la the Firefly. Maybe we should save the 6AU6's for pentode preamps instead of the more expensive 5879 and EF86.
I like much better 12bh7a & ECC99 triodes over 12au7 for Firefly-ish amps. Tone and headroom are two reasons. People need to understand that you only get so much from low volume stuff. Better to have the above two attributes and not have to be on "10" or "11" to fully squeeze that lemon. Some have to learn for themselves and that's okay. "You can lead a horse to water but..."
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> swing up 1.8v, and has to swing down almost 5v to get the other half of the current swing.
At -3.6V it gets down to 2mA. Not "off", but "pretty-off". We never really get to ZERO current in any amp. But the 6AU6 does less well than most Power tubes.
Perhaps a compromise between 1.8V and 3.2V would be as-good-as-it-gets.
> 3.25v of fixed bias feels silly.
Two LEDs.
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Hello, first post on this forum. I am the guy in the video in the first post. I have since built a version with 4 6au6 fed by a ltp pi and a simple 2 gainstage pre that goes GS-TS-Volume-GS-LTP- PPIMV-PA . I am still tweaking it and the info on here should help! will post a vid asap. this is a decent amp for practice and maybe a small gig. about 4 to 5 watts. sounds decent enough. I have each pair of power tubes going into a 100R cathode resistor bypassed by 220uf. I also have a presence knob off an 8ohm tap using a 100k nfb resistor in to the PI cathode similar to a liverpool/express setup. I used the PPIMV to stop the power tube overdrive, not really to control the overwhelming volume. :icon_biggrin: I will post the schematic as well. I am stuck on the bias point for the PA and not knowing an awful lot about math and electronics isnt helping a ton. I am using the scenario of 269ex with 263v on the anodes, about 170 on the screens through a 9k2 resistor combo and as i said earlier 100R bypassed with 220uf on each pair of power tubes. what would I need to bias this ab1? am I already close? Any help is welcome and I thank you in advance.
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Welcome to the forum and THANKS for posting about your amp! I am sure it will generate interest here.
Anything you are willing to share would be appreciated!
Please share a schematic if you can. Even a good hand drawn one will work and I can redo it in SCH format if needed.
With respect, Tubenit
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Hello all --
Out of town this weekend -- still ready and willing to proceed. Glad Blue Plate found the other Beam Pentodes -- hope we try those too. There are many sleeping beauties out there I think. Great to have some knowledgeable folks to point the way. Many thanks to Blue Plates et. al. for the effort so far!!
On the Iso xformer: It is a Triad Magnetics N-68X (I have 5 of them) - 50VA which should be more than enough since it is not supplying filament juice (right?). I have about a dozen old Radio shack 12.6VCT xformers -- forget their power rating at the moment (2Amps??). Will find out. Would be happy to get one of each into your hands Blue Plates -- more than happy!! Send me a PM and I will post them this week!!
Yes it is a bit of a pain to have two PTs -- but in this case I have them. Traid N-68xs can be had at Allied for about $12 -- filament xformers for about the same cost. For everybody else Antek has a 50VA 120V transformer with two 6.3V taps for about $25. This is a pretty cheap platform. True it can't play Shea Stadium next summer but -- I have given up on that little dream (LOL). Besides "the lads" already did that (LOL).
My interest in the 6AU6 was definitely peaked by Zambo's video. Great sound IMHO. Great of you to chip in here kind sir!!! Should be no problem getting some PA distortion from what I read here. The pp interest is partly curiosity - and partly wanting to hear some of that "Zambo" tone for myself. Guys -- 6au6s are dirt cheap and generally frowned on by everybody -- if good tone is possible -- lets use a few up -- whaddia think?? (LOL). Call it a rebellion if that helps -- lets all come to the defense of maligned tubes everywhere (LMFAO).
Plus I would really like to have a cheap platform for testing what tone changes really happen with different PA topologies. I think Sluckey's idea of a "plug-in" power section is inspired!! I'd love to have a pp cathode bias PA, a pp fixed bias PA, a sepp PA, and on and on. It wold be great to bolt on a bunch of different power sections to a particular preamp and listen for the differences. Shoot I really like the sound of old germanium treble boost pedals -- it would be great to have a 1v PA to hook one up to.
Sorry for the rant.
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Hello, first post on this forum. I am the guy in the video in the first post. I have since built a version with 4 6au6 fed by a ltp pi and a simple 2 gainstage pre that goes GS-TS-Volume-GS-LTP- PPIMV-PA . I am still tweaking it and the info on here should help! will post a vid asap. this is a decent amp for practice and maybe a small gig. about 4 to 5 watts. sounds decent enough. I have each pair of power tubes going into a 100R cathode resistor bypassed by 220uf. I also have a presence knob off an 8ohm tap using a 100k nfb resistor in to the PI cathode similar to a liverpool/express setup. I used the PPIMV to stop the power tube overdrive, not really to control the overwhelming volume. I will post the schematic as well. I am stuck on the bias point for the PA and not knowing an awful lot about math and electronics isnt helping a ton. I am using the scenario of 269ex with 263v on the anodes, about 170 on the screens through a 9k2 resistor combo and as i said earlier 100R bypassed with 220uf on each pair of power tubes. what would I need to bias this ab1? am I already close? Any help is welcome and I thank you in advance
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Is this close? Can you edit it to make it more accurate to what you have?
BTW, nice playing in the video you made.
With respect, Tubenit
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Thanks for the warm welcome!Ive been keeping notes on my original blueprint. I will redraw it and post it. I dont really know how to edit those ones. I will try to get that and a sound clip or video up today. This one sounds ok but if I were to do it again ( and i am sure i will...sigh...) i would use a much larger chassis and add one more preamp tube even if i only used half of it. This would make a sweet mini liverpool or plexi type amp i am thinking. So far it has been all the work and time of a big amp. Anyway, I am open source about everything i build so feel free to repost build sell etc. but check for saftey issues and errors first as I am a hobby guy at best. I am also known to draw things wrong. PRR catches everything though from what i can tell. Thanks again and pleasure to meet you all. Greg
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Greg/Zambo,
This is the program that you can use to edit the SCH schematics. Great program. Very intuitive and easy to use. Click on a component and you can edit the assigned value very easily.
http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm (http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm)
You could easily add a mosfet cathode follower behind that second gain stage and move the tone stack behind the CF before the LTPI ............... & then you'd be headed towards the Marshall Plexi area (sort of) This is the Marshallish 6AU6 idea I was referring to. I'd love to see some other Hoffman forum guys attempt one of the 6AU6 amps like you have. I think it could be a nice sounding amp.
IF you can even edit the GIF use "Paint" & repost stuff or make notes on it, ........... I can edit the schematics for you to get one that is accurate to your build. Again, THANKS for sharing your innovation! With respect, Tubenit
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I will have the schematic redrawn in a bit here. Here is a vid for sound reference. Sorry about the hum, my whole house is two wire with 3 hole plugs.... :BangHead: http://youtu.be/89-NaLKN5U8 (http://youtu.be/89-NaLKN5U8) cheers!
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zambo :headbang:
cool stuff!
--DL
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ok trying to post schematic.
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thanks zambo!
repost in .GIF format for all to see.
peace.
--DL
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Zambo,
I appreciate your post of the video. Have you tried the amp with 3 prong wiring outlets and if so, how much of the hum was
reduced?
BTW, great playing again!
Can you please clarify which approach you took to the power tube cathode resistor/cap?? You indicate in your post that each pair had their own 100R/220uf ................. BUT your schematic shows all 4 power tubes sharing one 100R/220uf? Which is it?
I have each pair of power tubes going into a 100R cathode resistor bypassed by 220uf.
THANKS for editing the schematic I posted!
I think your editing left an error on the 6AU6 screen resistor wiring. So I fixed that.
I also added the 2.2M resistors to the dual gang PPIMV for a "safety" feature if the pot fails.
PLEASE post if you see any errors.
Forum guys, ............... ANY suggestions to this amp?
With respect, Tubenit
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The cathode resistor and cap are back to single, all 4 into one. They were biased way to hot i guess with two seperate ones. screens need to drop more voltage to. i was just showing it the way its wired as of now. Thanks for the free ware tip btw and thanks for the compliments!
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also, if anyone knows the biasing math I posted all the number on here. anode to ground is 276v. anode to cathode is 271v. screen to ground is 208v. screen to cathode is 204. my voltages were lower yesterday by a bit. I think i had two cathode resistors wired up though. any way if someone can figure out the correct cathode R and screen R i would sure appreciate it. I am kinda guessing at it right now :help:
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> showing it the way its wired as of now
I do not see a path to ground for the 6AU6 grids.
They MUST return to ground through less than 1Meg.
I have attached a minimum fix. A more-robust fix is shown in Tubenit's plans.
> They were biased way too hot
Without a path to ground, the grids don't know what voltage to be. Cathodes follow grids. If grids float-up, cathode voltage goes high, cathode resistor and cathode current runs high, B+ drops, plates melt.
I kinda suspect you DO have this ground path; usually an amp without such a path is VERY erratic, not really playable.
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Zambo --
Really like the tone you are getting -- goodness sakes! That upper mid stuff when the pedal was on is inspiring.
Do you think you could tell me where I can get some fingers like yours?? I have been looking a long time for some. How do you get that pulsation on those bends? WOW. Great playing!! You should consider releasing a guitar lesson DVD.
Thanks for the posts!!
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See i told you PRR catches everything! :worthy1: I forgot the ground on the schematic for the dual 250k PPIMV but its the ground reference for the grids. Thanks for looking PRR!
@ Moonbird - I will put up some lesson vids to teach all 3 of my mediocre riffs if you would like :icon_biggrin: Honestly, thanks i am glad you enjoy and I am putting some of my lesson vids back on youtube ( Forbyn is my user name on there). Thanks for all the encouragement!
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Zambo --
Youda man!! I will be looking for them tonight!! :worthy1:
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Hey Zambo, you sound real good. You must play a lot and it shows. What gauge strings are you using in those two you tube clips?
How do you get that pulsation on those bends?
moonbird, I'm not Zambo and I don't know where your at playing wise, but this might help you. Try this, you take your left hand and hold it out with it's palm to the ground then pivot your hand left to right using the index finger as the pivot point. Let the pivot point run all the way up your forearm from the tip of your index finger in a straight line. Keep pivoting your hand while you slowly start turning your hand over to get to it being palms up. That's how I used to show guy's when I gave lessons. It's in locking up your wrist/fingers as a unit and pushing the strings that way. The strength is in turning/pivoting (up) the wrist, not in pushing with your fingers. Same thing for an "up" bend. You can think of it as many up bends (and release) all one after the other at the speed you set. Look at Zambos clip again, and I think you'll see it. One thing tho, the heaver the strings the harder it is to first build up enough strength to get it under your control, but you'll get it.
Brad :icon_biggrin:
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Brad --
I will definitely try that -- I think I understand.
I developed my vibrato from watching BB King. This sounds like more of a leveraged power vibrato approach. -- very cool. thanks.
Oh yeah -- me? Ima hack player -- but I really like music so I just keep trying. Thanks for the tips.
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I developed my vibrato from watching BB King. This sounds like more of a leveraged power vibrato approach.
BB's doing the same thing, shaking his wrist, altho he is lifting his thumb off the top of the neck. What's different is Zambos shaking it on an already bent note and you can't realy lift your thumb off the top of the neck if you want to keep the note/string bent. :think1:
tubenit, sorry I did'nt mean to hijack your thread, I'll stop now. Please carry on with your good work. :bravo1:
Thanks, Brad :icon_biggrin:
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All -
Back to business :icon_biggrin:
If we are heading toward a ~22k OT impedence, here is a more beefy OT that has multi speaker taps.
http://www.musicalpowersupplies.com/3.html (http://www.musicalpowersupplies.com/3.html) (look for the OT5PP)
Not sure anyone else has anything like this. Matt designed these specifically to beat the Hammond 125x series OTs for bass response. Fits the 2 tube version as well (44k I presume).
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if someone can figure out the correct cathode R and screen R i would sure appreciate it
Based on this information: http://oldradio.qrz.ru/tubes/foreign/01/6AU6.gif (http://oldradio.qrz.ru/tubes/foreign/01/6AU6.gif)
I would think a pair of 6AU6 tubes in push/pull would be about 32ohm ?
So in light of that, I personally would probably use a separate cathode resistor & cap for each pair of 6AU6.
And I'd probably try something like 32-50 ohms for each pair of 6AU6. For a quad, I think you'd use 16-25 ohms NOT 100 ohms.
Hopefully, someone more knowledgeable can better answer this question.
with respect, Tubenit
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yeah, i would love to get a good answer for one single resistor or a pair of them. :dontknow:
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one 100ohm, two 200ohm resistors, or four 400ohm
--DL
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one 100ohm, two 200ohm resistors, or four 400ohm
I don't understand what that means? Not saying it's incorrect, just not sure what it means?
I may be approaching this completely wrong since I am not that knowledgeable about this stuff?
For a SE 6V6, I would use 470R Like in the Fender Champ
http://www.el34world.com/charts/Schematics/files/fender/champ_5f1_schem.pdf (http://www.el34world.com/charts/Schematics/files/fender/champ_5f1_schem.pdf)
And for a P/P 6V6, I would use 250R Like in a Fender 5E3
http://www.el34world.com/charts/Schematics/files/fender/DELUXE_5E3.pdf (http://www.el34world.com/charts/Schematics/files/fender/DELUXE_5E3.pdf)
So I was thinking when you double the number of tubes, that you half the cathode resistor value?
Is that accurate or incorrect, please? I may be way off on this?
With respect, Tubenit
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one 100ohm, two 200ohm resistors, or four 400ohm
allow me clarify.
one 100ohm if you have 4 tubes sharing one cathode resistor.
two 200ohm if you have 4 tubes with one 200ohm under each pair
four 400ohm if you have 4 tubes each with it's own cathode resistor.
double the current halve the resistance. halve the current double the resistance.
--DL
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It makes so much sense when you put it like that. Thanks! :icon_biggrin:
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ok here is the latest updated schematic. I cant get my puter to make this a jpeg or gif. will do asap. Thanks again for the help.
Zambo, you need to edit it to a bitmap (BMP) then save to a GIF ...... then "shrink" it down to a postable size by hitting "stretch"
feature (under windows "PAINT") and stretching smaller to 45% Tubenit
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Hi guy's,All very interesting stuff and it seems that this idea may apply to other preamp tubes http://www.ampbooks.com/home/amp-technology/EF86-power-amp/bes (http://www.ampbooks.com/home/amp-technology/EF86-power-amp/bes) .
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ok trying to post schematic.
zambo, either the schematic is drawn wrong or the NFB ckt. is basically ineffective. as drawn the 100K NFB resistor has a direct path to ground and the 5K pot and 100nF cap are shorted to ground.
peace.
--DL
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zambo, either the schematic is drawn wrong or the NFB ckt. is basically ineffective. as drawn the 100K NFB resistor has a direct path to ground and the 5K pot and 100nF cap are shorted to ground.
Good catch!
This would be an option and something I've done on some of my amps.
Tubenit
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Hmm... aren't the 2k2's going to the screens shown wrong?
It looks like there's a 2k2 going from "B" to the screen on the "bottom" tubes, and then another 2k2 going from the screen to the screen on the "top" tubes....
should these resistors both go from "B"?
EDIT: corrected schematic above post Tubenit