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Push-Pull 6AU6 Build Thread. (warning: verbose!)

Discussion in 'Shock Brother's DIY Amps' started by ThermionicScott, Jan 29, 2009.

  1. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Hey folks,

    I'll preface this thread by saying that not long after building my 5F2A (which I love, don't get me wrong), I wanted to build something even smaller, but with the crunch of push-pull tubes. After gathering up most of the parts for a Firefly build, I decided to veer even further from the beaten path and build an output section with small pentodes or beam power tubes.

    A quick inventory of my stash revealed that I had a lot of 12AX7's and 6AU6's, along with some 6-volt rectifier tubes. Perfect -- the 6AU6 is a 3W pentode that was used in a lot of radios back in the day, so there are a lot of them left. (My conscience won't let me design something around rare tubes, anyway.) The challenge is that 6AU6's weren't often used for output sections, so I had to do a little bit of thinking:

    [​IMG]

    The quiescent operating point looks weird on the load line, but I think it'll work. It biases my tubes so that they are midway between saturation and cutoff, so it's essentially class A. The impedance for one tube turned out to be 25,000 ohms(!) due to the high plate resistance of these tubes -- doubling that gave me a plate-to-plate impedance of ~50,000. To accomplish this, I'll wire the Hammond 125A transformer for 22,000-ohm/4-ohm operation, then use an 8-ohm speaker to give a reflected primary impedance of 44,000. Whew!

    Here's a preliminary schematic. A lot of the resistor values have yet to be determined, and I'll add tone controls once I've heard it in action.

    [​IMG]

    I started off with cathode bias, but the variation in current draw between idle and maximum signal is such that my brain shut down, and I settled on fixed bias, using rectified heater voltage.

    Lastly, here's a shot of my test setup. Not much done yet except for the rough layout.

    [​IMG]

    Any comments/questions are welcome. Thanks for looking! :grin:

    - Scott
     
    Last edited: Feb 3, 2009
  2. Tele-Dave

    Tele-Dave Tele-Meister

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    You lost me right after "Hey Folks"
     
  3. sjhusting

    sjhusting Tele-Afflicted

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    PRR over on the Hoffman forum designed a 1/3W SE around a single 6AU6. I can't find the original thread, but here's another which does include the schematic. I realize you want push-pull, but there might be some useful info i there for you.

    http://www.el34world.com/Forum/index.php?topic=2958.0

    The original thread had a lot of useful info about the 6AU6, but it seems to have been deleted from the forum along with everything else older than 2008. I'm trying to get a copy of it.

    In any case, if you are going with push-pull class A, you have at least one real-world guitar-oriented reference to compare to. Start there and drop Rk to suit to taste (ear). The recommended values in the data sheets were not geared for guitar, so real-world best-sounding may differ considerably, as it did for my fixed bias 5902 amp.

    steven
     
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  5. <jbc>

    <jbc> Tele-Holic

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    OK, I'll comment.

    You've got 1/2 a triad leftover and no cathode bypass caps?

    Does the ax7 cathodyne have the power to drive 6au6 pp?

    How about a paraphase or ltp with that extra half-triode?

    Once I get the decks cleared I'd like to build something simular.
     
  6. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Neat stuff, thanks for the link! The reverb transformer has a 25K primary, so that's encouraging. :cool:

    - Scott
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    It ought to. 5V peak-to-peak is all I'll need to drive this thing. The tweed Deluxe (like many other tweed amps) only has two gain stages, and it has enough voltage swing to not only get 40V p-p, but go beyond that and drive the output tubes well into clipping. I might add a bypass cap to the first stage if I go with a TMB tone stack or something else that eats lots of gain.

    BTW, I thought about using a 6AV6 for one of the stages, since I knew I'd only need three. Actually, I still might, as it looks like they aren't as rare yet as I originally thought. ;)

    - Scott
     
  8. wnorcott

    wnorcott Tele-Holic

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    My hat's off to you Scott I love to see people actually design something from scratch instead of just recreate one of the old chestnuts.

    Keep us posted man.

    Bill
     
  9. mattdean4130

    mattdean4130 Tele-Afflicted

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    +1
     
  10. sjhusting

    sjhusting Tele-Afflicted

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  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Wow, that's an awesome book. I had seen the link on that other site, but hadn't checked it out.

    And by the way, keep asking questions if something doesn't fit, so we can all keep learning. :cool:

    As for my build, not much to report, except that I found the perfect power switch.

    [​IMG]
    :cool:

    - Scott
     
  12. sjhusting

    sjhusting Tele-Afflicted

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    Yeah! I had that idea too - you could do it, you know, if the blade switch controlled a relay for the real power.

    Then some magic eye tubes hooked up to whatever for a mysterious light show, and maybe some Nixie tubes displaying the bias voltage on the output tubes - of course, they would have to be great big coke-bottle shaped 807s with plate caps

    Would be a great stage amp.

    Maybe two of those electrodes with a discharge between them, like in Frankenstein.

    steven
     
  13. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Well, I got enough hooked up to test the rectifier. (The transformer measured 268-0-268 with nothing attached but the primary wiring.) The 10uF cap is temporary -- I have a more orderly layout planned for the power-supply chain:

    [​IMG]

    I'm obviously not on the short-list for UL certification, but the results are good so far.

    - Scott
     
    Last edited: Feb 3, 2009
  14. sjhusting

    sjhusting Tele-Afflicted

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    What is that, a JX you're using? I can't really see with these old eyes ...

    steven
     
  15. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Yeah, a 269JX. Supposed to measure 250-0-250. I'm either going to have to buy another transformer (maybe a 269GX) or play around with resistors to get my voltage down where I want it.

    - Scott
     
  16. sjhusting

    sjhusting Tele-Afflicted

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    instead of resistors, put a series of 5W zeners in the HV CT. less heat.

    steven
     
  17. celeste

    celeste Friend of Leo's

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    Try a psudo choke input filter, diode-> very small cap->resistor->normal cap feeding ot. the size of the small cap will be well under 5uf and likely under 1uf. you can sim it with PSUD to find the right value. By playing with the first cap you can go from AC*1.14 down to AC*.9. Do not take any power from that node, there is no real filtering there.

    I also worry about getting your bias from your heater windings. By doing so you have locked the winding to a ground reference and thus made a noise path for heater noise to get to your cathodes.
     
  18. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Not a bad idea -- a true swinging choke would drop my voltages too much, but something in-between might work well. I've had good success with adding a CR stage to the front of single-ended amps like the Champ to reduce B+ and PS noise, but since I only have an estimate of current draw for my project, it's kind of a crapshoot until I get all the toobs in there.

    True also. I figured it would make a good source since I only need a couple volts, but I haven't gotten far enough to tell whether it'll be noisy or not. If need be, I can always take the bias off one of the HT windings, but I'd have to divide it down by a lot. For the sake of science, I'll try it both ways! :D

    Thanks,
    - Scott

    P.S. PSUD is awesome! It took me some trial-and-error to get results that looked right, but now it's getting to be fun trying all the models. :grin:
     
    Last edited: Feb 2, 2009
  19. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I was home sick today (Monday), so I got a little more done in between bouts of sleep:

    [​IMG]

    The 6AU6 sockets are completely wired except for the control grids, I wired a speaker socket to the OT (cable is long so that my speakers don't have to reach as far to hook up!), and I've got the rest of the B+ chain on the terminal strip. PSUD2 told me that a 2.7K dropping resistor would get me pretty close to my desired operating point, so that's what I'll use for now.

    Here's a close-up of that terminal strip, but it's not a great shot either. Sorry...

    [​IMG]

    (Those little 10-ohm metal film resistors are for checking current draw.)

    - Scott
     
    Last edited: Feb 3, 2009
  20. marshman

    marshman Friend of Leo's

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    So, I'd been led to believe that 6AU6 was basically 1/2 of a 12AX7 in a 7-pin bottle...is that correct? The reason I ask is that I'd been led to believe that 12AX7s were crap as mini power tubes, that was what 12AT7 and 12AU7s were for (though the 12AU7-6AU6 thing would be nice if it were applicable).

    I recently got a whole pile of those, as well as some 6AQ5s, and was thinkin' of buildin' myself a little 7-pin only Princeton sorta thing.

    By the way, I love the prototyping board--very slick!
     
  21. sjhusting

    sjhusting Tele-Afflicted

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    Nope, it's a sharp-cutoff pentode.

    Power tubes have to swing current; preamp tubes swing voltage. A 12AU7 can swing more current than a 12AX7, that's why it's preferred as an output tube.

    Don't be fooled by the similarity of the tube designation. The letters between the first and last numbers were pretty much sequentially or randomly assigned, and don't mean much. The numbers have a definite meaning, though.

    I can't think of any 7-pin dual triodes off the top of my head other than the ecc91, and that's a RF tube with a mu of 38; so if you go with 7-pin preamp tubes, you're going to need twice as many. Fender used the triode part of a 6AT6, with a mu of 70, in the tweed harvard.

    steven
     
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