PT for P-P Class A low power?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by amper, Feb 25, 2006.


  1. amper

    amper Member

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    I mentioned this breifly over in the Amps forum, but I thought I'd expand/expound here.

    I'm looking at putting together a design for a Class A, push-pull, relatively low power amp. The basis for my design will be the AX84 P1 High Octane, with a few twists and turns.

    Based on my research so far, I think that a plate voltage of about 250V is in order. Looking at the AX84 schem's, it would seem that I would want to use a power transformer that has about 190-0-190 secondary windings (from a 115V primary), but I'm having trouble locating a PT to do the job. What I want is something that will handle the current demands of two octal power tubes (figure 6L6GC's as a starting point, but if I could squeeze in KT88's, I'd like that) and say, 4-6 noval preamp tubes (12AX7/12AT7/12AY7/12AU7--obviously the AT and AU draw a lot more current than the AX/AY's).

    Can anyone suggest a suitable transformer? The closest thing I've found is the Hammond 363CX (180-0-180, 250 mA), but it doesn't have a 6.3V filament tap, just a 5V filament tap. Mercury doesn't seem to offer specs on their site, but maybe I'll drop them a line.
     
  2. brad347

    brad347 Member

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    i've never heard of class A push-pull. I thought push-pull amps were all class A/B. Educate me.
     
  3. hustist1

    hustist1 Member

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    I'd be interested in this too. There's not a lot of info out there about Class A push pull. I'd like to see an explanation of how to choose the bias point. I built a SE EL34 practice amp last year that I like so much, I'd like to have a bigger version. As I understand, there will be heavier current draw, and the OT has to stand up to constant higher current, right?

    Do you do class A push pull by putting a phase inverter in front of two SE output stages, so to speak? Same bias point as for a SE?

    steven
     
  4. samwheat

    samwheat Member

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    I used a hammond filament tranny when i built a self split el34 high octane ... i used that same PT and had 250V on the plates

    Heybour could make you what you need
     
  5. Swarty

    Swarty Member

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    Allesandro makes an English Coonhound which is the only p/p Class A amp that I know of (although there are a lot of companies that make this claim). The Coonhound has 4 EL84s at about 200V good for about 15w.
     
  6. amper

    amper Member

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    Yeah, I was thinking I might have to go to a separate filament transformer, but I was trying to see if I could do it without one, and of course, with a commonly available PT rather than a custom job. Not that I'm ruling out a custom design, it's just that my resources are limited at the moment. The plan is to eventually go to production, so at that point a custom design might be worth the trouble and expense.
     
  7. amper

    amper Member

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    The thread over in the Amps and Cabs section has somehow turned into more of a Tech Info thread, so I don't want to beat the horse, but there's an awful lot of information out there on Class A1 P-P output topology. For a quick overview, you can read Randall Aiken's oft-quoted "The Last Word on Class A" article, as well as his article, "Is the Vox AC-30 really Class A?".

    The classic text is of course the Radiotron Designer's Handbook, 4th Ed. (I also have the 3rd. Ed.). BTW, the RDH4 even makes short mention of a "push-pull single ended" topology, but doesn't discuss it. Now *that* I'd like to see!

    Anyway, the most common reason to use Class AB P-P as opposed to Class A P-P is greater power, but I'm striving for the minimum power using a P-P output stage, which sort of dictates Class A rather than Class AB.

    I really need to go give Kevin O'Connor a bunch of money...
     

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