Any *real* Class A push-pull amps out there?

Discussion in 'Amps and Cabs' started by amper, Feb 25, 2006.


  1. amper

    amper Member

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    So, does anyone actually make any push-pull amps that are truly Class A?

    I've been thinking about building one myself, but I'm having trouble finding a suitable power transformer. I'm using the AX84 P1 High Octane schematics as a basis, and I'd like to do a push-pull Class A, variable cathode bias octal output stage. I'm thinking that a plate voltage of around 250V should do the trick for the output tubes, so I guess I need a PT with around 190-0-190 secondaries that can put out enough current to handle two octal output tubes and 4-6 noval preamp tubes (2-4 for the preamp, and 1 each for FX loop and reverb drivers). Any suggestions?

    In any case, I'd like to hear a real Class A push-pull, so I know whether or not to even bother.

    I played a Carr Rambler recently, and at 28W in pentode mode, I think it may be true Class A P-P. Nice amp...it was a used one at a local GC for $1500. Too bad I don't have that kind of dough lying around. Not quite the feature set I'm looking for, though.
     
  2. scottcw

    scottcw Low rent hobbyist Silver Supporting Member

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    Yes, several of the Alessandro amps - Beagle, English, Plott, Black 'n Tan come to mind.
     
  3. amper

    amper Member

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    Hmm...I'd forgotten about Alessandro. Funny, considering he's a local. But, I was thinking something more along the lines of an amp that would stand up to gigging abuse rather than a fine furniture type of amp.
     
  4. cnardone

    cnardone Supporting Member

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    Just because the cabinet is pretty doesn't mean that the amp won't stand up to gigging. George builds a durable amp. They also sound great.
     
  5. amper

    amper Member

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    No, no...I meant *I* would ruin the cabinet at a gig. If I even so much as scratched the finish of one of George's amps, I'd be depressed for weeks...:jo
     
  6. AlexF

    AlexF Member

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    Doug Sewell is making a version of his Wampus Cat which apparently is a long way in the direction of pure Class A, low voltage high current. He is very pleased with the results.
    Al
     
  7. v-verb

    v-verb Supporting Member

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    George make the black poplar cabs - no worries abou t scratching them - just get out the Sharpie!
     
  8. riffmeister

    riffmeister Gold Supporting Member

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    there are several multi-power tube true Class A (ie, single-ended) amps available.....the THD BiValve comes to mind.....
     
  9. amper

    amper Member

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    I've already got a Univalve, so I don't see any need for a Bivalve. The idea isn't to find single-ended Class A amps, it's to find push-pull Class A amps.
     
  10. Dave Orban

    Dave Orban Gold Supporting Member

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    +1000.
     
  11. number9

    number9 Member

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    I would bet Allesandro has some hard finish on there. Most of my new amps are
    "similar" in that I use real wood and coat them with one of a few finishes depending on
    if you are gigging or not. The ones I coat for "gigging use" I hve thrown on the ground,
    thrown down stairs and thrown off the roof onto the pavement... no worries.

    I would venture to say Allesandros amps are at least as durable as that, from what
    I have read and seen.

    On the class-A side, a few people make "true" class-A amps, Aleesandro, Aiken I think,
    I make one 19.5Watter myself (two 6L6's)... from reading your first post, I think you
    are looking at lowering the plate voltage to "go class-A", please note that you do not just
    lower the plate voltage to go class-A, that is not the criteria, but a side effect. You can
    cut the B+ in 1/2 or more and not be class-A, it also depends on the load, where you
    are biased, the grid input, etc, etc... ultimatly the only way to know you are in class-A
    is to check the output at the tube agains the input on the grid, or do the load line calculations
    for the tube(s) in question...

    I just wanted to point that out, as there is a big misconception about class-A operation.
    I suggest asking someone reputable (since no one belives anything I say :) such as
    Aiken, Allesandro, the guy at wavelength audio (whose name escapes me, oh, he makes
    class-A amps too), etc.

    Hope that helps some.
     
  12. Dave Orban

    Dave Orban Gold Supporting Member

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    My Beagle is in one of those Poplar cabs, with a typical amp handle on the top. I don't even bother to use the gig bag for that one. It is beat to sh*t, (a relic??? :eek: ) but it always works and it sounds like a million bucks!
     
  13. riffmeister

    riffmeister Gold Supporting Member

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    why? what's the advantage?
     
  14. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    With no offense intended...why do you care whether it's truly class A or just cathode-biased (like most so-called class A models) or class a/b or whatever?

    I've had true class A push-pull amps from Allesandro and Aiken and cathode-biased amps from Vox, Bad Cat, Matchless and others. There are a few generalizations I could make about all of them (e.g. nice rich, spakrly clean sound; great mid-level grindage) but none that I would make exclusively about the truly class A amps (and not the non-class A).

    I.e., why does operating class matter? I would think it would be tonal/behavioral attributes that one is interested in...not operating class. The operating class is just an engineering term (which some have apparently successfully turned into marketing blather)....

    :confused:
     
  15. number9

    number9 Member

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    I would think "mo power" unless we are talking about an 845 running SE at
    30 to 50 watts, but then you need over a kilovolt on the palte, and most DIY amp
    builders are not comfortable with that. Of course, if you could have an open
    chassis, there are not too many tubes that look as good as the 845 when on:

    http://www.cognitivevent.com/av_monstro.html

    Note you can get 40W easily out of a pair of 6550's in class-A... you can also
    (IIRC) read up on converting a vox AC30 rear end to true class A operation on
    Ailkens site under one of his information pages.
     
  16. amper

    amper Member

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    I wouldn't call myself an electrical engineer by any means (the closest affinity I can claim is a General class Amateur Radio license), but even I know that plate voltage is a fairly *major* factor affecting what output class you can use. There is a maximum plate voltage for every tube type beyond which you simply *cannot* run in Class A (BTW, Aiken's page on Class A is a good primer on this factoid). I wouldn't call a "low" plate voltage a "side effect".

    I've settled on 250V mainly because many of the tube data sheets show Class A characteristics at that voltage, so I figure it's a good starting point. For instance the GE 6L6GC data sheet gives values for Class A operation at both 250V and 270V, plus we know from Aiken's page that the EL84 can be operated in Class A in the same range. The Philips EL34 data sheet gives values for Class A at 265V.
     
  17. amper

    amper Member

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    Only because, as far as this discussion is concerned, I'd like to hear a push-pull amp that's really Class A.

    My ultimate goal is to build such an amp, because what I'd like to get is the sound of larger bottles in a push-pull topology at a relatively low SPL without attenuation. Of course, running in Class A as opposed to Class AB is going to change a few things.

    Anyway, I'd like to know if there are any amps like this out there so I can hunt them down and hear them in person.
     
  18. number9

    number9 Member

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    Hrmm, I am sorry I have poor verbage... I have a nasty habit of assuming people know
    what I am trying to get at... let me rephrase:

    If you have a class AB amp (AB1 or AB2), simply dropping the plate voltage by X volts
    will not ensure class-A operation. To ensure class-A operation, by whichever method you
    choose or see fit, you must check that for an entire 360degree input waveform which is
    unclipped and maximal, the output at the plate of the class-A output device
    (pair, SE, P-PP, PF-PP, etc, etc) is on for the entire 360degrees and unclipped.

    As an aside (or a note, I suppose, verbage getting in the way again) that is not my
    own personal definition, but a paraphrase of the definition by F. Langford-Smith from the
    Radiotron Designers Handbook, fourth Ed, 1953 pp 544-583... any and all complaints with the
    above definition of class-A operation can be directed at RCA and/or F. Langford-Smith.
     
  19. riffmeister

    riffmeister Gold Supporting Member

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    can't you just bias a push-pull Class A/B amp on the hot side and get into Class A operation?
     
  20. amper

    amper Member

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    You *could*, theoretically...but you'd more than likely blow your output tubes by exceeding the max plate dissipation, among other problems.
     

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