Class A Deluxe ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ambient1, Oct 25, 2004.


  1. ambient1

    ambient1 Guest

    Hi,

    I have recently started the search for a new amp to replace a reissue Twin. An Australian amp maker does some versions of classic circuits.

    Apparently he can do the amps either Class AB or A.
    What would be the effect of a class a output stage on a deluxe style amp.


    Thanks
     
  2. abergdahl

    abergdahl Member

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    I strongly doubt that it st either class A or AB. I have a Tweed Deluxe style amp tha have variable negative feedback. When some makers say Class A they MEAN Cathode bias and no negative feedback (ala Vox AC 30).
     
  3. ambient1

    ambient1 Guest

    Hi guys,
    Thanks for the replys. I do have a basic understanding about what is class A and what is not. However being stuck in Australia I haven't had the chance to hear many amps that are "class A" apart from bad cats.
     
  4. Scumback Speakers

    Scumback Speakers Gold Supporting Member

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    Class A amps (real Class A, that is) are not made in plentiful numbers. Usually they require more expensive parts & design and have a sweeter, fuller, rounder tone than a Class AB amp, such as a Marshall, Fender, etc.

    Of course, I'm biased, I make one. :D

    But there are others who make them, too.

    Mine is "Single Ended Class A", which means the power tubes are on all the time (360 degrees of the cycle), while other designs vary. Push/pull amps (Class AB for example) have their tubes on 50% or slightly more so that they don't run the power tubes as long as Class A.

    Good morning John... I see your Class A "hot button" got pushed this morning! LOL
     
  5. drbob1

    drbob1 Silver Supporting Member

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    One convenient way to tell if an amp is pure class A is look at the power rating. A class A 6V6 runs a maximum of about 7 watts, so if it's more than a 14w amp it can't be class A. I believe Alessandros stuff might be pretty close, he's getting 30w off of a pair of 6550s in some of those designs.

    I agree that the "Class A" Deluxe is probably meaning cathode biased, no negative feedback (like a Vox/Matchless/Badcat) which would tend to make it run hotter, have slightly lower power and perhaps more harmonic richness...
     
  6. Scumback Speakers

    Scumback Speakers Gold Supporting Member

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    drbob1, not going to argue your data sources, but possibly a single 6v6 runs at 7 watts (probably less than that with a clean signal). The Scumbag design is dual 6V6 single ended Class A amp. At 2 it puts out 10.8 watts with a totally clean signal, no distortion, at 8 ohms on an oscilloscope. I haven't measured a Scumbag above 5 on the volume knob (which goes to 11, by the way), but it measured out at 14.9 watts at 5 with an 8 ohm speaker load.

    The Fender Champ specs (according to a web page at Victoria amps I cut and paste this from) were:
    "The original Fender Champ was termed a "student" amp in the Fender literature, with around 8W of power and an 8" Jensen AlNiCo speaker appearing in the late '50s tweed models. Eric Clapton used a tweed Champ on the "Layla" sessions, and they are fun little amps with a lot all of raucous Class A muscle that belies their size."

    Doubling the power tubes used doesn't automatically double the power of a single ended Class A amp (found that out, too), but it gets close (at least from my amp designer's oscilloscope readings). My amp designer guru also says that after you hit "clipping" of the signal that the Class will change into a Class A "hybrid", as the waveform gets clipped at higher volumes (I don't pretend to know about all of that, though!).

    There's alot of Class A hype going around, from what I've read, heard, and so forth. Hopefully I've dispelled SOME of it. As for what other amp designers designate as Class A... well, there's lots of variables...obviously.
     
  7. ambient1

    ambient1 Guest

    Thanks for the replys - this seems to be a bit of a sore point out there. Its funny because Bad Cat wont let you forget they think their amps are class A in the product literature over here!?
     
  8. drbob1

    drbob1 Silver Supporting Member

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    Those numbers are a bit unusual. Mostly what I've seen quoted for the tweed Champ was 5w, perhaps as much as 7 for a BF Champ. The tube data is available lots of places, including the RCA receiving tube manual I have and on the net, but it is the rating for the tube at maximum clean volume (actually usually where the THD=10% or so), so I'd be very comfortable with a Scumbag amp putting out 14w clean, beyond that I think you'd get into clipping. You have to be pretty agressive with your biasing and input voltages to stay in class A once you start to clip. If the tube cuts off for part of the duty cycle (input voltage becomes high enough to prevent any current flow thru the tube) you've automatically moved into class AB. There is data for push/pull class A and it seems to me it's right around double the single ended numbers.
     

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