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Vintage threads from the first ten years

Class "a" amps

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9/23/2000 2:18 AM
Jim Price	Class "a" amps A snide Hi Fier questioned me on calling the Matchless Chieftain a quote"CLASS A amp. I`m sure its not "PURE" Class A but darned close. Can someone enlighten me, or my snide and uppity hi fi friend?? Thanks, jim

9/23/2000 10:01 AM

Jim:

There have been discussions on this here and what I gather is that with the traditional Vox AC-30 output section, it runs in Class A at lower volumes but when you crank it up, it is more like a hot Class AB. So I just call it "quasi-Class A" to avoid arguments with the audiophiles...

Steve Ahola

P.S. Mesa markets some of their amps as being "Pure Class A"... Are they doing something different or is it just marketing hype? BTW do we really want "Hi-Fi grade" Class A operation in a guitar amp? Seems like those amps would be too clean for the tastes of many guitarists... at least if you are looking for power tube distortion.

9/23/2000 7:26 PM
Lee M.	For a good technical explanation of class A, go to www.aikenamps.com. Lee

9/24/2000 7:50 PM
Paul Markwalter	For a simple explanation of "Pure" class A vs "Limited" class A go to: <http://www.soflotubeamps.com/soflo/smartale.htm> Paul Markwalter

9/24/2000 8:06 PM
tommy	That's a cool site. I have never heard that even order distortion is not cancelled out in a true class A push-pull output stage, as the above site claims. I'm gonna have to think on that a while. Anyone got a quick explanation? I thought that it was the nature of the 180% phase inversion that cancelled even-order harmonics, not so much the operating points of the tubes. Chris

9/24/2000 9:32 PM

Chris,

That Class A 'explanation' on the site Paul posted is very confusing; it's actually a mixture of Class A, Class AB, and push-pull, with a little cathode-bias thrown in as well. You're right about the operating point being unrelated to the harmonic cancellation (at least until you get close to Class B), which occurs in the output transformer. Go to www.aikenamps.com for the straight skinny.

Ray Ivers

9/24/2000 10:09 PM
Paul Markwalter	Pure class A can only be derived from a single ended output. Any class A derived from a P/P output can and will drift into A/B,or B 'cus it always will be P/P. Single ended can never be P/P.

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