• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

So what are some genuine Class A amplifiers?

#42
Jeff Scott said:
The [Interstellar Overdrive] preamp section is Class A, the tiny power amp section is not, it's Class A/B
Click to expand...

I had an interesting discussion with @Wasnex in SWR Interstellar OD - Retubed a while ago. He talked me around to agreeing that it's Class AB... but I think it's still a little ambiguous. There's no global feedback in the power amp, which is usually required in Class AB (to force one tube to work harder when the other one is cut off, thus minimizing distortion). The lack of feedback is why I originally said Class A. But the whole intent is to overdrive that little power amp and get some characteristic tube distortion, and that will probably drive the power tubes into cutoff - thus Class AB.
 
#43
Global feedback doesn't define operating class, conduction angle does.

The bias voltage is -15V, trimmed by R25, the plate voltage is 250V, without looking at the plate curves it looks possible to be biased pretty hot but I doubt that it's class A intentionally, or that it would sound better that way. The balance between the tubes is controlled by the matching of the tubes themselves, a little mismatch won't be the end of the world.
 
  • Like
Reactions: dkelley
#47
solodb said:
Is the Fender Champ 12 class A ? It has one 6l6 output tube and 2 12ax7 preamp tubes. It sounds pretty good for bass at low practicing levels and has a line level out and a headphone jack which can mute the 12" guitar focused speaker. I have been using it as a powered pre-amp sort of ?
Click to expand...

I'm pretty sure it's not, but damn if that isn't one of the best sounding guitar amps made by anybody. Cherish it. They're hard to find. Some guitar player will offer you a lot of money for it one day.
 
#49
arbiterusa said:
I'm pretty sure it's not, but damn if that isn't one of the best sounding guitar amps made by anybody. Cherish it. They're hard to find. Some guitar player will offer you a lot of money for it one day.
Click to expand...
A single tube must conduct over the entire 360 degrees, therefore must be class A
 
  • Like
Reactions: BassmanPaul, dkelley and JKos
#52
agedhorse said:
Did you happen to notice the weight and the quiescent power draw? It's not going to be operating in class A mode above say 300 watts into lower impedance loads either but MAY operate in class A up to rated power into 8 ohms.

This amp uses 2 x C19 AC power inlets on different branch circuits!!!
Click to expand...

First, I am posting as a music lover with a passing familiarity with some audiophile equipment. I have no formal education in electrical engineering and minimal informal understanding from reading posts such as yours--so, thank you. 2nd, what about the previously linked Gryphon Mephisto Solo Mephisto Solo - The Gryphon or the previously unmentioned Block Audio MONO BLOCK SE – Block Audio ? Block states that the Class A output is up to 200 watts/monoblock into 8 ohms (250 watts Class AB, so pretty close). I can't find it, but I think that Classe Audio (Class A--it's in their name) old DR2, DR3, and DR3 VHC (Very High Current) were low power pure Class A as was the Mark Levinson ML-2--they simply maxed out at their Class A rating without any Class AB additional power. They kept doubling output power with halving of the speaker impedance down to below 1 ohm. It's been said that Nelson Pass Class A amps (the XA series) are actually Class AB with extraordinarily high Class A bias. I guess it depends on how much power you use--likely a lot less than most people think except for dynamic peaks on inefficient speakers in very large rooms.
 
#53
I doubt you would find a class A, or want a class A, bass amp.

Usually it's something desirable by guitarists, because they are after certain distortion characteristics. And even many guitar amps advertised as class A actually are not. Additionally, I don't think I've ever seen a class A guitar amp rated at more than 30 watts. That amp struck me as having very high power for a class A amp, given that most I've seen are rated at 15 watts or fewer.
 
#54
From the quiescent power spec, calculate backwards quiescent output stage current for rail voltages that match rated 8 ohm power.

There is not enough quiescent output stage current to support true class A rated power at lower impedance except at lower power levels.

Doubling the output power with each halving of impedance is unrelated to class A. It's a function of stiffness of the power supply, output impedance of the amplifier and how rated power is defined.

Nelson has used novel bias schemes to extend class A operation, this was some of what made Theeshold unique back in the day. Iirc, Pioneer acquired this technology many years ago.
 
  • Like
Reactions: b/o 402
#55
I once played through a Vox AC50 that was class A. Well, kinda. I’d done some work on it, and didn’t connect the coupling cap to one of the power tubes. So, it was class A up until it distorted, which it did early and asymmetrically. I got compliments on my “angry” sound - it happened to work well for the music we played that evening.
 
  • Like
Reactions: bobyoung53
#57
There really only is one that I know that is a real class A bass amp. The BassBaby by Emery Sound. He used to have info on his website and I haven't heard one. Buddy sold it before I had a chance to visit him.

Link Removed

I have used my BiValve and UniValve by THD for bass but those amps are guitar amps per se and only usable for recording or low volume gigs with a super efficient speaker. They both go 2 ohms and can drive 2 4ohm EA Wizzy cabs
 
  • Like
Reactions: watt
#58
Vox amps are not true class A. Class A means low wattage 1/2 to 1/3 of AB. Frenzel amps made a mono block replica of the Craftsman C50 and they only output 30 watts using 2 TWO 6550 which is less than 1/3rd of the SVT output. The THD outputs 11 watts at most for a 6L6.
 
Last edited:

Latest posts

Top Bottom
Back