So what are some genuine Class A amplifiers?

[Bander]

I did a few searches and read a few threads, but really didn't see any product listed as a full up class A design? I read that tube pre-amps tend to be class A, and that most big iron designs are class AB. And then I read that class A can be very inefficient but still rewarding. Still have no examples, models or makers to compare? I even did a Bing search or two, but its all about "class D", couldn't find or identify any class A bass amps? I'd like to know of two or three examples?

 

[Bob_Ross]

I'm not aware (off the top of my head) of any dedicated bass amps that are true Class A. I know some of the MESA|Boogie guitar amps are (e.g., their Lone Star Special), especially when run in their low-wattage mode. Most of the Class A amps I've seen came from the hi-fi world.

 

[Jeff Scott]

I did a few searches and read a few threads, but really didn't see any product listed as a full up class A design? I read that tube pre-amps tend to be class A, and that most big iron designs are class AB. And then I read that class A can be very inefficient but still rewarding. Still have no examples, models or makers to compare? I even did a Bing search or two, but its all about "class D", couldn't find or identify any class A bass amps? I'd like to know of two or three examples?

Beyond preamps, you won't find many (if any) Class A bass guitar amps.

BTW1: tube preamps aren't the only ones that are Class A.

BTW2: a whole lot of tube guitars amps that claim to be Class A, are not; sadly, there is a lot of marketing hype in that sector.

 

[bobyoung53]

Beyond preamps, you won't find many (if any) Class A bass guitar amps.

BTW1: tube preamps aren't the only ones that are Class A.

BTW2: a whole lot of tube guitars amps that claim to be Class A, are not; sadly, there is a lot of marketing hype in that sector.

Guitar amp but isn't a Vox AC-30 a class A amp?

 

[agedhorse]

I don't know of any class a bass power amps.

The AC-30 only operates in class A mode up to about 10-15 watts then transitions into class AB. What makes this amp special is the combination of cathode bias, EL84 output tubes, the phase inverter and how the global feedback is configured. (IMO)

 

[LastJaguar]

Try a Krell KSA 250. Out of production for a decade or two, so it will need to be re-capped. Stereophile measurements:

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[JKos]

There are certainly single-ended guitar amps. But the wattage is usually rather low, say 5 W. Any tube amp with a single output tube is class A.

Epiphone Valve Jr is an example.

 

[agedhorse]

Try a Krell KSA 250. Out of production for a decade or two, so it will need to be re-capped. Stereophile measurements:

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It's a class AB amp biased with higher than average quiescent current which results in class A like operation into 8 ohms of a bit over 20 watts. Into 4 ohms, class A like operation transitions to class AB at a little over 10 watts.

I can't see how an amp like that would need new caps after 10 or 20 years, unless urban mythology and old wives tales drive that decision.

 

[JKos]

Is there any single ended class 'A' bass tube amps?

 

[Wasnex]

I don't know of any class a bass power amps.

The AC-30 only operates in class A mode up to about 10-15 watts then transitions into class AB. What makes this amp special is the combination of cathode bias, EL84 output tubes, the phase inverter and how the global feedback is configured. (IMO)

AFAIK the AC30 does not have global feedback. What am I missing?


I have a Matchless Thunderchief which was also touted as class A. Four KT88s and claimed to be 200W, with fixed bias. Pretty sure it's closer to 100W and definitely not class A because it pulls more current on my variac when I send a signal through it. No global feedback like the AC30, so it transitions very gently into OD. Nice sounding amp, but a bit quirky.

 

[Jeff Scott]

Try a Krell KSA 250. Out of production for a decade or two, so it will need to be re-capped. Stereophile measurements:

Invalid Link Removed

As Andy said, above, the Krell is not truly a fully Class A amp, as a nice an amp as it is. I own some poor-man's Krell gear, original Aragon 24k preamp and 2004 power amp that Dan designed.

However, if you ever stumble on what I am about to describe. There are only four of these, and I am not sure all four were completely built, I know of two that were and had been bench tested and listened through. Back in the late 1980s (remember the '80s?) my brother and a friend built up four prototype mono power amps that were true class A amps that could do 100 watts RMS @ 8Ω. They were pretty massive but very simply made, with a space frame of around 3/8" aluminum bar stock with a flat aluminum front panel on the front , the three other sides/back were nothing but massive heat sinks except for a small rear panel for audio in/out/AC connections. They were all black powder coated, and fairly heavy, IIRC. I have not seen any of them since around 1989 or thereabouts. I have no idea who had/has them, but I believe a pair of them were sold to another friend of my brother's back in the day.

Now, go find one (or more). The only hint I can give you is they are probably still in California. They were built in a garage in Oxnard that had a couple of PIE 4000 watt power amps living up in the rafters of.

 

[agedhorse]

AFAIK the AC30 does not have global feedback. What am I missing?

I may not be remembering this correctly then. It's been quite a few years since I worked with that circuit, I though I remembered a small amount of global feedback but maybe not. My point was that any global feedback was very low (or nonexistent).

 

[LastJaguar]

Ok, how about this one:

Mephisto Solo - The Gryphon

 

[abarson]

I'd venture to say that if you're not finding any examples its for good reason. Amp designers need to design amps that sell. If no one wants an inefficient heat sink that makes a bit of noise, then no one will design and build one.

 

[Spupilup]

I don’t know if Gordon Rankin, owner/creator of Wavelength Audio high end home audio gear is still making his “Guitar Engines” or not but a couple of his variations were single ended Class A designs.

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I had a pair of 300B mono amps 20 years ago and they were one of the most musical home audio amps I ever owned (I was an insane audiophile for most of my life going back to the ‘70’s). I know that doesn’t necessarily translate into musical instrument amplification but Gordon really has a talent with tubes and is a great guitarist. I also love that he took one of his amps to Swindon for Andy Partridge of the band XTC to audition.

 

[LastJaguar]

Or this one:

http://www.accuphase.com/technical_information/a-250_technical_information.pdf

 

[Bander]

That answers a lot of my questions. I'm reading thru Blair Jackson's book on the Dead's evolution of sound systems and gear. Just sorting out some of the differences in instrument, studio and live sound amplification, then and now, as well as recorded music amplification that consumes audiophiles. I got curious about the mystical class A guitar amplifier, and wondered if there was a bass equivalent.

 

[AlexanderB]

Well implemeted class A/B amplifiers will for use with bass guitar probably behave very, very similarly.
The very small amount of transient driven crossover distortion would probably be "masked" by the somewhat "chaotic" transients from a bass guitar. That especially on stage, where higher power is needed. I think Yamaha 2200/2201 is a good example of an amp that has very low distortion (typically 0.005%, medium power mid band and 0,01% THD, full continuous power, mid band). And that is a design that is about 40 years old.

While I have not seen class D amps with such impressive numbers, but modern class A/B, or H amps might do it too.

 

[ardgedee]

Nelson Pass has built and published plans for many different Class-A transistor amps for home hi-fi. I got to listen to a 30 watt per channel Pass Labs Aleph 3 once and probably the strongest impression it left on me is it was it was the only audio equipment I'd tried which might literally be able to cook an egg and boil water; it was a lot hotter than what I'm accustomed to considering as "hot" in home or pro amplification. It was explained to me once that Class-A basically means running at full power all the time, anything not used for signal is lost as heat. Crazy inefficient. Unless you're hungry.

 

[brianrost]

Hughes and Kettner made one that was sold as a preamp, but it was a class A amp with a speaker output. 5W at best.

Was the SWR Interstellar Overdrive class A? That was another unit sold as a preamp but had a speaker output.

Both were designed to capture the power stage distortion as well as any preamp distortion, so they had low wattage power stages driving a dummy load.

 

[Jazz Ad]

The H&K Bassmaster is a very potent bass amp given its 5 watts.

 

[beans-on-toast]

There are Class A tube and transistorized amplifiers. With an efficiency in the 25-30% range, they give off a lot of energy as heat. I’m surprised that California hasn’t banned them.

The best Class A power amps that I’ve heard are low power, 300B based. The hi-fi world loves Class A.

Class A is also used in some transmitting equipment, VHF is one example.