So what are some genuine Class A amplifiers?

[teemuk]

For the most part it [amplifier class] doesn't matter much imo.

This. The classes aren't in any order of superiority. They are in mere order of conduction angle and "A" happened to be first in alphabet.

So, all in all A isn't particularly any better than AB. Crossover distortion in linear operating region is inaudible in any reasonably designed amp and in overdrive class-A push-pull will have it regardless. Single-ended circuits naturally will not but then again they are principally different from push-pull B or AB circuits and react differently to overdrive to begin with. (So, that will be greater differentor than any amplifying class).

I don't know where rumours of Vox AC30 being a class-A amp came from, could have been The Beatles, some other guy or even -marketing- department of Jennings. Either way, the design is verifiably class-AB. Beatles also used wealth of other amps than AC30, such as UL series amps with solid-state front end and Fender amps. Whether you hear a class-A amp in action is least of your worries when identifying how Beatles got their tone.

 

[beans-on-toast]

Most people wouldn’t know Class A from a doughnut. So what does it matter.

Many early student amps were single ended Class A. People like the distortion of the guitar amps when pushed to the point of smoking because they run so hot and associate it with the term Class A.

Everything that Rupert Neve has done has Class A in it. He makes it a big selling point and the term helps with sales. Well that and his transformers.

 

[DJ Bebop]

Most people wouldn’t know Class A from a doughnut. So what does it matter.

 

[pfschim]

mmmmmm ... doughnuts

 

[Jeff Scott]

Most people wouldn’t know Class A from a doughnut.

Which of these is the Class A one?

 

[DJ Bebop]

 

[beans-on-toast]

Which of these is the Class A one?

View attachment 3970607

You can find doughnut making advanced classes (Class A) in culinary and baking schools. It’s a very complex process and recipes take years to master. Like Homer Simpson, you don’t need to understand what’s behind doughnut making in order to really like them. There are a lot to choose from, each with their own taste qualities. I don’t know if there’s a single answer for which doughnut is best. I suppose that’s why there are so many doughnut manufacturers.

Enjoy them, don’t overthink them.

 

[zombywoof5050]

Which of these is the Class A one?

View attachment 3970607

I would say none of those are.
These are Class A doughnuts (mmmmm, I'm getting hungry now):

 

[Heikki L.]

About ten years ago I was looking for a small tube amp for recording and practicing at home. I found an old Philips amp for sale locally and contacted the seller. The amp was already sold but the seller asked me what I was looking for in an amp. He told me that he builds amps for a hobby and could build me one with the features I was after. So I ended up ordering one. I wanted an amp which had small footprint, some eq and wouldn't be too powerful. I'm an ignoramus when it comes to anything with electronics, hence the short list.

The result was a single ended amp with hefty transformers, possibility to use different using octal base tubes (maybe 10-15 watts ??), pre amp with a 12ax7, eq with bypass, line out and a master volume bypass-switch. WHD 34x24x25 cm, weight ca. 14 kg (30 lbs. !!!).

Here are two samples (Youtube):

Clean

Distorted

The cab was Barefaced Compact (older model, using Kappalite 3015) IIRC. DI + the usual processing of course.

 

[Jeff Scott]

I would say none of those are.
These are Class A doughnuts (mmmmm, I'm getting hungry now):
View attachment 3972143

Those look like deep-fried bagels. Now, I'm hungry!

About ten years ago I was looking for a small tube amp for recording and practicing at home. I found an old Philips amp for sale locally and contacted the seller. The amp was already sold but the seller asked me what I was looking for in an amp. He told me that he builds amps for a hobby and could build me one with the features I was after. So I ended up ordering one. I wanted an amp which had small footprint, some eq and wouldn't be too powerful. I'm an ignoramus when it comes to anything with electronics, hence the short list.

The result was a single ended amp with hefty transformers, possibility to use different using octal base tubes (maybe 10-15 watts ??), pre amp with a 12ax7, eq with bypass, line out and a master volume bypass-switch. WHD 34x24x25 cm, weight ca. 14 kg (30 lbs. !!!).

Here are two samples (Youtube):

Clean

Distorted

The cab was Barefaced Compact (older model, using Kappalite 3015) IIRC. DI + the usual processing of course.

View attachment 3972282 View attachment 3972283

Nice! Good thing the amp's maker is Rikstone, not Risktone.

 

[74hc]

The result was a single ended amp with hefty transformers, possibility to use different using octal base tubes (maybe 10-15 watts ??), pre amp with a 12ax7, eq with bypass, line out and a master volume bypass-switch. WHD 34x24x25 cm, weight ca. 14 kg (30 lbs. !!!).

Likely the OT is hefty due to needing to get lower frequency response. The smaller ones are limited in that regard, and are usually used for guitar amps.

I suspect the preamp is based on the old Ampeg with a corresponding tone stack. You should be able to fiddle with the tone controls and get an idea of whether it is, or one commandingly found on Fenders.

Or, you can take the amp chassis out and post a pick of the internal wiring and someone here will come along and tell you which.

Some years ago there was a guy posting in one of the diy audio forums asking about designing an Ampeg preamp with a line out and DI out. Was developing the circuit and posting it. Same guy, or inspirational?

The power tube looks to be an Electro-Harmonix 6550 judging by the shape, the blurry internals, and the hint of their gold/beige logo.

 

[rickdog]

Likely the OT is hefty due to needing to get lower frequency response. The smaller ones are limited in that regard, and are usually used for guitar amps.

And it's a single-ended power amp. In a push-pull amp, the idle current for the two tubes is balanced between the two halves of the primary, producing no net magnetic field in the transformer core. In a single-ended amp, there's a field generated in the core by the idle current. This requires a larger core so that it won't saturate on peaks in the audio signal.

 

[agedhorse]

Yes, in a single ended tube power amp, the core of the OT must support the flux of the additional DC quiescent current.

 

[AlexanderB]

Hey, we agree!


Well, ok.

At $6k for a power amp, I think your argument is kinda-sorta absurd. Don't you think? I could be equally absurd comparing to a cheap linear amp with much the same results.

Here's a class D amp that's less than .008% THD (PLUS noise) for as high as it's measured (for bass applications, the second harmonic of 6.7kHz would be 13.4kHz which isn't important for normal players)
View attachment 3969281


And for dog whistle folks, here's an IM plot showing distortion with combined 18kHz and 19kHz, which is less than .005% at 1200 watts for any single harmonic (6dB -100dB = -94dB). Very low.
View attachment 3969283

I did not check the price, but it is not too similar from a pair of SVT Heritage amps (the Bryston has two channels), slightly more power, slightly more expensive, and probably as long lived as any SVT. There are other, less expensive, conventional amps with similar performance, but perhaps less powerful. (Peavey VX comes to mind.)
Could you please post the name of the class D amp of similar performance?

 

[agedhorse]

QSC, Crown, Crest are all in that ballpark measured at that power level.

I’m not going to convince you though, so why bother ?

 

[Jeff Scott]

Yeah, he's becoming a bit of a troll, isn't he?

 

[Heikki L.]

Likely the OT is hefty due to needing to get lower frequency response. The smaller ones are limited in that regard, and are usually used for guitar amps.

I suspect the preamp is based on the old Ampeg with a corresponding tone stack. You should be able to fiddle with the tone controls and get an idea of whether it is, or one commandingly found on Fenders.

Or, you can take the amp chassis out and post a pick of the internal wiring and someone here will come along and tell you which.

Some years ago there was a guy posting in one of the diy audio forums asking about designing an Ampeg preamp with a line out and DI out. Was developing the circuit and posting it. Same guy, or inspirational?

The power tube looks to be an Electro-Harmonix 6550 judging by the shape, the blurry internals, and the hint of their gold/beige logo.

IIRC, the builder said that the pre-amp was based on a Fender Champ. The power tube in the picture is Svetlana KT88. On this amp, power tube rolling makes quite a difference (IMO), especially when you change tubes between different types (6L6, EL34, KT88).