Just curious - EL34/84s

[Tube Guy]

Is there a simple reason why you don't get single ended, Class A amps using EL34/84s (or maybe I just haven't spotted them)?

Presumably the sound would be quite a bit different due to the lack of crossover distortion etc.

[aroman]

Isn't the Fargen MiniPlex a Cathode Biased Single ended Class A EL-34 Amp ?

[John Phillips]

Some EL84 single-endeds:

Vox AC4
Marshall Capri (and Mercury, but that has a solid-state preamp and is not thought highly of to say the least)
Selmer Truvoice (the little 'radio' one)
... and various other British budget amps by Elpico, Linear etc.

These were all small, cheap amps though and don't show off the 'tone' of the power stage very well - although the AC4 can be quite nice, and the Capri is not bad.

I don't know of any EL34 single-endeds over here, because there would (historically, anyway) have been no good reason to do it - you can get more power, more economically, from two EL84s in push-pull than an EL34 in single-ended.

The Mini-Plex sounds interesting...

[Scott Peterson]

The Matchless Clubman is a single ended cathode biased amp.

And it rocks. I love mine and have for over 5 years now.

[John Phillips]

Originally posted by Scott Peterson
The Matchless Clubman is a single ended cathode biased amp.

Is it an EL34 or 84? The only Clubman they list now is the Clubman 35, which is a 2-EL34 amp...

[Sam Evans]

Scott, I think the Clubman is push-pull.

The UniValve is single-ended.

Sam

[Scott Peterson]

Originally posted by Sam Evans
Scott, I think the Clubman is push-pull.

The UniValve is single-ended.

Sam

Hmmm. There is no bias adjustment on the Clubman and it sure doesn't sound like any push-pull amp I have ever heard. I could be wrong; perhaps I am.

And my amp is a Clubman 35 from 1996; I have no clue what Matchless is doing these days with the model. But it is a 2 EL34 model.

I was under the impression that the output tubes on the Clubman have no need to be matched at all because they are not in a push-pull setup. I was told that by Rick Seccomb of the original factory back in 1997.

I thought a single ended amp was where the output tubes do not work together.

Am I flat wrong here?

[John Phillips]

Yes.

Don't worry, that's not meant as a personal insult. There is a huge amount of confusion over these things (some deliberately introduced for marketing purposes).

In a single-ended amp, there is usually one power tube. This amplifies the entire waveform. It is possible to have two (or more) tubes also running in a single-ended configuration, but it is very rare - there is at least one old Gibson amp from the 50slike this. The tubes run in parallel, just as if there were two single-tube amps working side-by-side. It is not commonly used because it has the disadvantage of needing an OT which can handle the heavy DC current of two tubes and the large static magnetic field this generates without saturating. There really is no good reason to use it unless the idea is to blend two different tube sounds. I have no idea why Gibson did it.

Almost all two-tube amps are push-pull, which means that instead of being side-by-side in parallel, the tubes are coupled together through opposite ends of the output transformer winding and are fed out-of-phase signals (which is where the 'phase inverter' comes in) so that one is 'pushing' the waveform as the other is 'pulling'. This is much more efficient and does not require such a large and expensive OT since the 'static' magnetic component is zero (the two halves cancel out); it also has other advantages, including hum cancellation in the OT.

At 35W from two EL34s, the Clubman must be push-pull - or they're being a bit optimistic with the power rating. You simply can't develop that much power from those tubes in a parallel-single-ended application without exceeding the maximum tube ratings - 25W will be the limit.

However, you're not alone... I just read this on the Matchless website:

"Class A refers to a high performance style of circuitry used in vacuum tube audio devices to extract maximum efficiency from power tubes. A constant high voltage signal is sent through the power stage without the forced negative feedback (or "cool down cycle") inherent in the Class A/B design."

This is utterly and completely wrong. Class A is less efficient than Class AB - that's the whole point of Class AB. The presence or lack of negative feedback has absolutely nothing to do with the operating class - it can be used in Class A or AB amps - and is unrelated to the 'cooling' which is a bad way of saying that in Class AB the tube is shut off for a portion of the cycle. Equally cathode bias has nothing to do with the operating class either, which is also often confused (including by Matchless, none of their amps are actually Class A as far as I know).

The Phoenix I just tested was quite definitely Class AB - as any amp with only two EL34s delivering as much as 35W must be, since the power limit in Class A is 25W from a pair, the same as for a dual-single-ended.

The various factors (Class, bias arrangement, negative feedback) are independent and you can build an amp with any combination of them. The only certainty is that a single-ended amp is Class A - or even strictly, no 'Class' at all, since there is no other way of running such an amp. The Class definitions really only have meaning when applied to push-pull amps.

This is what is so stupid about the whole 'Class' issue - you have a push-pull, Class AB, cathode-biased, no-negative feedback amp that sounds great. It also sounds rather different from a Fender Champ, which is a true Class A (because it's single-ended - one 6V6), cathode-biased amp with negative feedback. And they sound great too...

[gitarzilla]

Carr HammerHead adn the Hot Cat are supposed to be class A and cathode biased.

[Tube Guy]

So if I've understood:

Single ended EL34s are rare because it's much cheaper to use 2 EL84s in push pull, presumably because of the expense of the OT.

Parallel single ended amps are rare because of the expensive OT and because the same power can be got for much less with push pull.

So are there any single ended EL84s?

And (naive question coming up), how different would the sound of parallel single-ended EL34s be from a push pull EL34 stage?

[John Phillips]

Originally posted by Tube Guy
So if I've understood:

Single ended EL34s are rare because it's much cheaper to use 2 EL84s in push pull, presumably because of the expense of the OT.

Mainly, and because EL34s are more than twice the price of EL84s (they may not look it in tube reseller's lists, but that includes the cost of testing, shipping etc - but in bulk (the way amp manufacturers usually buy them) they are, and certainly were in the past. They also require more expensive tube sockets (small cost, but significant if you're building a lot of amps).

Parallel single ended amps are rare because of the expensive OT and because the same power can be got for much less with push pull.

Yes, and because of the disadvantage of no hum cancellation in the OT which requires more filtering. The only extra cost disadvantage push-pull has is the need for a phase-inverter stage.

So are there any single ended EL84s?

Yes, the majority of the small British amps (all the ones I mentioned earlier and many more) are single-ended one-EL84.

And (naive question coming up), how different would the sound of parallel single-ended EL34s be from a push pull EL34 stage?

From a true Class A push-pull, probably not very different - except for having more hum. There may be a difference in the overdriven sound caused by the different transformer - the way it responds to sharp transients etc may be significantly different.

[Tube Guy]

Thanks John (and sorry I missed the EL84 heading in your earlier thread).

So, without wishing to restart the AC30 debate, are there many Class A push pull amps?

[John Phillips]

Almost none that I know. I'm not familiar with most of the 'boutique' US-made amps that are very popular around here , but from some data that was supplied by an owner, I'm pretty sure that Alessandro's amps which claim to be Class A really are. He does run the tubes very hot to achieve this though.

It's often a matter of biasing when it's a borderline case though - I was working on an old Selmer Zodiac 30 not long ago, and found that it was just possible to get it into Class A without harming the tubes - Mullard EL34s in particular seem a little tougher than the design spec, and I could get true Class A operation with tube dissipations of about 28W - a little hot, but no visible plate glow at that, and a clean output of 25W RMS in Class A. I don't know whether it was designed like that - the original factory bias was actually hotter still, with the tubes running at over 30W and noticeable plate glow. I could achieve very nearly a clean 30W output by biasing it a bit colder (tube rectifier so the B+ goes up as the current draw goes down, raising the point of clipping) - but that put the B+ voltage above the maximum voltage rating of the filter caps (hazardous) so I went for the compromise of running the tubes a little hot.

[Tube Guy]

I remember playing through a Zodiac I think. If I'm right it had snake skin tolex and radio push buttons.

This was a long time ago when I also had a JTM or Plexi Marshall 50W and the other guitarist had a 30W Marshall head, whatever that was. We also dabbled with an AC50 but I remember hating the sound of that!

[teleharmonium]

There is a version of the Trace Elliot Velocette which is a single ended EL34 amp , it is called the Velocette SE, I haven't seen very many around though and have not heard one. It uses the same cab and speaker as the single 10" version.