Matchless Chieftain

[Jazz P Bass]

Ah, but the Champ is single ended.

A piece of cake compared to designing a true Class A push/ pull circuit.

Aiken has the whole Class A thing covered pretty good.

The Last Word on Class A

[Leo_Gnardo]

So, at least the cathode voltage can be subtracted from the gross plate voltage. So, 510ish volts plate to cathode? Now less than 29W total per tube and the screens will be dissipating some of that. I'd guess that on the bench the actual bias is a proper class A. Still, what I'd like to know is what current production EL34's can be run like that without a light show?! I honestly don't think I'd use an amp that I had to feed a quad of NOS EL34's once or twice a year.

Didn't Matchless spell it "Cheiftain"? Anyway, in the few I've seen, I offered to bump up the cathode R value "or else be prepared to replace your output tubes early and often." Different answers depending on the owner's budget. Rich guys who want bragging rights can roast a pair of (real) Mullards, Amperexes or Telefunkens twice a year, small price to pay for that ultimate tone, seems only they can hear the difference. Same for Matchless' smaller amps with EL84 outputs. Tube toasters.

[Chuck H]

Same for Matchless' smaller amps with EL84 outputs. Tube toasters.

That's becoming an issue too! re: EL84's I use to say "Smoke 'em if you got 'em." They're not that expensive to replace, right? I'm changing my tune on that one though. Not because of electronic operating conditions though. Because of the physical. Good sounding EL84's still aren't expensive, but rugged ones are. In combo amps all the modern EL84's shake apart terribly. So badly anymore that it IS expensive to keep feeding them either the frail modern tubes often or the more rugged, older tubes less often. Dr. Z is introducing more models that use 6V6's instead and I've read that their EL84 amps are now shipped with those vibration dampers. Premature tube microphony on EL84 amps has become a common topic on the Dr. Z forums.

[Leo_Gnardo]

EL84's I use to say "Smoke 'em if you got 'em." They're not that expensive to replace, right? I'm changing my tune on that one though.

It's when failing output tubes wreck an output transformer or cause components on a PC board to go up in smoke, collateral damage, that's what gets to me. Most of my crustomers get a year or more out of a pair of EL84 in Blues Jr's & similar. I say more damage occurs off stage than on, amps getting rattled around in car trunks & thrown about like bowling balls. I have offered to install 6V6's instead of '84s but so far no takers. Hm, if I charged Dr Z money, think some amp owners would sign on? I work too cheap, not impressive, the big money guys get the gigs.

[cyclone]

Whoa! 29W each, is that what you still go with?

No, not with that amp or my Dads Eico HF-89 which is supposed to be biased so hot that the Mullard EL34's glow dull red similar plate voltages at that current.Run both at around 45 ma or so, plate voltages are higher but good old Sovtek el34's take a beating and the high voltages a real el34 is supposed to.

I know Chieftains are cathode biased but Superchiefs are fixed bias. other than that, they are a basically Champ with funky tone controls and a massive output section with no nfb. And a crappy afterthought effects loop. They sound glorious, like the chieftain but ever so much more so.

I was the tech at a big east coast matchless dealer in the early 90's. The output tubes did not hold up, actual working musicians would put NOS usually groove tubes in them immediately. I've got sylvania 6ca7's in mine rt now, they hold up as do the cheap Sovtek el34's from that era. The matchless branded output tubes generally did not. They were placeholders, really. They "sounded" fine

ditto on Juans comment on them being highly cab dependent. The Matchless cab for my Superchief was gigantic, had a 15 and a 12 I think, I passed on it. I always like old celestions with those amps. I gigged that SC 120 through a single 1965b straight cab with 4x10 35 watt celestions for several years.Best sound I ever had.

[The Dude]

This is bound to open up a can of worms, but here goes.

IMO, I don't hear a vast difference, after a certain realistic/normal point of dissipation, as far as a significant change in sound- certainly not enough to justify red plating or running the piss out of tubes. In fact, I'll go a step further and say that, if there is a difference, it's not a good one. Low end gets mushy, the amp gets noisier, etc. The idea of running the hell out of EL84's drives me bonkers. I would think reliability would be much more important to any gigging musician.

[Chuck H]

My favorite personal design runs a pair or a quad of EL84's at 355V plates, 330 screens cathode biased at about 12W for the whole tube. So probably about 90% plate dissipation. Just to qualify this, I did try several different bias points and finally settled with this hotter bias because I liked the way it sounded. Keep in mind that cathode bias cools as current increases and is somewhat self regulating. I've noticed since that this seems to be a popular bias point for cathode biased EL84 amps. Not so with fixed bias. Which brings me to the next piece of info on this design. I use a zener across the cathode resistor to "fix" the bias at the point where the wave form just starts to flatten. So this would be a cooler fixed bias point. And the place where, again, I think the amp sounds best. Most of what I've read confirms this, and your position on the matter with players biasing their fixed bias EL84 amps around 70%. All this, but...

EL84's are VERY versatile tubes (other than the fact that most rattle apart too fast). You can run them at lower voltage and higher current for a small. chimey and saturated tone or you can run them at higher voltage and lower current for an astoundingly large sound from small bottles. With lots of options in between. They're easy to drive, generate nice sounding intermodulations when distorting and don't cost too much. A really neat tube overall. Too bad they tend to be frail in combos.

[Dave H]

I only meant "proper class A" in the gross sense that the tubes are idling at about 100% dissipation and not actually over that amount.

I see what you have done there

"Class A amps run their tubes at 100% dissipation therefore an amp running its tubes at 100% is class A"

It doesn't follow like that. Class A amps (usually) run their tubes at 100% for maximum power output but you could design a class A amp with its tubes running at much less than 100%, or you could design a class AB amp to run its tubes at 100% or more like the Superchief.

[Chuck H]

It doesn't follow like that. Class A amps (usually) run their tubes at 100% for maximum power output but you could design a class A amp with its tubes running at much less than 100%, or you could design a class AB amp to run its tubes at 100% or more like the Superchief.

Yup. I know. I get that. The term class A is often misapplied. Class A, AFAIK, means a tube conducts the full signal being fed to it's grid. This is partly why class A push pull confuses me conceptually. Someone here tried to explain it to me once, but it came off sounding more like parallel operation than "push pull" to me.

There are design aspects that follow to make class A (single ended) more ideally functional like transformer design, running the power tubes at some lower voltage (*but see note) AND running the tubes at or near 100% dissipation. But no, you don't have to. Though this is virtually always the case WRT guitar amps, as you say, for maximum power.

*This lower voltage is often low enough to limit a tubes actual dissipation a little. Though a tube may be biased at or near 100% WRT current at idle. It may be technically more accurate to say that for class A, a tube is ideally biased to 100% of it's potential as dictated by the operating conditions.

Further, it's become common (and incorrect and confusing) in the guitar amp arena to call any EL84 amp with cathode bias "class A". Not that I think there's leeway WRT correctness but I at least thought I was doing better than that to call 100% dissipation at idle class A. Sometimes it's just faster, more concise and clear than to nit pick all manor of hybrid bias scenarios that happen in guitar amps. But I digress and will attempt to be more accurate in the future.

[g1]

I had thought (hoped) that all he meant was that even in class A, you should not be exceeding the max. dissipation spec.

[Jazz P Bass]

Chuck, have you read the article?
Clear as a bell.

The Last Word on Class A

[The Dude]

Chuck, have you read the article?
Clear as a bell.

The Last Word on Class A

Hadn't seen that one. Thanks for posting it, Jazz! It's well worded and puts everything in neat packages. Great read!

[pdf64]

class A push pull confuses me conceptually. Someone here tried to explain it to me once, but it came off sounding more like parallel operation than "push pull" to me

I think that from the perspective of the tubes, in class A parallel SE and push-pull are pretty much the same.