JJ 6V6s tube biasing

[surfsup]

I recently got a couple of these and it seems for a single tube 10W is the max plate dissipation. So at 350v, do I run it at 90% like an EL84?

10 * 0.9 = 9watts
9/350 = 25.7mA

I read where people say you can really push these tubes. Anyone have any opinions?

[JamesHealey]

I thought the jj 6v6 was higher plate Diss than that?

[surfsup]

14W as a pentode:

http://lib.store.yahoo.net/lib/thetubes ... -6V6-S.pdf

So is 90% a good value?

[Firestorm]

JJ's data sheet says 10W is a limiting value in TRIODE operation. So if that's how you're connecting it, 9W should be pretty safe.

[FunkyE9th]

What class are you planning on running the tubes? If you're doing AB, I wouldn't run it that hot.

[tubeswell]

JJ6V6S is not a 'real' 6V6, its more like a 6L6, and it is easily a 14W tube (some folks say 16W)! If you have it in fixed bias mode at only 350V, and no bias-vary trem, it will happily run at 70-85% IMHO. If you have bias vary trem, I would stick to under 70%.

If you have it in class A and cathode bias mode, then run it at full speed (say 14W)

[surfsup]

What class are you planning on running the tubes? If you're doing AB...

Doesn't the bias determine the class? ie if I am "cold" it is AB, if I am "hot" it is class A?

I am running an ax84 HO with a JJ6V6s and two 12ax7 preamps using a Hammond 269EX PT. The voltage B+ is about 350V

If you have it in class A and cathode bias mode, then run it at full speed (say 14W)

I admit I don't really understand the A/AB thing. I thought it was the place you biased the PT that determined the class. So hence what I should be shooting for. As I understand it, I can only really put out about 50-55mA max to the PT before the PT gets stressed. I pulled out the EL34 in favor of the 6V6s.

@90%
14 * 0.9 = 12.6
12.6/350 = 36mA

@75%
14 * 0.75= 10.5watts
10.5/350 = 30mA

So it seems there isn't much range anyway but I should be shooting for 30-35mA? But again, I read on some forums people said to push these JJ 6v6s tubes but I don't know what that means... 90%? 75%?

[Alexo]

I think 12.5 watts is pretty reasonable for a single ended amp using one of these tubes.

FYI, for class, unless you really have an unusual and specifically-designed amp, forr all practical purposes, class A = single ended and AB = push pull.

There are varying degrees of "A-ness" in a push-pull stage, and it is possible to have one that is 100% class A (or B for that matter) but almost zero guitar-oriented push-pull amps are actually designed that way.

For a cathode biased push-pull amp, you want to run them hotter than 70% (I usually shoot for 85% or so) because, under signal, the cathode voltage will rise and "cool" your bias setting, so you set them a little extra hot at idle to compensate.

[ToneMerc]

JJ6V6S is not a 'real' 6V6, its more like a 6L6, and it is easily a 14W tube (some folks say 16W)! If you have it in fixed bias mode at only 350V, and no bias-vary trem, it will happily run at 70-85% IMHO. If you have bias vary trem, I would stick to under 70%.

If you have it in class A and cathode bias mode, then run it at full speed (say 14W)

I agree on the JJ being more like 6L6 well. One of our more famous forum members got 52W out of quad of them with 490V on the plates. I was was curious on how much voltage they would take and I put well over 660V/325V screens on a pair in a MusicMan style supply.

TM

[FunkyE9th]

What class are you planning on running the tubes? If you're doing AB...

Doesn't the bias determine the class? ie if I am "cold" it is AB, if I am "hot" it is class A?

I am running an ax84 HO with a JJ6V6s and two 12ax7 preamps using a Hammond 269EX PT. The voltage B+ is about 350V

From your original post, I could not tell what kind of amp you were working on so I asked about class of operation. Now I know. You have a single ended amp, so it's Class A by default. You can't have AB with one power tube. You need two tubes.... One tube has pull while the other tube has to push.

[guitarmike2107]

If its cathode bias run it hot, 12 to 14watt,

When you overdrive a cathode biased amp the cathode voltage raises, the effect is that it pushes the valve towards class B operation, i.e. cooling off the bias/reducing the current.
In comparison when you overdrive a fixed bias power section the power valves are pushed towards Class A Bias.. hotting up the bias/raising the current i.e. opposite direction to class B. This is one reason why its good to bias fixed bias amps cooler, 70% say,

I recently learn't the reason for this by reading one of Richard Kuehnel's books, I biased my 6v6 pp cathode biased amp to 14w plate dissipation and it made a huge difference to the overdrive tone...

I guess there are no class B guitar amps for a reason, It also explains why some of the amp manufactures bias there cathode bias amps so hot, sometimes more than the plate dissipation

[surfsup]

Thanks guys, great posts! FunkyE, i thought on a SE you could still bias the tube hot to achieve AB. I guess i read that wrong elsewhere...thanks, i didnt think to mention the single tube because i didnt think it mattered.

[Firestorm]

You can bias (misbias) an SE amp so it falls out of Class A; but you sure don't want to. I can't think of any advantage in trying to set this amp up with fixed bias (although you could). Just stick a 2-watt 470R on the cathode, bypass it with a 25/25 or 25/50 and you're done.

[FunkyE9th]

You can bias (misbias) an SE amp so it falls out of Class A; but you sure don't want to. I can't think of any advantage in trying to set this amp up with fixed bias (although you could). Just stick a 2-watt 470R on the cathode, bypass it with a 25/25 or 25/50 and you're done.

How does it fall out of Class A and what class does it fall into?

You can bias it cold and all it will do is go into cutoff sooner so your max clean output swing is reduced (i.e. rated output power reduced). If you bias it hotter, then the grid conducts sooner and max swing is reduced again. Does that change the class of operation?

Even if you biased it smack in the middle of the load line, if you drive amp hard enough (i.e. outside of max clean output swing) it will go into cutoff or the grid will conduct.

Not trying to be argumentative, just trying to understand how it will fall out of class A and what class it ends up in. Maybe a class of its own?

[rdjones]

I guess there are no class B guitar amps for a reason, It also explains why some of the amp manufactures bias there cathode bias amps so hot, sometimes more than the plate dissipation

Most MusicMan amps are as close to Class B as you can get and still operate using typical tubes.
The exceptions are the early tube PI versions which operate with a standard bias supply.

rd