Single Ended - switch

[MWJB]

What Dave says, those are Dan Torres' suggested bias settings charts from his book, well known to be on the cold side. Those figures are his suggested bias settings and have nothing to do with actually running a tube in class A.

For an example, a 6V6 at 400vdc (corrected plate volts) should run at around 55mA for class A (halfway between cut off & saturation, according to RCA tube chart)...this is about what a stock SF champ runs nowadays & a SE 6V6 will be fine in this application but bear in mind that it's dissipating 22W!

"I believe some people talk about (especially) class A without actually knowing how it sounds (including myself)." Indeed, various folks have difering ideas as to what exactly meets class A perameters in a push-pull amp (plate conduction, grid conduction, presence of global NFB), many wrongly think that cathode bias means class A, some equally wrongly think that tube type signifies class ("All EL84 are class A").

"I mean it would be interesting to try out different bias voltages to run the amp in different classes just to learn how the sound changes." You certainly should try out different currents and see for yourself how it affects tone, watch for maximum plate dissipations and be careful not to burn up any OTs. Most P-P amps will still fall into AB...but the diversity of tones within this class really make defining an amp, by it's class alone, as meaningless.

[txstrat]

Well, the first place I look for those values is what is listed on the data sheet.
-g

That's right, but when you have a look on the data sheet and there are no values for a push pull configuration at 390 volts you might start to guess. The link I posted further down (or up - depending if you're member of this forum or not ) leads to some spec sheets of different tubes with the values I was looking for. With these sheets you can determine the class at any voltage (at least in the range for guitar amps).

@Dave H
I don't have all tube data sheets available, but for example the data sheet for a TungSol 6V6 indicates 34 ma at 315 v for class A operation (single tube). The bias chart reads 35.6 at 320 v and 36.8 at 310 v.
For a TungSol 6L6 the values are 54 at 350 v in the data sheet compared to 51.6 at 350 v in the bias chart. They both come pretty close, dont they?

@MWJB
Do you have a way to calculate the ma for a given voltage to make the tube run in class A?
But what for anyway? I think I'm gonna do it as you proposed (and what I did sometimes before) - trust the ears.

Matt

[MWJB]

Those bias charts are NOT indicators of class, they are ONE man's idea of at what point you should bias a tube for a given voltage, that's all. Many folk don't agree with those specific figures.

"They both come pretty close, dont they?" Close to what? Virtually no guitar amps made today, or made in the last 50years, currently runs a 6V6 at 310-320v, certainly none of the popular ones anyway. The currents you quote may well sound fine at the voltages you mention, but generally speaking, in the real world, many amps will be running higher voltages and higher currents in cathode bias.

Cathode biased P-P 6V6 amps might tyically run their power tubes at 35-40mA from 370-410v, maybe more current for some SE amps. Cathode biased P-P 6L6 amps might typically run 50-60ma From 400-430vdc. I am aware of some amps that run up to 70mA, but generally at a lower voltage (the SE Epiphone Tube 10 ran 70mA ish at 400v).

"@MWJB
Do you have a way to calculate the ma for a given voltage to make the tube run in class A?" Look at the tube charts for RCA tung Sol etc. But understand that many amps that sound good are based upon working examples, and that builders like to replicate conditions in those amps that they like the sound of...the original designers (of both tubes & circuits) had no idea that subsequent engineers were going to push aspects like plate volltage without reviewing bias resistors (notice that nearly every SE 6V6/6L6 amp you will see has a 470ohm cathode resistor, PP amps are usually 250ohms, sometimes 330ohms if on the higher voltage side).

I wouldn't bother with such a chart myself because you should be using your ears to get the tone you want (at a plate dissipation that doesn't damage components - experience is key here)...simply rebiasing an amp won't often push it up into the next class and if you try it, you'll probably find that it eats tubes/has an overly thick, garbled tone/suffers from power fade/spontaneously combusts!

Building a genuine class A push pull amp is something that you need to consider from the very outset, selecting voltages & transformers appropriately.

The generally quoted rules of thumb on bias points are 70% plate dissipation for fixed bias amps & 90% for cathode biased amps. Many F-B amps will sound fine at 50%, some may need to hit 80%. If you want to bias a push pull C-B amp much above 100% plate disspation, than have plenty of spare tubes on hand and be prepared for it to fade out before the end of a set.

[txstrat]

Virtually no guitar amps made today, or made in the last 50years, currently runs a 6V6 at 310-320v, certainly none of the popular ones anyway.

315v is what I found in the data sheet of a TungSol 6V6. I quoted it only for having a figure to compare with the bias chart.
I see what you mean. As I stated early before I'm not hunting for an amp running in class A.
I was just looking for a way to roughly explain it to myself. Like 70% plate dissipation is HOT for a tube (I believe this is easy to understand). I hoped there was sort of an easy way to determine class A too.

Matt

[pdf64]

Plate dissipation is one thing, class of operation is another. 100% plate dissipation (or 200% for that matter) does not in itself mean that the tube is in class A operation. Peter.

[txstrat]

Let me put it this way, just for the matter of knowing how to do it.
1. Say you have a push pull amp with long tail PI (don't know if it matters) and two 6V6 tubes running at 385v.
How would you bias it to have it running in class A?

2. Same thing just at 420v. How to bias?

Hope I don't annoy you with my curiosity.

Matt

[hasserl]

You probably can't run a 6V6 in class A at those voltages without over dissipating the plates.

[MWJB]

What Hasserl says...you'd be looking at around 50-60mA per tube, the amp would not likely sound right compared to one running 30-40mA. You would probably find that vintage PP 6V6 amps that run these kinds of plate dissipations have burned up their OTs. Type of PI is not relevant.