Help Biasing a Class A Amp

[planetal]

Hi,
Although I own and have set the appropriate bias on Class AB amps for quite a while now(on a few Rivera's, a Germino and a Flexi), I just recently purchased a fixed bias Class A amp(I think - a Fuch's lucky 7) and am a bit confused about biasing the amp..i.e. I'm confused at what dissipation should be..
I know for a class AB one normally uses the 70% of max rated power so it's simple math to get the bias range. Class A seems very different and I have no idea how to calculate what appropriate dissipation should be. Yes I know there are tube data sheets, but I'm not really sure I know how to read them. I'm obviously not an engineer, just a guitar player.

I have a BiasRite that works fine for my Rivera's, should it work on a Fuchs Lucky 7 (class a) as well? In reading the voltage, using the bias rite, it is reading WAY off from what I'm under the impression it should be (i.e its supposed to be 375Volts B+ I'm seeing 320Volts on the Bias Rite).. is the BiasRite not reading the voltage the right way? In terms of bias, how does one calculate what the proper bias should be for a el34, kt66 or 6v6?

I could simply write Mr Fuchs, but it's a Sunday night and I know there are many very knowledgeable people on here who could perhaps help enlighten me. (before I pester Andy in what are probably stupid questions during the week)..

Anyway, thanks for any insight.

Al

[John Phillips]

If you mean Class A, you need a scope to do it properly. You cannot go by dissipation since it is not related to the operating class in any way.

If you mean cathode-biased Class AB, which I assume you do since almost every amp marketed as Class A is in fact this:

You probably don't 'need' to bias it, since this system is self-biasing.

You probably can't bias it, without replacing a resistor inside, since most are not designed to be adjusable.

You certainly can check the bias. Usually you will find it's somewhere between 70% and 100% of maximum tube dissipation. It's impossible to know how correct or safe this is without knowing the operating point. If it's over 100% (some are, especially with EL84s) you're just going to have to accept shorter tube life, if you don't want to modify the amp to bring it down.

The 'plate voltage' will appear lower than the B+ voltage because you have to subtract the cathode voltage in a cathode-biased amp - which is typically 10-12V for EL84s, 30-35V for 6V6s and EL34s, and 45-50V for 6L6s and KT66s, although it varies depending on the other parameters. The true tube voltage (plate-cathode) is the correct one that you need to calculate the dissipation.

[Fuchsaudio]

Actually, I spent most of the afternoon on the PC....lol.

We've had a number of people having problems with outboard bias measuring devices. Not sure why, and not a slag on the meters themselves, but it may have to do with the power tube cathodes.

Most power tube cathodes are grounded to the chassis in something like a Fender or Marshall, and in our amps they are going through a 1-ohm resistor, to permit measurement with a digital volt meter. Perhaps this resistor "fools" these outboard devices ? One customer in Canada tried biasing an ODS-50 (the meter he owned, plugged into both power tubes), and got nothing but wacky readings. A simple Radio Shack digital meter did the trick.

I agree with John, that setting with a scope, rather than voltage operating points makes more sense, but for those without a scope:

The Lucky 7 is a fixed (adjustable) bias amp, which runs about 375 B+ (depending on current drawn by power tube and line voltage), and we ship it biased to 50-ma which is a safe point. We ship with JJ EL-34 power tubes, which work well at 50 ma. The JJ 6V6 sounds great at 30ma, although lesser tubes might not like that current.

The amp and both the power and output transformers can handle more current (it sounds great with an EL-34 at 70 ma btw), but a "hot" EL-34 is 47 ma/70% dissipation at 375 volts.

This is a good place to look: http://www.webervst.com/tubes/calcbias.htm

Happy Sunday....seems John is working today too.....lol

[John Phillips]

Aha... sorry for the assumption Andy, I should have known you'd not have followed the usual "Class A" (that isn't) pattern... :D

So is the Lucky 7 a fixed-bias, true Class A amp?

If so, that blows another hole in the cathode-biased = Class A myth :).


Sort-of working today... actually building a practical test setup to determine whether or not signal cables are directional, and posting the results. I wonder if you can guess what I found? ;)

[Old Tele man]

...electrons are slower heading north than when heading south, due to gravitational "drag"?

[planetal]

Gentlemen,

Thanks much for your responses. Andy, I didn't want to bother you with a phone call and now I don't (and won't) have to.. I will continue to use the Weber site for bias settings then. I just tried a GT KT66 I had at 70ma and it sounds quite good in the Lucky 7 by the way (although I haven't been able to turn it up for any period of time yet). There was an EH El34 in the amp when I bought it(used) and it was farting out like crazy (at 60ma). The KT66 isn't farting out at all so I guess I'm going in a good direction.

Something to do after watching the Giants lose..

Al

[Fuchsaudio]

Aha... sorry for the assumption Andy, I should have known you'd not have followed the usual "Class A" (that isn't) pattern... :D

So is the Lucky 7 a fixed-bias, true Class A amp?

If so, that blows another hole in the cathode-biased = Class A myth :).


Sort-of working today... actually building a practical test setup to determine whether or not signal cables are directional, and posting the results. I wonder if you can guess what I found? ;)

Yes, unlike some amp builders (one in particular I can think of), I have chosen to actually make a true class-A amp, rather than trying to convince the world with a messa floobydust, that my non-class-a amp reeeealy is class-a...lol.

Remember, the cable testing must be done with all cables perfectly level, so gravity does not affect your readings......right OTM ?

:rotflmao

[Old Tele man]

...I believe the word you were looking for is coriolis effect, e.g.: the rotational effect of earths gravity...ha,ha.

...nothing worse than "confused" electrons!

[ChickenLover]

Quick hijack: The Coriolis effect isn't based on nor due to gravity...it has to do with the fact that the earth is rotating.

[Old Tele man]

...damn! caught! but true, no gravity involved (it was a tongue-in-cheek statement, but inaccurate nevertheless).

Coriolis = apparent deflection due to rotating frames of reference, e.g.: precession of a Foucault pendulum