current and cathode bias SE

[kg]

and I'll just never go along with that... You bias a power tube for the text book definition of Class A, over-driven or not, it's still Class A.

-g

well, you go and rewrite all the textbooks for the last century then.

[kg]

i'll get the page/section numbers when i get home today.

13.2.ii.D rectification effects (p.551 in 4th ed.)

owing to the second harmonic component of the signal frequency plate current, the average plate current under operating conditions is greater than with zero signal.

[mooreamps]

Or I could have a push-pull amp that's "Class-A up to 1 watt output", say. Above 1W, one of the output devices cuts off at some point in the cycle: it goes into its Class-AB region.

because when the plate saturates, then you still only get 1 watt out of it... :|

or are you saying there is some bias shift going on when you dime the power stage ??


-g

[Steve Conner]

No, not really.

For instance, I just finished building a solid-state amp that puts out 150W into 4 ohms. To get the best distortion performance, it likes to be biased at about 250mA. So, the first watt or so is Class-A. (That's why I designed it that way: they say the first watt is the most important.)

But whenever the instantaneous current delivered to the speaker gets much above 250mA, the unused half of the output stage must be off at that instant. Therefore the remaining 149 watts are delivered in Class-B. That's why they are called Class-AB amps.

A push-pull Class-AB tube amp does exactly the same thing, except it's probably more like the first 10 watts that are Class-A, and the transition is more gentle.

In a push-pull amp designed for pure Class-A, the idle current is set so high that one half of the output stage will saturate before the other half ever turns off. But no musical instrument amps were ever made this way.

We've had long and bitter debates over this, you might want to search the forum. I proposed the concept of "design class" and "operating class" to try to work out a compromise between your school of thought and mine.

[mooreamps]

Here is what I did on my 36 watt jazz amp, running 2 6L6 push/pull... The self bias cathode resistor puts the S.E bias at -18 vdc... Let's call that Class A. Then, I inject an additional -24 vdc for a total of -42 volts for Class A/B.

-g

[Steve Conner]

And when you add that extra 24v of fixed bias, does the cathode voltage really stay at 18?

[Chuck H]

Why would anyone want to switch between class A and class AB without additional parameters like PI output and OT suitability being met. I'm pretty sure if you took a class A push pull amp and simply biased it AB most players wouldn't like it very much (think Vox AC30 with -10VDC added to the grids). Conversely if you took an average AB amp and just heat it up into class A it wouldn't work very well and could even cause damage. It seems to me that trying to draw a line in between two designs for all other parameters to facilitate a simple voltage change for operating class would have dissapointing results on both ends.

[kg]

in my experience, the difference in "sound" as you change classes is very subtle and difficult to discern.

that is, unless you go so far as to induce crossover distortion. the thing about my experiences with xod basically adheres to steven's quotation that "the first watt is the most important." since xod is by definition a small signal phenomenon it is, to my ears, more apparent than something that happens at max Pout.

[mooreamps]

And when you add that extra 24v of fixed bias, does the cathode voltage really stay at 18?

Yes, it does. But bias is not cathode to ground.. Bias is the delta from grid to cathode....


-g

[mooreamps]

Why would anyone want to switch between class A and class AB without additional parameters like PI output and OT suitability being met.

The "PI", or IPA as I call it, flips modes from S.E to P/P.
and the O/T really doesn't care as long as both tubes are sitting there idling at the same value of plate current...

:>





-g

[Chuck H]

The "PI", or IPA as I call it, flips modes from S.E to P/P.

+++ You can't really take advantage of the difference without more drive voltage.

and the O/T really doesn't care as long as both tubes are sitting there idling at the same value of plate current...

Well tube data is clear that you'll get more performance by optimizing the load for a specific bias, but guitar amps break those rules all the time. As kg pointed out the difference can be subtle. I think this is especially true of guitar amps that are often (if not usually) biased on the hot side of AB. I was just playing devils advocate for the discussion.

EDIT: Nice to have you back Ken. I rejoined late after the format change and I've missed the old crew.

[mooreamps]

Well, in Class A/B Push-Pull, electrically it's set-up like a fender twin.. In Class A S.E., electrically it's set-up more like a champ.

Now, I could use KOC's "body" control instead, but this is how I have it set-up for the moment....


-g