The Amp Bias Controversy

[Mark Hennessy]

Not impossible. Output phase is the anode load. If signal voltage on grid is postive, anode current increases. Multiply that current by the anode load resistor and the sun is higher. Therefore the anode voltage falls and is out of phase with the grid cathode voltage. Yet the anode load is in phase.

More precisely, what one might say here is: "Yet the voltage across the anode load is not inverted, relative to the input".

But at the same time, as you say, the anode voltage falls with respect to earth when the input rises. So, given that the input signal is also a voltage that is relative to earth, the amplifier is inverting if the output voltage relative to earth is what that matters (which is often the case), and that is what Terry meant.

You are thinking specifically about an output stage, where the anode load is an output transformer. Here, the concept of inverting or non-inverting is moot, as it depends on how you wire up the output transformer (and loudspeaker). If you're not taking negative feedback from the secondary, it doesn't matter. And you won't hear a difference in a mono setup.

I prefer "inverting" and "non-inverting" to "phase". Remember, a 180 degree phase shift is not the same as an inversion

[Murphyv310]

Hi.
Well I have seen sync separator circuits that have very high anode loads with the control grid at a positive voltage WRT cathode, the diode effect is used to advantage in this application.
I'll do a bit of searching and post it assuming I'm not pestered by the missus yet again!!!!!!

[pwdrive]

Not forgetting Mr Miller:

http://www.r-type.org/articles/art-136.htm

Lawrence.

[Terry]

As I said in an earlier post: "The ability of the grid and cathode to be used as a diode can be used very effectively in the right circumstances."

In another post I said: " The whole point is that grid current should never be allowed to flow in virtually all applications." - the emphasis being on virtually all.

Whilst it might be normal to discuss these exceptions here I'm concerned that, in this thread, it might give NWM the idea that his mistaken concepts of how audio amplifiers work were right in the first place.

These ideas, of course, are completely wrong as far as amplifiers are concerned but I think we need to be careful not to lead NWM astray here.

[Murphyv310]

Hi.
I agree Terry that in this thread discussing exceptions isn't helpful......... but I also believe that it has to be said that it can and has been done in a few cases and TBH without valves failing. No doubt NWM will find reference to this in his reading and if we say it can't be done then we look like idiots.
Yes there is no need to Labour the point about positive grid bias, voltage or current at this time as it is not relevant, good teaching though will make the pupil aware of it.

[Kalee20]

Very well put, Murphyv310!

[Murphyv310]

The Bell TV6 uses positive grid bias on the sync separator.

[ppppenguin]

The Bell TV6 uses positive grid bias on the sync separator.

Actual +ve bias? Or just driven into grid current for part of the duty cycle to clamp or clip a video signal?

[Murphyv310]

Bell seems to have used this method in a few sets, the grid is biased with a small positive voltage and the positive going video further turns the valve on, on syncs there is then a clipped sync pulse on its anode, I've hunted for a diagram but only found an explanation.
Pye also hold a patent for a very similar sync separator circuit.
Used in a prewar model.
I'm using a phone and can't seem to copy and paste either article.

[pwdrive]

+ve

http://korgnutube.com/en/guide/

Lawrence.