Cathode bias shift

[kg]

I'm still puzzled by how a cathode biased tube whose cathode voltage rises with conduction doesn't move the center bias value closer to cutoff due to the greater -V grid relationship. Isn't this what cathode bias is said to do? self bias? The circuit is always fighting for equalibium in power tube watts, regardless if the bias point must shift closer to cutoff to do it.

chuck, remember that cathode aka self bias works via degeneration (negative current feedback) at DC. so the average Ik is what determines the average Vk.

any time that average Ik goes up, the "effective" bias also goes more negative.

and average Ik will go up IF the tube cuts off during a portion of the negative phase--just as merlin mentions above.

so to answer your question, yes, this effect is nearly constantly occurring in nearly all tube guitar amps.

[kg]

I think what they really mean is - "a (class A/B) cathode biased amp because of its bias shift when driven, can run cooler than a fixed biased amp would under similar drive conditions", not that the cathode biased amp runs cooler under drive than it does at idle..

i tend to agree with dave H here... i think the comparison is really between a fixed bias and a self biased amp, given the same initial operating conditions. if looked at in this context then the statement becomes universally true, because it is impossible to find a situation in which average Ik goes DOWN with increasing power output.

Looking for a real world context here, very much looking forward to finding an amp that gets cooler, the more you drive it.

look at any true class A design. with class A quiescent dissipation is the worst case scenario, as no plate input power is being coupled to the load; as signal levels increase a greater proportion of PIP is consumed by the AC signal which then produces useful work at the speaker.

[MWJB]

Thanks KG, but how many PP true Class A instrument amps are there commercially available?

I am playing devil's advocate to some degree here...simply because it could easily be interpreted by the layman that the harder he drives his amp, the longer the tubes will last and the cooler it will run. Decades of practical experience suggests that this is a rather dangerous generalisation to be bandied about and certainly does not seem to be the case in the OP's scenario (cathode bias PP with moderate idle plate current), or any scenario so far put forward with any kind of measurement.

Also agree with Dave H. But bearing in mind that the cathode biased amp will be tangibly hotter at idle than a fixed bias amp (with the same PT spec) and it will have less swing in plate current compared to the fixed bias amp.

[Merlinb]

I'm still puzzled by how a cathode biased tube whose cathode voltage rises with conduction doesn't move the center bias value closer to cutoff due to the greater -V grid relationship.

To add to what KG said, although the bias voltage increases, the average anode current increase also. The whole load line shifts vertically upwards, so the valve doesn't (necessarily) get closer to cutoff, even though the bias voltage has increased.

[jpfamps]

Thanks KG, but how many PP true Class A instrument amps are there commercially available?

I am playing devil's advocate to some degree here...simply because it could easily be interpreted by the layman that the harder he drives his amp, the longer the tubes will last and the cooler it will run. Decades of practical experience suggests that this is a rather dangerous generalisation to be bandied about and certainly does not seem to be the case in the OP's scenario (cathode bias PP with moderate idle plate current), or any scenario so far put forward with any kind of measurement.

Also agree with Dave H. But bearing in mind that the cathode biased amp will be tangibly hotter at idle than a fixed bias amp (with the same PT spec) and it will have less swing in plate current compared to the fixed bias amp.

When calculating output valve dissipation you always have to take into consideration power dissipated in the load, otherwise you would be disobeying the laws of physics.

Theoretically if any "ideal" audio amp is driven to maximum clipping, ie producing a full power square wave, then dissipation in the output devices will be zero. It is this principal that make SMPS and class D amps small and light, because they need less heat sinking, although off course in a real device there will be some dissipation as switching is never instantaneous.

Thus it would seem that driving your amp continuously at clipping would potentially enhance valve life by reducing plate dissipation (although I expect this would not be a very musical experience for anyone listening.............).

However, in a valve amp, driving the amp hard into clipping, increases massively the screen grid current in the power valves, and this is definitely not good for reliability.

[MWJB]

jpfamps wrote "However, in a valve amp, driving the amp hard into clipping, increases massively the screen grid current in the power valves, and this is definitely not good for reliability."

Not to mention the strain on power transformers, speakers, etc.

[es345]

What I don't understand is why this is NEVER the case in any cathode bias amp I build! In every one, and I've built quite a few, I test as follows: I run the amp into a matched resistive load to save my hearing, I run a 1 khz signal into the input, and then I constantly measure the plate voltage and the cathode voltage as I increase the volume from 0 to max. When running the numbers, EVERY TIME, the calculated dissipation *increases* despite the amp B+ sag that obviously occurs.

I am not sure - independent of this interesting discussion - if we answered regarding the measurement methode explicitely.

I assume that you have measured the plate voltage with a VM in DC position. The averaged voltage at the plate you measure will be nearly constant with or without drive - SAG neglected -. If you would do the same measurement with an osziloskop you would see e.g the 1Khz full sinus curve with a positive offset of the plate DC voltage. The tricky thing is that during phase where the plate voltage is higher than the plate DC voltage without signal drive there is no plate current running through that tube. At that time the other tube is working. The voltage in that phase is just induced from the other tube through the OT. The VM is averaging the whole cycle and not only that part where the current is flowing.

So simply you cannot take this measurement to calculate the plate dissipation.

[kg]

jpfamps wrote "However, in a valve amp, driving the amp hard into clipping, increases massively the screen grid current in the power valves, and this is definitely not good for reliability."

Not to mention the strain on power transformers, speakers, etc.

... or ears, or marriages...

[imaradiostar]

... or ears, or marriages...

Wow- I can picture the strain on a marriage if one ran the BAGA to (unattenuated) clipping while a wife is upstairs taking care of the baby! My wife is due TODAY and she can hear an unamplified strat from bed with me plinking in the next room late at night.

I still contest that at a certain point (high enough overdrive situation) there is a peak then a drop in plate current relating to the outputs getting biased off more than they would be at rest.

jamie

[MWJB]

"I still contest that at a certain point (high enough overdrive situation) there is a peak then a drop in plate current relating to the outputs getting biased off more than they would be at rest."

Hi Jamie, I don't doubt there is possibly a "point" where this is the case, but is it a definite real world trend? And how would the player determine this point using his instrument & ears on a stage? If he can't, what is the practical use of knowing that this point exists? It seems to me that folks are using ever more elaborate ways to not quite say, "I don't know"! ;-)

[imaradiostar]

I bet Neil Young's Deluxe is doing that on some recordings- sure sounds like it anyway! On the whole though- no, that kind of bias shift is the minority case. I've lost sight of what this thread was really about anyway. Sorry to distract.

jamie