Why does power tube bias go bannanas when you're playing loud?

[leadfootdriver]

I'll have a bias tool on a power tube, and when you play loud, you can see the bias creeping up. Then when you're really loud, it goes off the charts..

Is this part of the power tube sound?

[Tom Phillips]

I'll have a bias tool on a power tube, and when you play loud, you can see the bias creeping up. Then when you're really loud, it goes off the charts..

Is this part of the power tube sound?

If you are monitoring the current through the tube while you are playing, the plate current will increase. However, that is the operating current while the tube is amplifying which is higher than the bias current. The "Bias current " is not rising. The bias current is only measured under no signal condition.

[The Dude]

A bias probe is made to measure and adjust bias. That is IDLE current- meaning the tube current at idle with no signal applied. Of course tube current is going to increase when you apply signal to the amp. True of either solid state or tube amps.

Edit: I guess we know who types faster.

[Enzo]

Agree. What you are measuring is not bias. Bias is an idle condition. Think of adjusting bias as like adjusting the idle speed RPM on your car. If you start driving around, the RPMs increase, but that is no longer idling.

[überfuzz]

I don't know what the small, well most likely under 0.5 W, resistor can take. Well, you might wanna leave it at measuring idea conditions.

The maximum power dissipation in an amplifier, specially for tube amps, is way over the rated dissipation. The reason being that the maximum dissipation is often rated as dissipation with 1%, or some other percentage, distortion.

[soundmasterg]

It isn't a good idea to play an amp while monitoring the current draw using one of the bias tools. The sense resistor in them usually can't take the current draw and will fail eventually. Weber warns their customers of this on their site for the Bias Rite tools as many people had to send them back due to failures because they played while the device was plugged in.

Greg

[leadfootdriver]

It isn't a good idea to play an amp while monitoring the current draw using one of the bias tools. The sense resistor in them usually can't take the current draw and will fail eventually. Weber warns their customers of this on their site for the Bias Rite tools as many people had to send them back due to failures because they played while the device was plugged in.

Greg

Thanks for the warning, but the Pro Bias One from Eurotubes is safe to use this way.

Eurotubes ?Pro One? Bias Probe

[Jazz P Bass]

The question is 'why would you do such a thing?'

Bias, by it's definition, is an idle current setting.
It's ultimate goal is to take the output section from Class B (complete tube cutoff) into a very slightly On, Class AB, which removes the crossover distortion inherent in a Class B setup.

There is nothing to be learned by watching the bias probe while playing the amp.
Nothing at all.

[Audiotexan]

The question is 'why would you do such a thing?'
.....
There is nothing to be learned by watching the bias probe while playing the amp.
Nothing at all.

I don't think it's necessarily wanting to watch the probe while playing...though I'm sure some would. (And that would bring up this issue, among others I'm sure, for those not understanding the intended application of the tool)

Well, I'd bet the reason most of them "would do such a thing" could be considered "lazyness", but it'd be nice if there was one that could be safe to use "on the fly....would be especially nice for educating customers I'd bet! But most of us learned these things the old fashioned way. (Trial and error, and major amounts of repetition!)

Too impatient to:
1. set the bias, turn off, wait to cool, remove, reinstall w/out bias probe, test audio, think "ok, it's set right and this is the sound I get."

2. wait for cool down, remove tubes, reinstall bias probe, adjust it hotter, turn off, wait to cool, remove bias probe, reinstall tubes, test audio, think "ok, it's set a little hot and that's the sound I get."

3. wait for cool down, remove tubes, reinstall bias probe, adjust it colder than step 1, turn off, wait to cool, remove bias probe, reinstall tubes, test audio, think "ok, it's set slightly cold and that's what it sounds like."

4. etc etc until they learn what they actually like in that amp.
(repeat for each amp you own, and per each brand/batch of tubes, until you get a general experience of roughly what you expect.)

5. It could be quite a time-saver (over the medium-long haul) if you could be able to leave it all safely assembled.

Not to mention the longer the wait in between "auditions" the fuzzier the memory gets.

I have to give the idea/product credit (if it actually is safe and works). It's not without it's merits!

[J M Fahey]

Well, if you're lazy/impatient, it's your problem.

The tool was intended to be used in a certain way, any other is your choice but don't come here to complain.

[g1]

While there is nothing to be seen as far as playing, checking with a sinewave input at various power levels could show how well matched the tubes are beyond idle. (gain matching?)

[J M Fahey]

And please don't mix/confuse things.

That tube current increases while playing, is not "bias variation" by itself, simply the tube doing its thing under drive voltage ... what it was intended to do by design.

Simply can NOT call that "bias current" , which as correctly mentioned above must be measured at idle (i.e. with no signal applied).

Now to something completely different: in some amps (not necessarily all) , when driven real hard, there may be a bias voltage variation, caused by power tube grid rectification, which creates a negative voltage which can add to the static bias one.

It will shift working point, and can be see on scope, but as soon as you stop playing it returns to normal.

Time constants involved are in the order of a second or less.

[Audiotexan]

Well, if you're lazy/impatient, it's your problem.

The tool was intended to be used in a certain way, any other is your choice but don't come here to complain.

Bad morning? *slides you a cup of coffee*

Maybe I should have said/added what g-one did to make things clearer "While there is nothing to be seen as far as playing", but I thought that was fairly clear already.

I wasn't on JPB's ass, nor anyone elses. Just made a side observation since leadfootdriver mentioned that the possibility of one that has apparently been designed with such in mind (that adjustments can be safely made while in circuit, between play tests) without risk to amp nor test equipment in the process.

I was simply marvelling at the idea that such a tool might finally exist. Especially after doing it the old fashioned way (as I clearly noted) for all these years.

Not sure what set you off, but no one I saw was complaining.

Alll the best man.
-A.T.