running a preamp tube in class B! - diyAudio
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Old 20th April 2013, 08:53 PM   #1
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Default running a preamp tube in class B!

I have a tremolo circuit here using a 6au6 with no bias at all. There's no cathode resistor and no other source of bias in sight. After puzzling over it, I've come to the the conclusion that it was intentionally biased into class B in order to seriously deform the waveform.

It sounds really, really good. I'm looking at basically wholesale stealing this design to go in an very low noise, low gain amp I'm designing right now, but not only is the pentode noisier than need be, but it would be that much simpler as a triode. So I'm looking at revamping the circuit with a 12ax7 instead.

BTW all I know about using tubes for tremolo oscillators I learned from the Valve Wizard site.

Is this just like a power tube load line, where the class B load line is 1/2 the resistance of the class A line?

This 6au6 has a 68k anode resistor right now, which would provide a gain of around 100 on the usual class A loadline, but I'm thinking it's really operating in class B and the gain is that of a 34k class A loadline.

Also, what kind of voltage swing does an oscillator like this have?
Pretty much the whole way? Does it go into grid current limiting or cutoff at all? And is there any way to control all that?

Sorry I'm unable to post any schematics anytime soon.
Thanks all,
Ted
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Old 20th April 2013, 09:41 PM   #2
DF96 is offline DF96  England
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What is the value of the 6AU6 grid leak resistor? If particularly large then it might be using grid current bias. On the other hand, oscillators often run in Class C.

Note that a valve with zero bias is not Class B, but Class A with grid current. You would probably get distortion but for a different reason.

Without the circuit we are just guessing.
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Old 20th April 2013, 09:51 PM   #3
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The grid leak resistor is fairly large- it's 2.2M.

Again, sorry I can't post a schematic right now, but I may be able to at some point.

The screen is at 80V, the B+ is about 250v. Suppressor tied to ground.

"Note that a valve with zero bias is not Class B, but Class A with grid current."

So does this mean that the usual loadline applies?

Thanks again!
Ted
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