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Re: Varible 12ax7 cathode resistor in preamp?

3/1/2000 3:28 PM
ken gilbert Re: Varible 12ax7 cathode resistor in preamp?
"having to change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing."
yes, but that of course assumes that that's what we WANT to do with our plate voltage swing! i agree that's what it seems dan is saying.  
call me bizzare, but i don't see anything wrong whatsoever with a 100k plate resistor only dropping 20v @ idle. the tube will last a long time.  
"At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand."
come on randall, don't let that engineer in you get out of hand!!! there's only one test we need to concern ourselves with and that's the with the ears. who cares what the operating conditions are, if it sounds good?  
now, having vented on you a bit, i will say that i often use my scope to SEE what a pleasing tone LOOKS like, so that i might more easily repeat it later, and correlate the waveform shape with what i hear.  
i say keep your operating conditions reasonable (ie. don't put 500V on the plate of a 12ax7) and your power dissipations within spec and let 'er rip.  
3/2/2000 3:46 AM
Randall Aiken

"here's only one test we need to concern ourselves with and that's the with the ears. who cares what the operating conditions are, if it sounds good?"
Calm down, Ken! Re-read my post and you'll see all over it where I said "which may or may not be a good thing,depending on your needs" and "unless you are designing for an asymmetric bias point". I was just trying to explain what Dan meant with his comment.  
However, if you blindly change something and don't know why or how it changed the tone, what good is it if you can't repeat it or apply it to a slightly different circuit? Do you want to be forever forced to design by trial and error? I would never mod or design a circuit without checking the result on a scope for accidental unwanted oscillations, or exceeding some circuit parameter that could cause early failure. You may choose to just listen to the result and ignore any possible problems, but, in my opinion, that's a rather foolish way to go about amplifier design.  
Randall Aiken
3/1/2000 8:19 AM
Tracy Teague
Re: What Dan Torres bias chart means
Hi John  
Dan has a chart that relates 12ax7 tube operating voltage to a plate resistor value and the appropriate cathode resistor range. Your question might be better stated another way. If you change the plate resistor or V+ voltage, you may need to change the cathode resistor value. Dan also talks about this as changing the tube bias. you could also think of it as finding the operating point you want for that stage. some characteristics might be differing gain, band width, touch response. My own experience has shown me that the cathode to plate resistor relationship is much tighter at 160 volts V+ than say 350 or 400 volts.  
Randall explains it much better than I can. But having seen and experimented with that chart, I throw in my 2 cents.  
3/2/2000 12:48 AM
Hey everybody, How about a varible plate load resistor to go with it?

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