I've built this stereo amp some years ago which I think it's a modified Fisher design, it works fine however I have a couple of Phillips ECC82 and RCA 12AU7 nos , which I would like to try instead of the Chinese ECC83-b I have now, so I'm guessing the biasing of the first triode + plate and cathode resistors of the PI + the network feedback, the firsts two things I think I can manage by looking at the datasheets or by using the values of other amps, however I'm not sure about the NFB.. I don't know how is that calculated. 
An externally hosted image should be here but it was not working when we last tested it.
That's not at all correct. Gain is principally set by the feedback loop. It will be slightly lower, like maybe a dB or so.
The two tubes have similar bases and heater voltages. Similarity ends there. Substituting will greatly increase distortion. Bad idea. It's not like ECC83/12AX7 is a difficult tube to find, so why not use the correct replacement?
The two tubes have similar bases and heater voltages. Similarity ends there. Substituting will greatly increase distortion. Bad idea. It's not like ECC83/12AX7 is a difficult tube to find, so why not use the correct replacement?
My old HK A224 is a very similar amp, that used 12AX7's for the concertina. I subbed in 12AT7's as an experiment, and was surprised how much better it sounded. Didn't even bother to change the load resistors.
jeff
I have a question about this circuit, and how the 12AX7 is used in it. The voltage amp (1st stage) is biased with only 0.5V dropped across its 1k cathode resistor. A -0.5V grid bias is very close to contact potential, is it not? What effect would grid current have on that input tube? Would it load down the volume control in an undesirable way?
Also, the voltages don't look right. On the 2nd stage phase splitter, the voltage at the grid is shown as 85V, while the voltage at the cathode is shown as 80V. That would mean the grid of the second stage is at +5V from its cathode, so is deep in grid current.
VMUNIX, are you sure those voltages are accurate?
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Also, the voltages don't look right. On the 2nd stage phase splitter, the voltage at the grid is shown as 85V, while the voltage at the cathode is shown as 80V. That would mean the grid of the second stage is at +5V from its cathode, so is deep in grid current.
VMUNIX, are you sure those voltages are accurate?
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I saw mention of the JJ tubes, for replacing the generic 12AX7 I find the JJ ECC803S to be a nice sub in most voltage amplifier stages. Their ECC83S is pretty good in high gain preamp stages. Both seem to have better noise performance than you would expect. The gold pin versions don't seem to be significantly different in any way except a bit more costly.
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