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replacing ECC83 with ECC82 in stereo amp

VMUNIX

Member
2009-09-09 2:55 am
south
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. :confused:


[IMGDEAD]http://i68.tinypic.com/23n1go.gif[/IMGDEAD]
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
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?
 

VMUNIX

Member
2009-09-09 2:55 am
south
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?

I agree they are substantially different, I've seen/read that the 12AU7 is "very good" for HIFI audio so why not, the ECC83/12AX7 I have are Chinese or those JJ more suited for instruments amp I guess. Ok then I will leave it as it is until I find other valves.
 
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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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.