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replacing ECC83 with ECC82 in stereo amp
replacing ECC83 with ECC82 in stereo amp
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Old 8th April 2016, 04:51 PM   #1
VMUNIX is offline VMUNIX  Argentina
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Default replacing ECC83 with ECC82 in stereo amp

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.


replacing ECC83 with ECC82 in stereo amp
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Old 8th April 2016, 04:56 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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It will be fine, just lower gain.
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Old 8th April 2016, 05:15 PM   #3
SY is offline SY  United States
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replacing ECC83 with ECC82 in stereo amp
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?
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Old 8th April 2016, 06:24 PM   #4
VMUNIX is offline VMUNIX  Argentina
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Originally Posted by SY View Post
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.
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Old 8th April 2016, 06:31 PM   #5
SY is offline SY  United States
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replacing ECC83 with ECC82 in stereo amp
I'd try them- some of the JJ tubes are excellent.
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Old 8th April 2016, 06:47 PM   #6
vinylkid58 is offline vinylkid58  Canada
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Originally Posted by VMUNIX View Post
Ok then I will leave it as it is until I find other valves.
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.

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Old 8th April 2016, 06:58 PM   #7
SY is offline SY  United States
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replacing ECC83 with ECC82 in stereo amp
100% feedback in that position helps.
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Old 8th April 2016, 07:15 PM   #8
vinylkid58 is offline vinylkid58  Canada
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C'mon, what's wrong with a bit of feedback?

jeff
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Old 8th April 2016, 09:12 PM   #9
Ketje is offline Ketje  Belgium
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If you put 82 in place of 83 the gain is so low there is no room for feedback.
For the 82 replace the cathode resistor by 1k8 (was 1k) and the anode resistor (270k) by 68k.Like that the concertina will get it's usual voltages back.
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Old 8th April 2016, 11:59 PM   #10
rongon is offline rongon  United States
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replacing ECC83 with ECC82 in stereo amp
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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