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I'm currently debugging an amp I put together, which is essentially a Marshall 1987 plexi circuit. It also has a "1 tube reverb" circuit added, but that's not important here as the amp behaves the same with the reverb circuit disconnected. The amp sounds good and has no noise issues, but does not have the volume it should - it's loud enough to hold its own in a rehearsal space but a 50 watt Plexi should be call-the-cops loud. So I set about re-checking everything against the schematic and measuring voltages. I found and corrected a mistake in the tone stack which would have been losing me a few decibels (wrong value slope resistor), but when checking DC voltages I came up with something which is puzzling me.
The DC voltages at the valve pins are all within a few percent of the expected values except for V2b, the cathode follower. V2 is set up with the standard 100k plate and 820 ohm cathode resistor on V2a and a 100k cathode resistor on V2b. With a JJ ECC83s in place I measured the following voltages.
JJ ECC83s:
p6 256V, p7 146V, p8 86V
It strikes me that pin 8 (V2b cathode) is far lower than it should be - all of the measurements I've found online place it within a few volts of pin 7 (V2b grid). I can find no errors in my component choice or layout around this stage, and the DC resistance between pin 8 and ground is 100k as it should be. Thinking it might be an issue with the valve, I tried a couple of old ECC83 valves which I had lying around. They are an old Mullard and a Brimar salvaged from used equipment, which pass signal but are otherwise untested. The voltages around V2b with those were as follows.
Brimar:
p6 251V, p7 146V, p8 112V
Mullard:
p6 252V, p7 137V, p8 94V
Clearly the voltage at pin 8 does vary when changing valve, but all of these are still coming out lower than they should be. Because it varies between valves and I don't know how tired my spares might be, I think my next step may be to try a fresh, tested ECC83 in V2 and see what I get. I'd be grateful for any other pointers about this - am I barking up the wrong tree? I'm also not certain how much variation should be expected here.
The DC voltages at the valve pins are all within a few percent of the expected values except for V2b, the cathode follower. V2 is set up with the standard 100k plate and 820 ohm cathode resistor on V2a and a 100k cathode resistor on V2b. With a JJ ECC83s in place I measured the following voltages.
JJ ECC83s:
p6 256V, p7 146V, p8 86V
It strikes me that pin 8 (V2b cathode) is far lower than it should be - all of the measurements I've found online place it within a few volts of pin 7 (V2b grid). I can find no errors in my component choice or layout around this stage, and the DC resistance between pin 8 and ground is 100k as it should be. Thinking it might be an issue with the valve, I tried a couple of old ECC83 valves which I had lying around. They are an old Mullard and a Brimar salvaged from used equipment, which pass signal but are otherwise untested. The voltages around V2b with those were as follows.
Brimar:
p6 251V, p7 146V, p8 112V
Mullard:
p6 252V, p7 137V, p8 94V
Clearly the voltage at pin 8 does vary when changing valve, but all of these are still coming out lower than they should be. Because it varies between valves and I don't know how tired my spares might be, I think my next step may be to try a fresh, tested ECC83 in V2 and see what I get. I'd be grateful for any other pointers about this - am I barking up the wrong tree? I'm also not certain how much variation should be expected here.
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