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Is this circuit ok or has it been hacked? Please advise (vintage tube amp)
I have a '60 Guild 99-J amp that I am overhauling. The actual circuit is different from the schematic that I have retrieved from another forum.
Attached you find the schematic with the differences that I have noted in my amp (red lettering = my amp).
As you can see there are many mismatches which were caused by one or more of the following: (a) a poorly drawn schematic, (b) a different version of the amp, (c) the result of a previous tech's job. Surely several components do not look original.
Besides the different value of many resistors, which is probably a minor issue, the following issues look to me like the most critical:
- input jacks wiring (note the 470k grid stoppers);
- treble/bass pot junction;
- the 220k resistor on the cathode of V3;
- a 25/50 cathode bypass cap on the power tubes instead of a "100P" cap;
- an additional 30u filtering stage (corresponding .05 cap in the original schem missing);
- missing .001 cap on the pin 1 plate of V3;
- two 10u filter caps increased to 30u;
- solid state rectifier marked "GZ34";
- a replacement power tranny with 230V primary (this amp comes from the UK);
- additional second speaker jack (4ohm?).
The power tranny has got three 6.3V secondaries with different current supply but no 5V secondary for rectifier tube heaters. Thus for the moment I cannot use a GZ34.
I'd love to have your comments on the mismatches, i.e. may the amp live as it is or should anything be corrected?
So far I have replaced all the electrolytic caps (including the additional 40u stage that is missing in the schem) and a few drifted resistors.
The original old Jensen speaker unfortunately is blown or shorted. The amp sounds good through another speaker, but definitely less loud than expected from a 2x6L6 p/p power section with a rather beefy output tranny.
I want it to sound like it should, i.e. not through a modified or less than optimal circuit.
Attached you find the schematic with the differences that I have noted in my amp (red lettering = my amp).
As you can see there are many mismatches which were caused by one or more of the following: (a) a poorly drawn schematic, (b) a different version of the amp, (c) the result of a previous tech's job. Surely several components do not look original.
Besides the different value of many resistors, which is probably a minor issue, the following issues look to me like the most critical:
- input jacks wiring (note the 470k grid stoppers);
- treble/bass pot junction;
- the 220k resistor on the cathode of V3;
- a 25/50 cathode bypass cap on the power tubes instead of a "100P" cap;
- an additional 30u filtering stage (corresponding .05 cap in the original schem missing);
- missing .001 cap on the pin 1 plate of V3;
- two 10u filter caps increased to 30u;
- solid state rectifier marked "GZ34";
- a replacement power tranny with 230V primary (this amp comes from the UK);
- additional second speaker jack (4ohm?).
The power tranny has got three 6.3V secondaries with different current supply but no 5V secondary for rectifier tube heaters. Thus for the moment I cannot use a GZ34.
I'd love to have your comments on the mismatches, i.e. may the amp live as it is or should anything be corrected?
So far I have replaced all the electrolytic caps (including the additional 40u stage that is missing in the schem) and a few drifted resistors.
The original old Jensen speaker unfortunately is blown or shorted. The amp sounds good through another speaker, but definitely less loud than expected from a 2x6L6 p/p power section with a rather beefy output tranny.
I want it to sound like it should, i.e. not through a modified or less than optimal circuit.
Attached Files
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