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03-10-2024, 08:37 PM
Hello Physics
That is an interesting circuit which should be very pedal friendly as is. I would suggest that you simply get it working first and have your friend assess the tone. It should be very "high-fidelity" as it was intended to be a clean public address amplifier, and not anything approaching the kind of tone altering beast modern guitar players use.
The capacitive-coupled bias supply alarms me. I am in agreement with KOC that such bias supplies should be replaced with proper transformer-generated supplies, and a true independent bias-set network applied with cathode current-sense resistors for the output valves. At the moment there is a mix of fixed-biasing with cathode biasing. This is not bad unto itself, and if I recall, Mr. O'Connor did that in previous versions of his Studio power amplifier.
Regarding the capacitors: I think it is safest to replace the ones that say "PMK" on them as the "P" may mean paper? I do not know this as fact, but the age of the unit suggests it could be true. All paper dielectric capacitors have a limited life regardless of what oil is used or how well sealed the package may seem. In fact, both the oil and the paper are organic and cannot have the durability and stability that plastic has.
This is obviously a hand-built amplifier with large solder connections that themselves are quite durable over time. However, as experts have warned us, solder is eroded by electron flow, so you may want to check certain connections that seem dubious.
Also, for completeness, you may want to replace 20% tolerance resistors with 5% or 10% as the former are cracked carbon and the latter would be carbon film. All carbon resistors exhibit distortion unique to carbon, but the earlier construction forms were quite poor and lacked stability, and lacked even the accuracy of manufacture one expects today. These deviations introduce nuisance noises, snaps, and pops, that would simply be annoying especially after investing time and effort to restore an interesting amplifier.
Cheerio
That is an interesting circuit which should be very pedal friendly as is. I would suggest that you simply get it working first and have your friend assess the tone. It should be very "high-fidelity" as it was intended to be a clean public address amplifier, and not anything approaching the kind of tone altering beast modern guitar players use.
The capacitive-coupled bias supply alarms me. I am in agreement with KOC that such bias supplies should be replaced with proper transformer-generated supplies, and a true independent bias-set network applied with cathode current-sense resistors for the output valves. At the moment there is a mix of fixed-biasing with cathode biasing. This is not bad unto itself, and if I recall, Mr. O'Connor did that in previous versions of his Studio power amplifier.
Regarding the capacitors: I think it is safest to replace the ones that say "PMK" on them as the "P" may mean paper? I do not know this as fact, but the age of the unit suggests it could be true. All paper dielectric capacitors have a limited life regardless of what oil is used or how well sealed the package may seem. In fact, both the oil and the paper are organic and cannot have the durability and stability that plastic has.
This is obviously a hand-built amplifier with large solder connections that themselves are quite durable over time. However, as experts have warned us, solder is eroded by electron flow, so you may want to check certain connections that seem dubious.
Also, for completeness, you may want to replace 20% tolerance resistors with 5% or 10% as the former are cracked carbon and the latter would be carbon film. All carbon resistors exhibit distortion unique to carbon, but the earlier construction forms were quite poor and lacked stability, and lacked even the accuracy of manufacture one expects today. These deviations introduce nuisance noises, snaps, and pops, that would simply be annoying especially after investing time and effort to restore an interesting amplifier.
Cheerio
