Trying this on DIYAudio since I think there's more amp people here. 🙂
I picked up a dead late 60's Tremolux amp and am looking to restore it. It had a dead power transformer, so I swapped in a spare and it came back to life. It's essentially a Deluxe Reverb minus the Reverb.
https://schematicheaven.net/fenderamps/tremolux_ab763_schem.pdf
However it has a pretty pronounced 120Hz buzz on both channels, and I haven't been able to pinpoint what is causing it:
1) I measured about 2V peak ripple on B+ after the tube rectifier. All of the power amp voltages measure within spec, and the two tubes are balanced to within 0.25mA of each other.
2) With nothing plugged in to either channel, there is no noise, just a very slight 60Hz hum signature which is completely normal.
3) Plugging anything in to any jack and turning any volume up past 2 and you can hear the buzz. It's quite a bit above the background hiss and 60Hz hum. It's clearly visible on a scope. There's no way the amp could be used with a microphone in front of the speaker.
4) With a shielded 4.7K ohm input jack you can hear the buzz (which kind of eliminates the guitar as a problem).
5) All tubes have been exchanged with ones from a known good Deluxe Reverb and still no change in buzz.
I systematically went through the circuit:
1) Swapped in a filter cap board from a known quiet AB763 DR, that had new bypass and series resistors replaced. No change in buzz.
2) Confirmed all of the DC voltages at every plate, grid, and cathode.
3) Measured the values of all resistors (all within 10%).
4) No measurable DC leakage anywhere in the tone stack or coupling networks (all the controls work and there are no scratches when tuning any of the controls).
Starting at the end of the chain:
1) Bypassed both power tube grids to ground through a 1uF/630V film cap. Amp is silent.
2) Lifted the input to the PI (the 500pF coupling cap), and put the 500pF coupling cap right to 0V. Amp is silent. In fact, injecting a test sine wave directly into the 500pF cap and I hear a clean tone with no buzz sound (this tells me the issue is 'upstream' of the power amp).
3) Went back through the circuit, and bypassed all grids to 0V through the 1uF cap. So long as one of the grids is bypassed, there is no buzz (however it doesn't matter which one!)
4) Added 47uF/500V extra bypass caps across every gain stage, from the top of the plate resistor to the cathode. 4 in total. No change in buzz.
5) Measured 0V resistance from every point to it's ground reference on it's bypass cap, and measured nothing more than 0.1ohms, and low resistance from every 0V node back to where the PSU sends out it's 0V reference. Just to be sure, I reflowed every 0V node on the board, and also where it bridges over to the brass plate behind the pots: no change.
6) Bypassed the plates (pins 1 and 6 of every preamp tube) to ground via the 1uF: amp passes signal (although severely muffled), and there is no buzz.
7) Pulled the trem tube: no change.
8) Scoping with AC coupling on all PSU voltages after B+ shows quiet power rails.
Despite the buzz, it amplifies correctly, you can hear the guitar and all of the controls work properly.
I'm stumped!
I picked up a dead late 60's Tremolux amp and am looking to restore it. It had a dead power transformer, so I swapped in a spare and it came back to life. It's essentially a Deluxe Reverb minus the Reverb.
https://schematicheaven.net/fenderamps/tremolux_ab763_schem.pdf
However it has a pretty pronounced 120Hz buzz on both channels, and I haven't been able to pinpoint what is causing it:
1) I measured about 2V peak ripple on B+ after the tube rectifier. All of the power amp voltages measure within spec, and the two tubes are balanced to within 0.25mA of each other.
2) With nothing plugged in to either channel, there is no noise, just a very slight 60Hz hum signature which is completely normal.
3) Plugging anything in to any jack and turning any volume up past 2 and you can hear the buzz. It's quite a bit above the background hiss and 60Hz hum. It's clearly visible on a scope. There's no way the amp could be used with a microphone in front of the speaker.
4) With a shielded 4.7K ohm input jack you can hear the buzz (which kind of eliminates the guitar as a problem).
5) All tubes have been exchanged with ones from a known good Deluxe Reverb and still no change in buzz.
I systematically went through the circuit:
1) Swapped in a filter cap board from a known quiet AB763 DR, that had new bypass and series resistors replaced. No change in buzz.
2) Confirmed all of the DC voltages at every plate, grid, and cathode.
3) Measured the values of all resistors (all within 10%).
4) No measurable DC leakage anywhere in the tone stack or coupling networks (all the controls work and there are no scratches when tuning any of the controls).
Starting at the end of the chain:
1) Bypassed both power tube grids to ground through a 1uF/630V film cap. Amp is silent.
2) Lifted the input to the PI (the 500pF coupling cap), and put the 500pF coupling cap right to 0V. Amp is silent. In fact, injecting a test sine wave directly into the 500pF cap and I hear a clean tone with no buzz sound (this tells me the issue is 'upstream' of the power amp).
3) Went back through the circuit, and bypassed all grids to 0V through the 1uF cap. So long as one of the grids is bypassed, there is no buzz (however it doesn't matter which one!)
4) Added 47uF/500V extra bypass caps across every gain stage, from the top of the plate resistor to the cathode. 4 in total. No change in buzz.
5) Measured 0V resistance from every point to it's ground reference on it's bypass cap, and measured nothing more than 0.1ohms, and low resistance from every 0V node back to where the PSU sends out it's 0V reference. Just to be sure, I reflowed every 0V node on the board, and also where it bridges over to the brass plate behind the pots: no change.
6) Bypassed the plates (pins 1 and 6 of every preamp tube) to ground via the 1uF: amp passes signal (although severely muffled), and there is no buzz.
7) Pulled the trem tube: no change.
8) Scoping with AC coupling on all PSU voltages after B+ shows quiet power rails.
Despite the buzz, it amplifies correctly, you can hear the guitar and all of the controls work properly.
I'm stumped!
Please post a scope pic of the hum at the output (with scope input and time base settings).
The pattern might tell something.
The pattern might tell something.
Essentially you replaced the mains transformer.
Did you connect the filament centre tap to ground ?
Agree with post 2 and 3.
Did you connect the filament centre tap to ground ?
Agree with post 2 and 3.
The filament and HV secondary center tap is soldered to the chassis just as in the original (just to the right of where the PT mounts). The only other thing I changed was removing the death cap. Just to be sure, I also re-soldered the two wire jumpers that go from the 0V side of the two sets of cathodes over to the brass bar under the pots at the front. Still same noise. 🙁