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. 🙁
Scope pic of noise, with a shielded 4.7k ohm 1/4 plug (it's just a Neutrik 1/4 plug, with a 4.7k ohm resistor soldered between the tip and sleeve, contained with the shell of the plug, with the open end covered by shielding tape). Volume on the amp is set to 3, which is when the noise comes out cleanly from the amp (and gets louder as you turn it up). Output is taken from the main speaker output jack connected to a 4 ohm speaker.
Scope pic shows HF oscillation, modulated by 60Hz and 120Hz.
Your scope has a zoom function. You could use it to see the HF and measure the frequency.
What happens if you disconnect the power stage NFB?
Your scope has a zoom function. You could use it to see the HF and measure the frequency.
What happens if you disconnect the power stage NFB?
So this was an interesting insight.
I opened the feedback loop (disconnect the wire from the output jack, and connect the 820 ohm feedback resistor to 0V instead). I sent a 1kHz sine wave at 100mV at the input and observed this across the speaker terminals (with the volume control on 3):
There is a lot of 'hair' on this signal at high frequency, even with no global negative feedback.
I then switched to a 1kHz square wave:
There is a MASSIVE overshoot on the output, up between 800mV to 1V.
So I lifted the input to the phase inverter (the 500pF), and injected the test tone directly into the capacitor, to completely bypass the preamp. Again, this is the output with the same 100mV square wave, directly into the pi input:
So there's more of a semblance of a square wave here, but still massive overshoot.
When injecting into the PI, yes, there is no audible buzz on the output (just a faint 60Hz signature that's barely audible).
Post 12 second screen picture : correct interpretation is not overshoot, but "treble" only, no reproduction of low frequencies.
Picture 3 : signal with strong treble enhancement and impaired low frequencies.
Picture 3 : signal with strong treble enhancement and impaired low frequencies.
This amp is completely stock, other than the fresh PSU bypass caps. Winding the sine frequency down I hear a clear tone down to about 60Hz.
Any idea what would cause such an effect? I quickly checked a functioning Deluxe Reverb, and it can reproduce a square wave with only minor overshoot, and this amp is essentially the same circuit. I've never seen an amp producing such a whacky square wave response, especially with the NFB lifted.
As far as I can see you opened the feedback loop or something like this and so the amplifier characteristics will change, as seen in your scope pictures.
Without reading everything again I seem to remember the hum problem was located in the input stages, not in the power section, so why would your attention change to this part of the circuitry ?
Without reading everything again I seem to remember the hum problem was located in the input stages, not in the power section, so why would your attention change to this part of the circuitry ?
After re-reading I tend to think that the buzz is not produced by the amp itself - especially as the amp is silent without anything connected to the input.
What looks like oscillation in the scope pic could be switching noise from a nearby SMPS (switching power supply).
I would try the amp in a different place.
What looks like oscillation in the scope pic could be switching noise from a nearby SMPS (switching power supply).
I would try the amp in a different place.