I have pretty much got the core of the amp - the clean and the OD - singing now. The clean channel has a very low sound floor, sustain, bloom, all there after playing with lead dress and some different component choices. That part of it is good to go and sounding great.
I have been playing around with reverb because I am getting something like a 50Hz AC hum whenever I turn up the Return pot beyond about 25%. This is amplified by the Master of course so testing this with the Master at a nominal 50% the hum starts to become apparent at around 25% on the Return. The Level pot controlling the amount of reverb works fine, is quiet, and has all the range of delay I need all the way to 100%. In the beginning it also had a HF component to the noise like white noise which with a couple of component changes was fixed. The tank is oriented in relation to the PT correctly, the LF hum is present with or without the tank hooked up though.
Things I've tried so far:
- Scoping the traces.
- Pulling the tubes.
- Tapping components listening for any microphonics.
- Adjusting component sizes.
- Lead dress.
- Relocating the reverb board grounds.
- Playing with the shielding of the reverb tubes
I couldn't locate the frequency of the noise with FFT. I get something like a small 150Hz spike but this is visible even when the probe is not connected. The scope is down to its minimum setting and probes on x1 for components after the coupling caps and x10 for the HV points. Likewise, I used the DMM to check that I wasn't getting any DC leakage from the caps - none, or at least just a few uV - nothing significant.
Pulling tubes
Pulling V5 (the mixer) eradicates the noise. Also tried swapping the mixer and recovery tubes with another 12AX7 but this didn't have any real effect - the 12AX7 was slightly noisier with a HF whitenoise. Didn't try pulling the driver (V3) or the recovery tubes (V4) but I did roll the recovery tube, with no difference.
Tapping components
I had been through the whole amp anyway as part of the earlier tuning, had found a couple of dry solder solder joints and replaced a bad cap in the PI, and on the cap leading to the Return pot (C64) heard a microphonic "thud". Replaced the cap and no difference.
Adjusting component sizes
Focusing on the recovery tube (V4), I had replaced the 1.2Meg snubbing/biasing resistor in the delta net at the plates for a Piher CF and this had reduced the HF component of the noise. It was further reduced (fixed) when I replaced the 22k grid resistor with a 220k. In my opinion, the tone is more 3D with a richer harmonic content with the 220k. These didn't affect the LF hum though.
Lead dress
I've repeatedly chopsticked the leads and tried separating and moving all the leads on the tube side and the pots side - no effect on the hum, during or after.
Relocating the reverb board grounds
Looking at the grounds, which (in the #060) I am convinced lead from the board to the isolated can cap case and then on to the first ground point near the input jack, I tried disconnecting these from the can and using an alligator clip lead attaching them to the first ground point. I understand can caps can be noisy, this one has its case isolated as well but it didn't make any difference if the rvb board grounds were attached via the case to GND1 or directly to GND1.
Playing with the shielding of the reverb tubes
Made no difference. I only recently saw photos from the bottom of the #060 chassis and thinking of the different orientation of the reverb transformer I am using, I listened for any difference with the tube shields on and off. None.
That I can't identify the frequency of the humming noise is annoying and could be down to my inexperience with the scope but it sounds like 50Hz. The heater wires are all in good condition and moving them around made no difference either. The only time I could make the hum stop was when I shorted the output side of the Return pot (the wiper) to the bus bar ground. So even if I haven't been able to correctly identify the source of the hum yet, it is definitely in the rvb circuit.
Presently, as long as the Return pot isn't turned up beyond 25% I could live with it and use the amp fine, but it is not much use with this limited range. The only thoughts I have now are directed towards the dry side of the signal and the mixing resistors on that side of the circuit. I've seen several evolutions of the reverb circuit but nothing quite like the #060 so it's hard to say if these have any bearing on the noise being generated. I seem to recall somewhere, I saw the output side of the mixer (V5) with a more isolated LNFB resistor in parallel with the mixing resistor. This isn't in the #060 though, so it's only a hunch that this will filter out any AC hum coming from the plate B+ in that location. It's coming via a coupling cap if it is, so there is little (if any) DC component to it. I'm just not sure that altering this part of the circuit would make any difference to the hum. A higher value resistor in a parallel LNFB net would certainly affect reduce the gain of that stage and that might be what it needs. However, it is a completely untested theory here.
I am stumped. Is it possible this is what the amp does anyway? That I am chasing problems that don't exist? Any advice/insight will be gratefully received - as always. Thanks
Stephen
