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I never figured out what PGM5 did, and I forgot to note the chip numbers. I have some photos of the board though, I can get the numbers that way.
I did some more experimentation, and found that when I dimed the channel volumes, the ADC would clip horribly and make the reverb break up with a disgusting noise. I changed the gain structure to cure this (5x attenuation before the reverb, 5x more recovery gain) but now I get a lot of hum and hiss with the reverb control up full. Also the reverb sounds rough at low volumes. No amount of messing with grounding fixed the hum completely. I'll try an isolated supply instead of the present one derived from the heater winding, to see if that helps.
The amp is a channel switching non-master-volume design, with the reverb after the channel volume controls, so I guess it demands a lot of dynamic range from the reverb if it's to be usable at quiet levels as well as dimed. Adding a reverb send level control as well as the existing one (which is on the return side) would help with this, but spring reverb amps seem to get by fine with just one control.
Maybe spring reverb just has more usable dynamic range than digital reverb? Any thoughts on this? Maybe I could use some zeners or MOSFETs or something, to compress the reverb send signal at high volume. Or maybe the structure could be rearranged to put the reverb before the channel volumes somehow, but I can't picture how to do that. I think I have it wired similar to the reverb channel on the Deluxe just now, except the last stage before the PI is an anode follower that mixes the wet and dry signals, with a gain of 5 for the dry and 25 for the wet.
I have an old London Reverb lying around, and I'm thinking maybe I should just lose the DSP and steal the reverb pan out of that. Now I think about it, I could even gut it, build my tube project into the chassis and cabinet, and pretend I have a BFDR
I did some more experimentation, and found that when I dimed the channel volumes, the ADC would clip horribly and make the reverb break up with a disgusting noise. I changed the gain structure to cure this (5x attenuation before the reverb, 5x more recovery gain) but now I get a lot of hum and hiss with the reverb control up full. Also the reverb sounds rough at low volumes. No amount of messing with grounding fixed the hum completely. I'll try an isolated supply instead of the present one derived from the heater winding, to see if that helps.
The amp is a channel switching non-master-volume design, with the reverb after the channel volume controls, so I guess it demands a lot of dynamic range from the reverb if it's to be usable at quiet levels as well as dimed. Adding a reverb send level control as well as the existing one (which is on the return side) would help with this, but spring reverb amps seem to get by fine with just one control.
Maybe spring reverb just has more usable dynamic range than digital reverb? Any thoughts on this? Maybe I could use some zeners or MOSFETs or something, to compress the reverb send signal at high volume. Or maybe the structure could be rearranged to put the reverb before the channel volumes somehow, but I can't picture how to do that. I think I have it wired similar to the reverb channel on the Deluxe just now, except the last stage before the PI is an anode follower that mixes the wet and dry signals, with a gain of 5 for the dry and 25 for the wet.
I have an old London Reverb lying around, and I'm thinking maybe I should just lose the DSP and steal the reverb pan out of that. Now I think about it, I could even gut it, build my tube project into the chassis and cabinet, and pretend I have a BFDR
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