Drop in digital reverb for a Silvertone 1484

[olddawg]

I just started restoring a 1484. The reverb module, as usual, has issues. I could probably kludge it into working again.... But.. I'm curious. These reverbs notoriously suck. Has anyone ever built up or drawn up a drop in digital reverb circuit on a perf board that could match the send and return parameters with no mods to the amp other than removing the old module? (I know, the module uses piezo elements) I'm aware that a low voltage DC supply would have to be used. A wallwort velcroed to the chassis would be fine. My apologies if this has been discussed previously.

[Enzo]

Schemo?

The problem here is levels. The output of the IC is probably enough for the recovery stage in the amp. The drive side wwill have tube signal levels, you'd have to drop them.

[olddawg]

Schemo?

The problem here is levels. The output of the IC is probably enough for the recovery stage in the amp. The drive side wwill have tube signal levels, you'd have to drop them.

Thanks for your response Enzo. I hoped you would chime in. Lowering the signal level should be too hard? Voltage divider? The problem I see is band pass, phase, impedance matching, and hiss. Unity gain op amp after voltage divider to drive the Reverb chip? Mental napkin block diagram...

[Leo_Gnardo]

Lowering the signal level should be too hard? Voltage divider?

Maybe drive your digi reverb with the signal at the grids of the 6CG7/6FQ7 that normally drives the piezo reverb, then you could leave that tube out entirely.

I'd think simple. Maybe back to back diodes at the digi's input to limit input signal amplitude so you don't wreck it that way.

[R.G.]

I did a paper design for something similar, up through the PCB layout stage. I was solving a slightly different problem, but it may help.

I decided that what the (guitar amp) world needs is a drop-in digital reverb that looks to the amp as much like a reverb tank as possible. Conceptually, you could unplug a tank, plug in the cables to the "tank" and go looking for a power source in the amp. That meant that I had to worry about input impedances, input levels, output levels, and power supply sources.

The BTDR-2 has an input impedance of a few k. So it needs an input buffer.
The BTDR-2 has an input level for lowest distortion and digital grak of about 3V p-p. So it needs two sets of clipping diodes to impart a soft overdrive sound by not letting the input go outside the linear range of the input digitizer.
The BTDR-2 has a gain of about one, so it has an output level of about 3V p-p, so it needs an output level change.
The BTDR-2 has an output impedance of about 200 ohms. Probably OK to drive a resistive divider.
The BTDR-2 needs about 100ma of power supply current. UGH. That means you either play games with filament doubler/rectifier/filter/regulators or put in a separate power supply something or other.

So I designed and stuffed that all onto a PCB. No biggie.

Well, except for the power supply. 100ma of anything except heater current is tough in a tube amp. Frankly, the best choice is probably to "capture" a quiet, well behaved pedal power supply inside the case, feed it 120Vac, and then regulate the 9V down to what the reverb needs. Simpler and less error prone than dinking with heater AC. A darker horse candidate is to feed heater AC to a small transformer that makes maybe 12Vac out of the heater and then rectify/filter etc that. Last choice IMHO is spending a lot of time trying to make low voltage DC out of not-quite-enough-voltage heater AC and solve the grounding problems.