BF PR Clone Tremelo footswitch motorboating

[Grun]

I built a BF Princeton Reverb clone using the ab1270 schematic for reference and at this point I have all the bugs out, everything works, except when I try to turn "off" the tremelo. I don't have an output jack to use with a footswitch so I'm using a switch until parts arrive.

According to the schematic there's a point where two .01u caps and a 1M resistor meet that gets switched to ground to stop the tremelo effect. When I switch it to ground I get a motorboating sound. I've gone over the circuit again and everything looks like it's in the right place. The tremelo is working as intended, but it would be nice to be able to turn it off as it still works slightly with the intensity turned all the way down, which I think is normal for this circuit.

Can anyone give some advice on how to turn off the tremelo without the motorboating?

Schematic link:

http://www.ampwares.com/schematics/P...ERB_AB1270.pdf

[Grun]

Nevermind, I figured something else out. The 1M resistor going to groound from the grid pin has very little DC and most of the ac osilation. Moved the leads from the switch to ground out that resistor and problem fixed.

It would be nice to know why the stock schematic switch caused a motorboating sound though.

[loudthud]

The resistor to the cathode is intended to give the oscillator a little kick-start when the switch is opened. In some amps it goes to the bias supply. How is the startup of your? Does it seem slow?

[Grun]

It takes a second to start up but grounding the 1M resistor connected to the grid and a .01u cap is working to turn it off and on.

I have another issue with the tremelo I'm working on now, with the tremelo on if I turn up the reverb I get a thumping sound that gets louder as I turn up the reverb. At max Intensity the thumping stops, at that point I believe the Intensity pot is 0 ohms to a .1u cap, and I wonder if turning the Intensity pot down and adding resistance to that side of the .1u cap is creating a faint RC waveform that is causing the thumping sound.

Maybe the RC waveform is either being cancelled out or summed into the tremelo ocsilation waveform and coming back out through the NFB loop into the wet/dry mix circuit? With the reverb turned all the way down the thumping stops.

I don't have a scope (yet, saving up for one) but I'm going to try looking for odd bits of ac with my MM around those areas.

[Grun]

I found a spot on the tremelo circuit where the leads of my MM stopped the annoying thumping when the reverb was turned up, between a .1ucap and the intensity pot. I ended up putting an old 100k metal film resistor in series with a .1u cap to ground, problem fixed! Then the leads on the old resistor broke All of the resistor's I ordered for this build were carbon comp. I tried a 100k CC in where the metal film was and it didn't work. I took a break and thought about it for an hour, and came up with an idea that it might be shot noise in the CC resistors causing all this trouble. I scrounged up a 10k metal film resistor, installed it and it worked

I can't say it was CC resistor shot noise for sure but using a metal film has worked where the CC's didn't.

The next build I do I'll try carbon film in most of it with CC in key areas to try for a more vintage sound.