Check out this tremelo circuit

[hasserl]

Got a Valco-Supro amp on the bench, came in for a non-functioning tremelo, I found a failed resistor in the circuit (one of the leads broke). I couldn't find a schematic anywhere for it so I traced it out and drew one up myself.

This is a bit different than the typical Fender circuits I'm used to seeing, either the roach circuit on the bigger BF/SF amps, or the bias wiggle as in the Princeton Reverb, or the brown face trem circuits as in the 6G12 Concert. This circuit uses a single triode that ties to the cathode of the input gain stage. The trem action is very nice, but without an intensity control it's a little much. The range of speed control is good though, allowing a nice slow tremelo action up to faster than I'd typically use.

Anyway, just thought I'd share it here. If you could add an intensity control to it it would make a nice way to add tremelo to a cathode bias build, using only one triode to get there.

[Chuck H]

It's still a bias vary trem. On an amp with more gain stages I think it would be better to inject the voltage from the trem triode on a later preamp stage cathode. I'm pretty sure I've seen it before, but I can't remember where.

The intensity control would be easy to add. Use a pot as the affected triodes cathode resistance and inject the trem on the wiper. You would probably have to use a 5k pot paralleled by an 18k resistor to get the 3.9k value on that amp. Since I don't know where to get 3.9k pots.

Chuck

[Albert Kreuzer]

I'm pretty sure I've seen it before, but I can't remember where.

Look at the Marshall 18W tremolo. Very similar.
And it has an intensity control

Cheers,
Albert

[52 Bill]

I guess Supro/Valco used that style of trem in a lot of their amps, I've seen it before.

Thanks for sharing the schematic. And for the sake of completeness, your schematic is missing a value for the treble input cap.

[hasserl]

Look at the Marshall 18W tremolo. Very similar.
And it has an intensity control

Cheers,
Albert

Yeah, you're right, very similar.

[hasserl]

I guess Supro/Valco used that style of trem in a lot of their amps, I've seen it before.

Thanks for sharing the schematic. And for the sake of completeness, your schematic is missing a value for the treble input cap.

I couldn't ID that value, the info on the cap itself was not readable and I don't have a cap meter that will measure the capacitance in place. If I was to build this circuit and wanted that feature I would experiment with values <.1uf.

What do you think of that odd rc filter on the loading of the input gain stage? I went over that numerous times to make sure I had it right. This is a ptp wired amp and it's not neatly done at all. I guess Valco specialized in the Rat's Nest school of lead dress. I had to poke and prod all the components to trace it out, all the while muttering WTF under my breath. But I'm certain that is correct.

The amp sounds very nice though, in a tweedish sort of way. Much more civil that a 5E3, yet not refined, still very 50'ish. I'm sure part of that comes from the 6X9 speakers. It's got a very sweet tone for low distortion bluesy work. The owner plays harp thru it and loves it.

[Chuck H]

It's got a very sweet tone for low distortion bluesy work. The owner plays harp thru it and loves it.

I built a small (13"x13"x8.5") 1x10 combo from the guts of an old VOM phonograph. Two 12ax7 two 6v6 and one 5y3. I voiced it for a harp player in my neighborhood and loaned it to him. He had it about six months when I started asking him about it. I had already been patient. After more than a year I still almost had to wrestle him to get it back. I guess low voltage cathode bias
6v6's sound good for harp.

I do remember adding a low pass filter to the front end of my amp when I voiced it for harp. Many harp mics are really bright. That 470p shunt on the input is probably part of why the Valco amp works well for harp too.

Chuck