Supro vibroverb tremolo

[pontiacpete]

This amp is working well except for the tremolo. It's not working at all. There is no schematic that I have been able to find.
It uses a 6DR7 for the trem and it has this unusual looking light(bayonet style like the 47 commonly used in pilot lights)and 2 photo resistors mounted to a tube socket behind the chassis.
The light inside this thing was hooked up to the heaters of one of the other tubes. This looks like someone had tried to get the trem work in the past. The heater voltage gets the light lit but not that bright and it is not flashing. When checking the signal around the sockets like V1 I can see the signal is oscillating but i'm getting no tremolo.
Has anyone every seen one of these ?
Schematic anyone?
And are there any suggestions as to where the light should be connected?
Maybe its the wrong light?
Thanks!
pete

[Tom Phillips]

...The light inside this thing was hooked up to the heaters of one of the other tubes. This looks like someone had tried to get the trem work in the past. The heater voltage gets the light lit but not that bright and it is not flashing. When checking the signal around the sockets like V1 I can see the signal is oscillating but i'm getting no tremolo..

Pete,
I agree that someone screwed up the circuit during a botched repair attempt if the bulb is connected to the heater supply. The Valco company manufactured Supro Amps. Attached is an annotated Valco schematic which uses the 6DR7 tremolo circuit. It has three LDRs shown and your circuit presently has two per your post and I suspect that there were many design variations. The first task is to restore the light driver and maybe the Low Frequency Oscillator (LFO) as necessary. The attached schematic should help you do that. Note that many of the incandescent lamp tremolo circuits have some current flowing when the tremolo is OFF so you may see some faint glow at all times. However, when the trem is ON you should, of course, get strong pulsing light.
Cheers,
Tom

[52 Bill]

Thank you Tom for posting that schematic. I really could have used one a few years ago.

Pete, have you tried shining a light onto the photocells to see if that causes a change in the signal? Remember that this circuit is a vibrato (pitch shift) and not a trem (volume change) circuit.

[pontiacpete]

I pulled the photocells and did a light test and they reacted to my covering them with my hand.
Thanks for identifying this as a pitch shift vibrato. Didn't know that. When I was looking at the wave on V1 the wave was getting smaller and larger, but that would be a volume change. I think this means the caps in the oscillator are doing their bit without the light and photocell resistors.

It's really nice to have that valco schematic thanks Tom!

[pontiacpete]

The light bulb that is in there now is a GE1893. The voltage on that bulb is 14vac (64vdc not sure which to read). This is a 14v bulb, but the schematic says 28v.

edit: should I get one of these:
https://www.zoro.com/ge-lighting-min...64/i/G2571502/

[52 Bill]

The light bulb that is in there now is a GE1893.

The schematic sez the bulb is a GE1819, why not get one of those?

[pontiacpete]

duh,,, my light bulb isn't on Geez it was right in front me

Thanks for pointing that out.

[pontiacpete]

Sure enough it was just the wrong light bulb that was keeping the vibrato from working. The 1819 bulb did the trick. This amp is also wired a bit differently than the Valco schematic around the intensity pot but it works.
Thanks for the help!!
pete