Ampeg Reverbrocket R-12RT Tremolo

[rocknronnie]

I have a 1965 Ampeg Reverbrocket R-12RT amp. The Tremolo does not work. It seems all the caps are fine. The resisters are OK. I don't think this has a tremelo oscillator. Does anyone have any experience with these old amps.

[Tom Phillips]

Ron,

Yes, it does have an oscillator. However, it doesn't use a Light Dependent Resistor (LDR) to modulate the signal.

The foot switch is hardwired to the chassis and is needed to turn the oscillator on. Is it still there and working properly?

There are different versions of the circuit. One version is posted at http://www.schematicheaven.com/ampeg...ocket_12rb.pdf
(Note: First check that your amp uses the tubes shown on this schematic. If there are differences you have a different version of the amp) In the schematic listed above the second half of V1 (a 6SL7) is the tremolo oscillator. You need to start troubleshooting there and verify that the oscillator is running.

Tom

[rocknronnie]

Ampeg Reverbrocket R-12RT Tremolo

Thanks for the reply. I don't have the amp the schematic showed. This is my amp here It has 7868 tubes in it.
http://www.schematicheaven.com/ampeg...ocket_12rt.pdf

I know on this amp the tremolo circuit in in 1st half of V2(tube 6U10). I don't know where the oscillator would be in this circuit. Like off pin 5,6 or 7. It does not have a foot switch (It is missing) but I have jumped across there and the tremolo comes on for a second then fades away. I have a new tube in it so it's not the tube. The caps seem OK. Resisters tests OK. What next? There is a 40uf filter cap. Might it be bad.? I don't have one to jump but I have ordered one for a different amp build. Thanks.

[Tom Phillips]

Just on my way out but V2 is the Tremolo Osc. The Cs & Rs between pins 5 and 7 and the associated parts form the osc components.
Most likely problem could be weak tube or drifted parts values.

[Enzo]

Joe Piazza strikes again. He redraws the schematics but leaves out the part numbers so we have nothing to refer to in discussion.



You say all the caps are fine, then later the caps SEEM OK. WHat did you do to test the caps?

Your osc is pins 5,6,7 of V2 as Tom said. Those three .05 cap in the feedback loop are always suspect in a trem that doesn't move. Did you determine they are not leaky? Can't tell by looking. Is that a fresh 25@25 cathode cap?

Is the speed pot 100%? an open there will kill the motion.

And get a scope or AC voltmeter on pin 5. That is where the oscillation should be. it might be there, and the problem at the point where it is injected into the signal path. That is,those sections of V1, down and to the right of the trem osc.

[J Martin]

Don't waste time checking the oscillator caps, just replace them. That circuit is very sensitive to leakage.

[Enzo]

I agree with you, I was wondering what he might have done to determine they were OK. I suspect mostly just assume.

[J Martin]

Yes, Enzo, I was wondering also, but it didn't seem to be worth a long discussion.

[rocknronnie]

Caps

I will just replace the three .05 caps. I tested them with a DVM capacitance tester. They were off a little.I know that doesn't detect a leaky cap. Thanks for the help.
To respond to Enzo. Pin 5 voltage 308 volts holds steady. New cathode 25/50 cap. Speed pot %100.

[rocknronnie]

I replaced the caps and bingo it works great. Thanks for all the help.