1965 Vox Cambridge Reverb footswitch problem

[jvm]

I have a 1965 Vox Cambridge Reverb with a strange noise when the footswitch is used. Sounds like someone running a blender on the same AC line! Only does it with footswitch connected and tremolo turned OFF. I just re-tubed, cleaned all pots and jacks, re-soldered any suspect solder joints. The amp works beautifully except when using the footswitch. I get this buzz saw when the trem is turned off with the footswitch plugged in. I used a stereo plug and jumper wire instead of the footswitch and had the same issue so the footswitch is not the problem. If the reverb is on, the reverb pot increases and decreases the level of the buzz. With reverb off, reverb pot has no affect. I can also stop the noise by turning the speed control wide open. Any suggestions?

[52 Bill]

There is audio carried on the reverb footswitch line, just like a Fender BF amp. The audio line is picking up the noise from the tremolo oscillator.

Try a separated cable like the one that Fender used on the BF amps, where the reverb cable is a separate shielded cable and the trem line is outside of the shield.

[R.G.]

Unless your amp has been re-capped in the last decade, it is telling you that the caps are wearing out and need to be replaced.

This is not terribly difficult on the Cambridge.

The Cambridge reverb was my first amp. Missed it. Bought another one about 10 years ago. Keeping this one.

Edit:
I realized I should elaborate. The reverb and tremolo circuits in all the Thomas Vox amps are similar designs. Both reverb and tremolo have an initial emitter follower which provides the "dry" signal for the effect depth pot, and drives a second transistor stage to work the effect section. The electrolytic caps in these circuits are critical to the proper function. I had a Beatle head that oscillated on its own until I tracked down and replaced the emitter cap in the second transistor of the reverb effect.

There is a dark horse here. It is possible that the power filtering caps are going high impedance and the oscillation is leaking through the power supply too. Same fix for all these.

Do not settle for replacing one cap. The next one to fail may be in a day, a month, or a year, but it will happen.

[52 Bill]

I think he has a tube version here. At least that's the schematic that he posted.

[R.G.]

Doh!

Sorry. Got solid state on the brain right now.

In my best archaic Emily Litella voice "never mind".

[jvm]

Yes, I have the tube version. I replaced all cathode bypass caps but have not done the main filters yet. I don't think a shielded cable will help as I have eliminated the cable and footswitch by using a stereo plug and jumper wire in its place. When the noise is present, all of the controls have varying degrees of affect.

[52 Bill]

I don't think a shielded cable will help as I have eliminated the cable and footswitch by using a stereo plug and jumper wire in its place.

Sorry, I didn't understand that in your earlier post.

Then R.G. could still be right in that it could be coming in from the power supply lines. If you have a filter cap of the correct voltage, you can parallel it across the sections of the can cap and see if the problem clears up.

Also watch the lead dress of the wires going to and from the trem controls. Try and move them away from any signal lines.

The fact that it only happens when a footswitch is used, should be a clue here. Maybe a bad ground on the footswitch jack?

[jvm]

I unhooked the circuit from the footswitch jack and used a jumper to switch the trem on and off. Same problem. I then started moving lead wires and discovered that if I pushed the V7-pin 2 grid lead from C20 against the chassis, the noise stopped. I replaced it with a shielded wire grounded at one and no more noise! Actually a pretty cool little amp. I don't see the 17 watts it was advertised as making. About the same clean headroom as my 10 watt Gibson GA9. Sounds very Voxy! Thanks for your help diagnosing the problem, 52 Bill and RG.