Vox AC15TBX Noisy Reverb Circuit

[AndyinNYC]

Hi - new to the forum so be gentle …

This is one of the English Korg-era AC15’s made by Marshall using the green PCB’s. Overall it’s fairly quiet, just a slight hum - but when I use the blend knob to introduce Reverb that hum gets significantly louder and even buzzy.

Does anyone have any experience of this type of problem and what I should be looking for? I know these amps had ground Loop issues so perhaps that’s it but wondering if there’s anything obvious I’m missing? I’m an amateur but can do basic repairs, and failing that I have a great amp tech.

Appreciate any help!

Cheers

Andy

[nosaj]

Check for DC on the reverb input as you blend it? If you get some you have a dc blocking cap failing.

nosaj

[J M Fahey]

Reverb tanks hum and buzz.

Maybe itīs unbearable in a silent room, not that much in a live environment.

If removing tank return RCA plug and shorting the female RCA jacks kills hum, then itīs coming from the tank, not an "electronic" problem.

Just for full confirmation, unscrew tank and move it around, any hum/buzz change?

The pickup coil is VERY sensitive to external magnetic fields, specially coming from power transformer, you might find more silent position.

[AndyinNYC]

Thanks for the responses, much appreciated.

The buzz was still there after I disconnected the reverb tank so I assumed it was electrical. I ended up putting some better quality ECC83 tubes in there, including the ones responsible for the reverb, cleaning the connectors to the reverb tank and the tube connectors and it is MUCH quieter. I’m assuming it was a valve on the way out it a a poor quality valve. It’s a good point about bedroom volume vs stage volume, but at least now I won’t get from complaints from the sound guy.