Mark IV reverb noise

[Kirk A]

I've discovered, via my newly acquired FS-5L footswitch, that the reverb circuit on my new-to-me 1995 Mark IV has noise. The noise is directly proportional to the amount of reverb dialed in (on the control next to the footswitch jack). Could this be as simple as a bad tube, or are there other commonly known problems?

Thanks in advance,
Kirk

 

[Boogiebabies]

How long is the cable to the footswitch and what type of cable connects it
to the FS-5L?

 

[Kirk A]

The cable connecting the FS-5L switch was simply a 3 foot long speaker cable that I would normally patch to the Thiele 1x12 ported cabinet. Given your concern about switching the reverb off/on, do you you believe that the cable and footswitch are part of the reverb signal circuit?

It sounds as though the noise could be 60Hz, so the next thing I plan to check is the ground connection to the reverb tube's shielding.

Thanks.

 

[Boogiebabies]

The switching is a simple tip to ground, but it completes the circuit through the cable. I am wondering if the reverb circuit is picking up hum from the cable. If you have an A/B switch, I would try it with a guitar cable and see if it does the same thing. I am thinking you are creating a ground loop hum somehow.