I just bought a used Marshall DSL401 40W tube amp.

Great amp, except one problem. When reverb is on ANY setting other than zero, when switching from either Overdrive channel back to Clean there is this huge volume swell that at first sounds like feedback. It lasts a second or two and ends with a reverb spring type ring. Right away I knew it was a problem with reverb. I turned reverb to zero and switched channels several times (it only happens when going from Overdrive to Clean not the other way around). With reverb on zero there is no problem at all. But even with reverb at 1 I get this big noise. The first time it happened I thought I had the volume on Clean maxed out and was getting feedback. But the noise only lasts about 2 seconds and then the Clean channel was fine.

I am not a technical type at all. I can change tubes and bias an amp but not a whole lot more.

Does anyone here have any idea what would cause this and how to fix it? If it is simple I could do it myself (replace a resistor etc...)

Thanks in advance for any help you can offer.

Wes