Marshall JVM410C Problem

serven · Mar 21, 2015

Hi All,

Having a funny issue the last 4 days with my JVM410C combo amplifier.

I'll switch it on, take it off standby and try to play but only a slight feint very soft heavily distorted tiny noise comes out the speaker. If you switch it off for a few minutes and back on again, it is fine.

I use both FX loops (First for GT-100 Multi FX in 4 Cable Method and Second for RC-300 Looper). Am I right in saying the problem must be somewhere in the pre-amp section because while it is giving this particular issue, if I play something pre-recorded out of my looper pedal it is fine?

What can this problem be and does it have to go in for repair?

Hasie · Mar 21, 2015

Maybe you have read the following already.

http://www.guitarforum.co.za/amps-speakers-and-valves/general-valve-amp-care-tips/

It might give you some clues as to what the problem is or what to do.

ezietsman · Mar 22, 2015

I had similiar symptoms with one of the amps I made. Mine turned out to be a dirty standby switch, causing the voltage on the valves to be way too low. Working the switch solved it for me. The other likely explanation is that one of your preamp valves are on its way out. If you have spares (usually 12ax7) swop them out one by one and see if the problem goes away. You can safely exchange them, no biasing necessary for preamp tubes, they're all cathode biased.