Help me troubleshoot a hum in my Mark 4..

[billyg121]

i need advice on isolating a hum issue with my mark 4 version a.it seems to be a loud 60 cycle hum,of course its much more noticable on the lead channel.ive swapped guitars,cables,disconnected my pedals..etc.the hum does disappear when i unplug the guitar cable from the amp...not sure if that means anything.you guys have any trouble shooting ideas?

 

[swbo101]

How old are your tubes? You may want to look in that direction.

 

[stokes]

Probably a noisy V1 tube.It goes away when you unplug the guitar cable because the grid to V1 is grounded at the input jack,basically taking the tube out.

 

[GD_NC]

I had a similar problem last year with my MkIV. It was caused by a bad pentode/triode switch.

 

[billyg121]

awesome.that gives me a good place to start.i will first check the v1 preamp tube.