Mark IV hum problem - remote diagnosis needed

[avoelp]

I have a Mark IV B which has recently developed a hum problem that i want to eliminate (naturally ;-) ). Here ist what I have observed so far:

• - With the amp turned on, the hum appears even when no cables at all are connected.
  
  - The volume of the hum is dependent on the output level control.
  
  - The volume of the hum does not depend on any of the 3 input hgain controls.
  
  - With channel Rhythm 1 turned on, the volume of the hum is also dependent on this channel's master volume control.
  
  - In the Rhythm 2 and Lead chsannels, the volume of the hum is independent of the channel's master volume control (the MV controls increase or decrease the noise but not the hum).
  
  - Finally, the volume of the hum also depends on the reverb level. With the reverb level at zero the hum is gone, regardless of all other settings.

What could be the cause for this issue - and the remedy?

Thanks for your suggestions!
Andreas

 

[bgh]

Have you tried replacing the v3 and v4 pre-amp tubes? One half of v3 is used for the reverb return and one half of v4 is used for the reverb send. If you have some spare 12ax7s, swap out the v3 tube and see what happens. If the hum persists, put the original v3 back in and swap out the v4 tube. Unless one of the spare tubes is bad too, that will let you know if either v3 or v4 is causing the noise.

 

[bgh]

To the OP,

Curious as to whether or not you were able to get this issue resolved. And, if you did, what was the solution?