Amp hums gets worse with reverb turned up - mark iv A

[tonedover]

Mark iv A - 1990

So I have an annoying hum. It gets way worse when you turn up the reverb.

I have tried
- new RCA cables for the reverb tank
- unplugging the reverb tank
- brand new 12at7 in the reverb tube spot

Nothing helping so far

Suggestions?
Replace all the preamps?
Take it to a tech?

 

[mysterblues]

Sounds like tech time. Most likely a simple fix. Pretty sure in the IV there is more than one tube involved in the reverb section. Does the noise go away when reverb off?

 

[mysterblues]

This quote from another thread, hope this might send you on the right path :arrow:


Is it a Rev A or B? You can easily tell by the loop jacks: four jacks = Rev A, three jacks = Rev B. Reason being, the Rev B does not have a single dedicated tube for reverb; the duty is split between two preamp tubes.

 

[tonedover]

No unfortunately it's still present even with reverb at zero

 

[tonedover]

This quote from another thread, hope this might send you on the right path :arrow:


Is it a Rev A or B? You can easily tell by the loop jacks: four jacks = Rev A, three jacks = Rev B. Reason being, the Rev B does not have a single dedicated tube for reverb; the duty is split between two preamp tubes.

It an A.

#693 off the line, June 1990

It's all written in marker on the inside chassis

 

[mysterblues]

Bummer. well, a good tech should have no trouble. Gives you peace of mind,too

 

[BlackBoxy]

cables ground wiring problem

 

[DaveP]

Attach a ground wire to the sleeve of the RCA cables connecting to the bottom of the amp. Tie them together.
This should correct the issue. Sounds like a floating ground problem.
There is a MESA doc on this somewhere.

 

[ReverbIsANoun]

Mark iv A - 1990

So I have an annoying hum. It gets way worse when you turn up the reverb.

I have tried
- new RCA cables for the reverb tank
- unplugging the reverb tank
- brand new 12at7 in the reverb tube spot

Nothing helping so far

Suggestions?
Replace all the preamps?
Take it to a tech?

Is the loop ON or OFF? Kindly ensure the loop is OFF. Next, turn the reverb to 0. Still have hum?

 

[McBarry]

As above, and then disconnect the reverb tank briefly (not so good to run the reverb circuit without the tank) - any change??

Sounds like it's bench-time for some fault-finding though...