MKIV Hum / Buzz presence control induced

[austinlegro]

1994 MKIVB
Since I've owned it it has always had a little more background hum than I would have liked but I was noodling around on R1 and fiddling with the presence control last night and almost like a switch going lower than No.3 on the dial induced what can only be described as the same sound as touching the tip of your plugged in guitar lead, that deep throbbing buzz / hum.

Bizarrely (or logically if you know the insides of Boogies perhaps) turning down the presence control of either R2 or the lead channel, even though playing on R1, induced the very same terrifying buzz.

Thinking I'd better not touch those knobs again I carried on strumming away until I decided to add a little reverb. On turning the reverb knob the massive buzz came back and wouldn't go away this time.

Now, with channel masters and amp master at zero the buzz (almost identical to the touching your hot guitar lead) is there whenever the amp is switched from standby. No knob alters the buzz, no leads plugged in other than speaker output and the same buzz, just slightly quieter when using tweed or class A.

I'll ask the obvious question, has anyone ever had this problem or know of a solution?

..a problem shared and all that...

 

[Restless Rocks]

Take a look at your output tubes and make sure you don't see any or all of them having a bright red glow. Or "red plating" as we say. Even if you don't see redplating try swapping out the output tubes and the V5 phase inverter tube.

 

[austinlegro]

Oh dear.
Amp tech says output transformer goosed. 16v bias voltage?

I'm now just very worried..