Squealing Tubes in Mark IV position

[daverose]

Hi folks. New to this tube diagnostics stuff. Need some help figuring it out. Run my new (used) Mark V though some high volume paces at band practice a couple of days ago. Having a problem in Mark IV mode only where if I turn the Gain up to about 3:00 or higher and the master volume past say 2:00 I get a high pitched squealing, even with the guitar volume all the way down. If I turn the Global Output Master all the way down it has no effect (other than lowering the volume of the squeal) This only seems to happen in Mark IV mode with the Bright switch on. If I switch to IIC or Extreme or turn the bright switch off it stops immediately. Also happens at all wattage settings. Unfortunately I like the Mark IV with the bright on sound the best. Any ideas?

 

[4406cuda]

Turn the master on channel 3 down to about 11am and turn up the global master to desired volume.