Laney VC30 112 bias problem (?)

[MobileYankee]

Heh, I noticed this amp is quite a star here
so anyways,

this amps started making this weird small explosion noises and volume trouble and i suspected the power tubes.
according to the schematics the 7W 56ohm bias resistor voltage is suppose to be about 9v, when i checked it it was 20.7v, means every tube gets about 20W (!!).
its the amp from the studio i play in, the owner asked me to check it out, he mentioned that the tubes were never replaced and the amp is 5+ years old (and its a studio/rehearsal room amp, so it's working about 4-5 hours a day at least)
so do i need to replace the tubes only or there's some other things i need to check or replace?

thanks a bunch you guys!
peace

[Randall]

I would say those output tubes have paid their dues and are past retirement age, being under biased and used that much. And I would measure the actual value of that bias resistor. If it's drifted replace that also.

[Mick Bailey]

Whenever I get a VC30 in I always check the OPT for shorted turns. Those little transformers often fail in these amps, especially if the tubes have gone.

[Highanddry]

Whenever I get a VC30 in I always check the OPT for shorted turns. Those little transformers often fail in these amps, especially if the tubes have gone.

Sorry to resurrect this old thread. What does ‘check the OPT for shorted turns’ mean? I have a VC30 with V6 and V7 occasionally red plating.

[Highanddry]

QUOTE=Mick Bailey;n467401]Whenever I get a VC30 in I always check the OPT for shorted turns. Those little transformers often fail in these amps, especially if the tubes have gone.[/QUOTE]

Sorry to resurrect this old thread. What does ‘check the OPT for shorted turns’ mean? I have a VC30 with V6 and V7 occasionally red plating.

[Helmholtz]

When an OT has shorted turns this typically results in very low output.
Do you see redplating at idle?

Here's a method to test a transformer for shorted turns:
neontest-1.pdf

[Mick Bailey]

The reason I mentioned checking the output transformer is they're very prone to faults, especially so when there's an issue with the tubes. These transformers are built to a bare-minimum specification. If the amp has good volume, then the OT has likely survived, otherwise you'll get very low volume and excessive distortion. Best to get the red plating problem fixed before using the amp.

The problem with a shorted turn is you can't pick this up with DMM - a single turn is of such low resistance that when its shorted it's not detectable out of the hundreds or thousands of turns on the transformer primary.