No dc voltage

[kansas-terry]

I have a 1980's Mk 2 100watt lead, one of the output tubes burned a hole in the glass and the mains fuse blew. I cannot get any dc voltage reading on the standby switch, anyone have any ideas?

thanks

[neikeel]

I have a 1980's Mk 2 100watt lead, one of the output tubes burned a hole in the glass and the mains fuse blew. I cannot get any dc voltage reading on the standby switch, anyone have any ideas?

thanks

Start at the beginning!
Likely a tube went and shorted, blew a hole in the tube so maybe it was not the correct rated fuse (too high??) and blew late, that will have fried the screen resistor on that socket (maybe open or shorted) if you were unlucky it will have taken the OT with it.

From the beginning:
check all screen resistors (replace as needed), put in the correct rated fuses
output tubes out, on standby
Switch on, does the mains light come on? Check for ac HT on the diodes and dc HT other side of diodes as far as HT fuse. Next do you get dc HT throughought the amp (plate resistors).
Normally on that amp you will not get dc on the standby it is ac between the PT HT secondary and the diodes (look at schematic to check I am right).

Next step is output tubes back in with a load (speaker) and see what you have.

If not right post back and we will review.

[kansas-terry]

Thanks for the help, finally got it. Seems the master volume models are wired different then the jtm 45's.You will not get dc voltage at the standby switch until you turn on the standby switch. All the voltages came out okay no harm to the trannies.The man at eurotubes told me he had seen a few tubes do this. Installed new quad set and preamp tubes and the amp is ready to go!
The tubes were right at one year old, and the power spiked during the gig, that might have caused the failure.
Once again thanks for the reply!!!!!

Terry

[demonufo]

Bob at Eurotubes is one person you shouldn't really be listening to about ANYTHING!

If that bias supply isn't permanent from the standby switch, you need to rewire it to the hot side of the standby to protect your tubes (and PT) from further failures. BIG surges can occur.