Mark III Died... Tube short?

[timedgar]

Hi, I'm new to this forum. Been a Mark III enjoyer since the mid 90s...

My Mark III Blue Stripe Combo has been acting funny... Seemed to lose volume... Crackles, etc.

Finally at practice a power tube was glowing red hot, made a dramatic flash and then the fuse blew. My first guess is to replace the tubes, possibly all 4 since they are all a few years old.

I decided to measure some voltages before I did this, to make sure it is not a problem with the amp itself...

Heaters on all PWR tubes are 3.5v AC (I thought it was DC heaters?)

The inner pair grid (pin 5) measures -59.8vdc
and the outer -48.3vdc

All the plates are at 497vdc

(All these measurements are with all PWR tubes out)

Do these sound ok? Are there other measurements I should do (e.g. screen grid)?

Thanks in advance for your help!

Best,

Tim from BK,NY

 

[Boogiebabies]

Hi Tim,

Measure the screen resistors to be sure they were not taken out or lost resistance valuye when the tube redplated. Other than that, new tubes, new fuse and it should be fine. The readings are o.k. with no load, but the bias readings are a bit high which may be due to a no load situation. Measure again with new tubes.

 

[kdorsey]

Hi Tim,

Measure the screen resistors to be sure they were not taken out or lost resistance valuye when the tube redplated. Other than that, new tubes, new fuse and it should be fine. The readings are o.k. with no load, but the bias readings are a bit high which may be due to a no load situation. Measure again with new tubes.

+1
A visual inspection is usually enough, because when they blow, they really blow!

 

[stokes]

What boogiebabies said.Be aware that when measuring the heater,you are obviously measuring from the pin to ground,you should measure from pin 2 to pin 7 and you will see 7vac which is right with no tube loading it down.Be careful when poking around in there.