Fender PA 100 issue

[Mathias]

Having a little work on a Fender PA100 and seeking some advice.

Amp is from a friend who bought this, I believe it was not starting up at the time of the purchase. He told me it keeps blowing the fuse.
Mains transformer was blown. Replaced mains transformer, recapped and new output valves. Sound is coming out of all channels.

However... I adjusted the hum balance pot and it went toast. I replaced it and once more upon adjusting it burns up.
It seems like some heater short is occurring. I measured continuity on the valves and no heater shorts are present.

What would cause the hum balance pot to go toast like that? Any advice on how to proceed is welcome.

Pink highlighter is what has been replaced so far. Green to point out hum balance pot.

 

[Di_Sousa]

Whats the wattage rating of the Pot you are using?

 

[heatwalk]

How long did you have output for before it went up?

Cooking those fender hum pots isnt as expensive as blowing a transformer but for the time being remove the hum bal and use some 100R resistors (or maybe an unmatched set?) to ground and monitor if those start to smoke. I know you said it was on adjustment but, initially, I would think that whatever the fault is would take out those resistors too.

Probably pull the power tubes first and idle it for a bit to see if the fault is in the preamp section before putting power tubes in and hooking up to a load. Continuity at the socket doesnt prove something isnt funky when current is rippin’ through tubes

 

[CJ]

Put in a new balance pot, leave the wiper disconnected and see if you have any DC superimposed on the 6.3 wind.
Measure from ground to either side of the pot.

 

[Mathias]

Whats the wattage rating of the Pot you are using?

I don't have any specs on the rating. It's a Fender replacement part 100 ohms hum balance pot. CTS made.

Put in a new balance pot, leave the wiper disconnected and see if you have any DC superimposed on the 6.3 wind.
Measure from ground to either side of the pot.

I'll take a measure, I've actually found pin 6 from the far right output valve being dangerously close to the heater pin 7.
To be sure... Leave the wiper disconnected as in leave the middle leg to ground disconnected?

How long did you have output for before it went up?

Cooking those fender hum pots isnt as expensive as blowing a transformer but for the time being remove the hum bal and use some 100R resistors (or maybe an unmatched set?) to ground and monitor if those start to smoke. I know you said it was on adjustment but, initially, I would think that whatever the fault is would take out those resistors too.

Probably pull the power tubes first and idle it for a bit to see if the fault is in the preamp section before putting power tubes in and hooking up to a load. Continuity at the socket doesnt prove something isnt funky when current is rippin’ through tubes

A couple of minutes. Good chance it was cooking slowly and the adjustment was a sort of (bad) luck on both occasions. I'll do some measurements and install pre valves one by one while keeping an eye on the dc / ac measurements. Thanks for the resistor tip.

Thank you all in advance!

 

[CJ]

Yes, middle leg,

You can also check for a leakage path from either side of the 6.3 wind to ground, do this with the middle leg disconnected also. Turn it off and stick an ohm meter from ground to the 6.3 winds.

if the pot instantly turns to toast then it is a HV problem. If it is a slow bake and smoke then it is a 6.3 problem.

Ampeg used a wire wound pot for hum balance.