Premier Model 120 Blowing Fuses,,Many Fuses

[zozoe]

Good day all~ I'm writing on behalf of a dear buddy & phenomenal, award winning pro guitarist who has this olde as the hills(early 60's) Premier model 120 amp that is forever blowing fuses, most of the time upon turn-on. I hooked him up with a literal old geezer (but 2 yrs younger than me & the sweetest guy) who can fix & rebuild anything from an old Crosley radio to a large LCD, & modern day 400 wpc Phase Linear ss amp, circuit board, HW, & even fixed my ultra high-end SACD everything player that needed a $20 laser replaced. To date, he's built me almost 10 tweed killers made from old 40 & 50's mono power/P.A. amps,
w/or w/o schematics, & then custom tailor & tweak each project to my tastes,,, He's worked on that rather basic Premier 3 or 4 times, doesn't blow fuses on his bench, but my buddy gets it home, plays for a minute, sometimes an hour, & the amp invariably pops a fuse... No smoke or flames, just proper value fuses... Me & my genius tech are vexed, while my bud is super frustrated, & at his wits end!! Since I'm the blogger, I thought I'd reach out to all you folks who mostly know more 'All Things Amp' than I ever could handle. So, Premier 120, great sound, pretty as hell, but has it out for fuses. I know enough to almost be dangerous under the hood, & I could easily follow along with any suggestions or fixes. Sorry for the ramblin' post, but I had to give some background.
Thanks in advance, & stay well everyone!!
🕉

[R.G.]

There is a time and a place for rambling posts...

Short answer: there's a stickie on "blows fuses" debugging at the top of the technical discussion forum (that is, this one).
Shorter answer: I would start by replacing all the electrolytic capacitors in the beast. Electros simply wear out with time. If it then keeps blowing fuses, I'd use either a light bulb limiter to quit having to replace fuses so often - replacing a fuse stops everything, and it gets expensive. Or I'd use the "blows fuses" approach and combine that with a light bulb limiter.

[Stevem]

Have the output tubes been changed out and the amp then taken home by the owner?

It sounds to me like one of the output tubes has a thermal run away problem that may only show up when it’s hot enough.
Believe it or not my favorite way to tests for this is to play the amp with it sitting upside down.
If it then does not fail in the time frame it usually craps out it then you know it’s a thremal issue.
It could also be the PT going south when hot enough!

[Stevem]

Note that this amp takes a 1/2 amp slow blow fuse, not a fast blow type, and also with these cathode biased output stages if the bypass cap goes leaky that resistance acts in parallel with the bias resistor and lowers its value .

This will make the output tubes especially when hot pull far more current then they should.

I always test new bypass caps I use in output stages for ESR and many times over the last 38 years I was happy that I nipped that potential problem in the Bud!