Premier Model 120 Motorboating Trem & Blowing Fuses

[zozoe]

Hiya all, & hoping all are well!! I have a buddy who has this ancient Premier Model 120 1x12 combo, and he brought it to my tech, who's not a musician, but can fix everything I've thrown at him.
ISSUE#1, is the vibrato, which is motorboating like crazy, with techman saying "that none of the schematics match the actual circuitry of your amp. The tremolo feeds into the volume control and blah, blah, blah"
ISSUE #2, The other day he used the Premier for an hour. It sounded pretty good except for some crackling noise that would come and go. At one point he tapped on the chassis about 2 inches from one of the 12AX7's and the noise stopped for another 30 minutes. Then all of a sudden the fuse blew. He put another one in and that one blew too. I'm pissed, because this tech has done relatively well w/modding my old 50's mono tube pa amps, & armed with schematics, he's able to make screamers out of them, but him not being a player, & I not being a tech, I need lean on him, & with my guitar handy, I don't leave until until it rocks.

Anyway, motorboating probable cause, & cure? W/O the proper schematic, techman is not pulling thru & if I had suggestions, I could lead him along. And the fuse situation is another problem. As an aside, the previous owner of the amp had done some unsanctioned changes to stuff, making everything so much worse.
Thanks in advance for all your assistance, & do stay healthy~
Kenny
P.S. to further help, 2-6v6, 3-ax7, 1-at7, 5y3
The pot codes read 140525,140534,140439?
Serial #247412

[R.G.]

Motorboating is classical gain-phase oscillation. It is usually caused by some combination of grounding problems and/or poor capacitors in the B+ grounding/bypass string. If the amp hasn't had its electrolytic caps replaced in the last decade, it would make sense to just do that. It might fix the motorboating, and is a good idea in general.

The fact that whacking the chassis made it quit screams "bad contact" at me, and probably a bad ground contact for one of the stages. Slogging through each and every ground connection might find it. ON older amps with poorly-thought-out grounding (like using the chassis as a ground bus) you can have oxidation under the nuts, bolts, bushings and standoffs over time. Removing them exhaustively and cleaning the metal where it makes contact will generally fix this issue. The real cure is a better grounding scheme, but that's a lot of work.

Blowing fuses is a whole other problem. That could be a bum filter cap finally shorting, could be a wire contacting chassis, shorted output tube, loss of bias, and on and on and on. I wrote a post on blown-fuse debugging that is listed in the sticky posts at the top of this part of the forum... the ones that nobody reads.

[Stevem]

Blowing the 1/2 amp fuse now sounds to me like the 7591 outout tube has a short now, and these can be intermittent!

[zozoe]

It uses 6v6's, but possibly the same issue.
Thanks all~

[TUBEDUDE]

1/2 Amp fuse seems a little small. I'm not surprised it would blow with the turn on surge.