Gibson GA-15 Goldtone - Easy Diagnosis?

I bought this Gibson GA-15 Goldtone/Trace Elliot Velocette for pennies on the dollar. The original owner had tried to sell it on Reverb and the buyer shipped it back for a refund after he apparently smoked it. Opening it up revealed evidence of a pretty significant flame out in the power section. After repairing some traces, this old guy was up and running pretty easily as demonstrated by my shitty guitar playing at the end of the video.

[@goodun2974]

For me, "Eighteen" came and went 47 years ago! Great tune; I love the original Alice Cooper Band. There's a video of them on KZbin playing "Black Juju" and several other songs from side 2 of "Love It To Death" on a small stage, like a high-school auditorium!

[@OhmicronIndustries]

Haha...Thanks for sticking with the vid long enough to see my lousy guitar playing. I love Alice Cooper and he still puts on a really great show 'til this day.

[@goodun2974]

@@OhmicronIndustries , I didn't care for "Welcome to my Nightmare" or anything that came after, but the original ACB was great. By 1979, Alice was getting falling-down drunk before going on stage (there's video on KZbin). He did get sober eventually but it seems he had to take up golf (ugh 😖) in order to do it, and I don't know which addiction is worse!

[@goodun2974]

PS, a somewhat unusual circuit, with signal ground lifted above the chassis ground; you hardly ever see that done in tube type guitar amps, although it's fairly common in pro audio gear, to precent ground loops. The way it's drawn out is strange; note that they didn't show heater voltage going to pin 4 of the 12AX7 tubes, only to 5 and 9....and one leg of the high voltage AC secondary feeding the fullwave bridge is shown as "zero volts", which is definitely not true!

[@OhmicronIndustries]

Yeah, not a particularly complicated circuit but definitely unusual as you stated. There were certainly some inconsistencies in the schematic and several places on the pcb that were unpopulated. I'd have to look at the board again but I wonder if pins 4&5 on the preamp tubes are tied together on the board?

[@goodun2974]

@@OhmicronIndustries , they would have to be in order to use both halves of the triode. Trace Elliott products were not known for consistency, reliability, or repairability. "Uh-oh, the infamous Trace Elliott bass amp is fucked up again". David Lindley, from an ElRayo-X gig around 1988 (bassist Jorge Calderone was using one). I'm amazed that replacement boards are still available, especially as Trace Elliot has been through several owners/distributors (Ovation, back in the day, IIRC).

[@davekeating.]

The first question I'd ask is, "Why did amp fry without blowing a fuse?" 'Cos it should have...

[@OhmicronIndustries]

Agreed. I'll never know for sure but maybe the buyer blew the fuse that it came shipped with (calls for a 1A) and then just replaced it with whatever he had on hand? Total speculation on my part, would love to hear any theories you have.

[@davekeating.]

@@OhmicronIndustries Increasing the wattage of resistors that burn out is akin to putting in a bigger fuse. The six watt screen resistors, R25 and R26, should be 1W or 2W, flame resistant. Trace Elliot never thought that someone might start upping or bypassing a fuse that keeps blowing - possibly due to a power tube “disturbed’ during transit? R32, the 4W screen dropping resistor fried, also R36, 1/4W resistor, which, according to schematic, is someway related to heater supply? Two 1W screen resistors would have stopped all that nonsense.

[@OhmicronIndustries]

​@@davekeating. I still can't get my head around how that heater circuit cooked. Interestingly, the original R25 & R26 were undamaged. Also of note, the schem lists R32 as 4w while the BOM says 6w. At the time the 10w was what I had but perhaps I should order a more suitable replacement. FWIW the amp has been working properly for at least a year now with no problems.