Gibson Skylark GA-5 Weirdness

[flythair1]

Hello MEF Community,

I just bought a 1960-61 GA-5 (5Y3, 6V6, 12AX7) that has a couple of issues that I'd appreciate your opinions on. First it needs the filters replaced & a three prong power cord. I figure I'll go discrete with the caps. The originals are a 20uf & two 10's. I have 20's & 33's in stock, can I up the values to a 33 & two 20's?

After inspecting the wiring, I found a heater wire for the 12AX7 to be unsoldered but twisted to the 4-5 pins. There is also a ground line running through the 4-5 heater connection. This is the weirdness, as I have never seen this wiring detail before in servicing quite a few amps, Gibsons included. The schematic does not show this wiring. When I bought the amp, we plugged it in and made sure it worked and it did, but obviously needed caps. Another thing is that the 12AX7 is wired backwards from the schematic. This may have no effect but is kind of confusing.

The last weirdness is a wire from the tiny OT that is just hanging out with a stripped end but no sign of ever being soldered to. I looked in the archives for threads about the Skylarks and found one mentioning the OT's with extra wires and there is another wire connected to a feedback resistor then to the first filter. Should I just secure and insulate the hanging wire?

Thanks for reading and for any insight to this weird circuit. Here is a pic of the 6V6 and 12AX7 bases. The black wire is for the 4-5 heater on the 12AX7. The white wire is a ground line going from 4-5 to the ground side of the input jack???

Scott

[Chuck H]

It looks like the 12ax7 filaments may be wired parallel and backwards. It doesn't matter much since all the filament does is get hot. It may be a sort of hum reduction attempt. I don't see the bare wire from the OT in the photo. If it looks stock it may be a shield ground or some such since anything connected to the winding would cause obvious problems floating bare in the chassis. For that matter a sield ground could contact a voltage source and cause problems too. If the amp is working correctly just snip it flush and shrink tube it. You can change the filament winding to the stock arrangement if you like but it doesn't really matter to the tube.

[flythair1]

Thanks Chuck for your input.

The OT has 6 wires total with one wired for the feedback. The loose wire is just sticking up in the air and now that I have just checked, it only has continuity with both sides of the secondary so it must be another output tap. I'll go ahead and install the larger filter caps and see how it sounds. I suppose both cathode caps should be new also.

The grounds on this amp look kind of crazy, just not what I'm used to. The unused pins on the bases are full of tie-points. It seems like the main ground point is the input jacks.

Scott