Looking for amp rebuild advice please

[StratSlim]

I'v just relocated the page not long ago and need help rebuilding my GA 20 RVT
I know back a while ago on the old Amp page i had trouble getting advice for this amp. I just had back surgery and have been out of work for a while and should have some more time to get this thing up. I've had it for along time and ordered a weber speaker and some tubes about 8 years ago. Went through schematic problems "Gibson has many deferent tube configurations".
I also had trouble with NOS tubes going micro phonic and the R/T went out i got frustrated and shelved it and built a couple of Tweed kits "Deluxe/Champ which i love" butt Now i want to get the Gibby up and running.
I had been told some of the problems i was having sounded like speaker but where i had put the new speaker in with NOS tubes felt it was OK. After i got the Tweeds running i switched the new weber with the orig CTS and the amp sounded much better though i still didn't have reverb or trem. I have had the amp more than 35 years and know some components must be borderline. What should i do for a complete rebuild/restore on this amp. Which parts should i replace and what substitutes may i use. I have replaced tubes and recapped twice while i have owned this amp.
Sorry about the long wonder post.

StratManSlim

[52 Bill]

What can you tell us about the amp, is it working?, do all of the controls and effects work? Do you have the right schematic now?

What do you want to do, get the amp working right or rebuild it as a project? If the amp has real vintage value you will probably want to repair rather than rebuild, as the value will be an issue.

If you've owned it for 35 years and you've re-capped it twice already, it probably doesn't need caps again. But, what caps have been replaced? Filters and coupling caps or just the filters?

[StratSlim]

I plugged it in this afternoon after a few years idle and it's not bad. I let it run a good 5 hours and it remained stable . I have the choke/trans that the reverb connects to removed so it has no reverb. I remember the lug had broken from the windings and i tryed to repair it but couldn't i'll have to search for the part and the correct schematic.
The first schematic i had i got from Gibson online and it had the wrong tube layout i installed the nos tubes and it was screwed up i noticed the tube socket was wired incorrectly then discovered they had changed tube types mid model.
When i got that corrected i was having trouble with the reverb and trem working then not working and going microphonic. I swapped tubes all over the place till i got it all quieted then the reverb went out because of the choke. I'll have to replace that choke if i can find another (don't know if the schematic has it's value in the footnotes or not). The tubes and power caps are all that's been changed so what else would you recomend all caps and resistors?

StratSlim

[52 Bill]

There was a thread here just a few weeks ago about replacing the reverb transformer in Gibson amps. I think that the bottom line was that the commonly available Fender reverb driver transformer will replace it with no problems. You will need to add a two lug terminal strip for the output wires, as the Fender style has wires instead of the built in terminals that your original one had.

You should check any resistor that is in the power supply sections of the amp for signs of stress. These are the voltage dropping resistors and the plate resistors in the amp. If any of them are way off in value they should be replaced.

If the amp has any cathode caps, they should be checked, as they are one common source of problems. I'd also suggest checking all of the coupling caps for any leakage.

[J Martin]

I have found that the Fender reverb driver transformer is not a direct replacement for the old Gibson transformers. It will work fine, however, if you also replace the Gibson reverb tank with the Fender "AB" type of tank.

[StratSlim]

Thanks guys for steppin up.

I found the schematic on my desktop put it there when i setup my new computer after a big crash last spring. I'll have to look it over for possible replacment parts.
As i said it runs ok very quiet no hum or cracklin. I also replaced the rectifier circuit as the diodes exploded when i installed the wrong tubes. It has three 6EU7's where the schematic i had there were only two. I was tracing back from the diodes when i noticed the preamp tubes were not all the same pin out 6EU7's don't cross with 12AU7's at all. What do you think (i was considering rewiring all preamp tubes to 12--7 pinout as the 6EU's are rearbirds) ?

I would like to replace any resistors or caps that may change tone or present a threat to other components. This amp has always been way bright for my taste so this will be a consideration though i wonder how much drift there might be in values.

How would you go about checking actual values of each component by lifting one leg? Or should i just change them and check values as i go.
Can i get an accurate reading on the caps with a meter? Which caps/resistors should i go for.

Thanks again guys
StratSlim

[52 Bill]

I don't think that the 6EU7 is all that hard to find NOS, and Sovtek is making one currently with reasonable quality, so why bother to re-wire the for 12AX7.

Most parts should be testable in circuit, and I find that a good quality cap meter will cover most all of the commonly seen values in guitar amps.

Again, as far as other components to check/change, any cathode bypass caps, any leaky or off value coupling caps, and all of the power supply dropping resistors. The caps in particular will have tho most influence on the overall sound of the amp.