The Amp Garage

It might interest you to see the schematic for my 54 GA77. This amp sounds amazing and especially when you jump both channels. Anyone of the 30 or so that have played it want to buy it. This model amp was made in true Gibson fashion with like 3 different tube complements this version is the most ra...

Just for fun I went through a few available GA series amp schematics (same-ish era) and found interesting differences to a seemingly similar tone stack. These might be drawing errors from Gibson or others (i used diagrams from the el34 vault). Omitted the values and focussed on the arrangement. Damn...

Could it have been drawn more clearly? Absolutely. But that's not the Gibson way! :mrgreen: Interestingly the later schems been drawn much nicely, at least to my eyes. So I think my tone stack understanding is right per the original circuit? The GA-Custom designated amp schemo (attached) is almost ...

GA50 aspirations Gibson GA50 Clone Noise Issue Pay close attention to the struggles and ultimate success in that second one. Some lessons certainly. Thanks Ian, I missed the second one somehow. I basically used your annotated schem for figuring out the component values and voltages, so thanks for t...

Thank you for the kind words...I generally figure no one is paying attention to what I do. I think folks do here. I've been lurking around for a while, especially looking for the Gibson discussions. I'm not so sure about using a 6N7 in place of a 6SN7. The 6N7 has a higher plate resistance and twic...

I hope this looks better now.

Oh my... You are totally right, I was looking at the T version :D The long wire goes to the other volume pot I omitted from the tone stack circuit as shown here on the correct schem, (thanks again!): https://www.prowessamplifiers.com/schematics/Gibson/ga50.pdf I'm planning to build a 2 channel versi...

Hey, I'm struggling a bit to piece together the mechanics of this tone stack. To be honest I'm lost. Could you guys elaborate on what is going on here? Attached the "cleaned up" schematics of the circuit too (might contain errors). The Gibson style schematics are a bit hard to follow, at least for m...

Hey,

Sure, please share photos and any info you are willing :) I still want to tinker with the circuit quite a bit. I like the amp as its is but i'm not 100% satisfied yet :)

Thanks a lot!

J.

I believe these mods are basically a sum of Rob Robinette's favourite mods. I don't have experience with this particular kit, but their kit designs in general vary in quality. If the PCB is designed and produced well, imo there is no reason to disregard them. Component wise most of the kits are on t...

Yeah, it is an interesting question. I'm not sure how this affected the amps and "tone". So far the tubes are not stressed but I'll keep an eye on the 6V6 just to be on the safe side, especially now when these bottles are expensive, even the old russian ones (hot little devils). Funny thing is I nev...

Gotcha. I'll keep experimenting with the circuit. Strange thing is I've seen different values on the PT secondary side ranging from 250-0-250 to 360-0-360. Now I'm guessing its all about biasing, still it seems odd. Anyhow 320-0-320 looks like a nice in between level.

Thanks again.

Those transformers could be Electro-Voice. I've seen them in Sound City amps, not sure but HW might have used them later on too.

Not surprisingly it was a bad solder joint on the V1B grid. Also the 500pF mica cap was faulty so I replaced it with a ceramic one (probably will use these in the future instead of the micas). No extra grid stopper or 1M reference resistors added. Might tinker with the stopper tho. Much better! Ever...

I would suspect a damaged pot and/or a bad solder joint. You shouldn't lose the ground reference at max volume if everything is wired as it should. Easy enough to measure, with the amp off, what resistance you get at max volume. Could be an oscillation, but my guess would be a bad connection. You c...