Gibson GA 40 T - Les Paul Amp - repair, overview and sound demo 

Yepp!

Thanks a lot! In fact, it’s a privilege to work on these old beauties and bring them back to life.

You are absolutely right! It sound gorgeous even with my good old Telecaster! These guys knew what they were doing, when they designed amps.

@@martinstubeampworkbench2299 I've got a Princeton and a Tremolux, both Tweeds. The Tele is my favorite with both of them, and I swear I can get any amount of headroom I want, and I can put the "edge of breakup" at any volume level I like. The whole thing is totally controllable by the pressure of my right hand. That's what tubes are all about. Pedals? What pedals?

@@jpalberthoward9 Yes sir! I agree! A Telecaster beats them all . The bridgepickup and the tone pot is all you need to get any sound you want. Pedals? Nono!!

@@martinstubeampworkbench2299 Have you ever listened to the interview with Steve Cropper? He said he used to have about 20 guitars, and he dragged them around with him to every session, but the producers invariably said "Steve, break out the Tele, it sounds the best!" So he sold all the other ones. He said that Teles sit in the mix better than anything else, and the clarity you get from them makes them cut through when they need to. The combination of a Telecaster and a Tweed amp is like peanut butter and jelly, or beer and pizza, or jeans and a T shirt. A complete and total natural perfect match.

This time I was playing a homemade Telecaster with a Mahagoni body equipped with Tonerider PU. Actually it’s the guitar you can see (and hear ) in the openings of my videos.

Mmmmmmh….. this Gibson amp is cathode biased. In my oppinion it makes no sense to calculate „backwards“. Here is why: different tubes have different current draw, even if there are the same type, brand etc. Tubes are individuals. Output Transformers have different resistenses, cathode resisters vary as well. So, to get a precise outcome, it’s better to know all these values. But there is a very helpful bias calculator on robrobinette.com. Check that out. Greetings

Hi there No, this version of the GA 40 has no tonestack at all, just a single knob to bleed of high frequencies . Simple, but all you need to dial in the sound. This simple solution was used on almost every Valco amp from the 50th and on the famous 5E3 Fender Tweed Deluxe. I never had a G45 Maestro in the shop, so I can’t tell, what kind of toneshaping Gibson did there. And yes, I think a solid wood cabinet does matter. It acts like an acoustic guitar, it resonates. Chipboard or Plywood doesn’t. And solid wood is in comparison to chipboard lightweight. Greetings

Yes, I did . Coupling caps , bypass caps and the lfo caps for the tremolo. I didn’t change the tone caps , they stayed behind the bord. Normally they won’t fail, because they never see any high voltage. But if your treble control don’t work, maybe you should check them.

@@martinstubeampworkbench2299 Neither bass nor treble are barely audible whether they are at 0 or 10. I’m playing through an old Magnavox large alnico speaker comparable (exact same as I understand) to a Jensen P12Q. Gave me a bit more headroom which this amp needed as I had been running a 60’s rola 15 watt alnico. Unfortunately the gold Jensen 4 x 8” speakers were gone long ago. It would get rowdy at 3-4 on the volume pot before I swapped to the P12Q. Glorious sounding amp… just would like a bit more flexibility to eq different guitars. I imagine it was a perfect match for the P-90’s of their day. I dread pulling that board. Has those old white Cornell Dubilier oil caps from what little I could see on the back of the turret board. Thank you as I think this is the only vid that I’ve seen posted of maintenance on this amp. Almost a dead ringer for the G-40.

Well, in some of these old amps, the tone circuit is not very efficient. But when there is almost no audible change , there is definitely something wrong.

Discharging the caps is a must! But it’s only a 15 minute video and I have to decide what to show, because a repair takes hours. I’m shure, every amptech discharges or at least checks the voltage before sticking his fingers into the circuit. The Gibson had a deathcap, but with the three prong cord I installed it’s obsolete anyhow. I removed it. Thanks for your interest!

@@martinstubeampworkbench2299 thanks for replying.