Gibson GA 35RVT Lancer

[rgorke]

So, a buddy of mine had this amp given to him. Evidently it had been stored for 20-30 years. He tried it a while ago and he said it didn't work. He asked if I would take a look at it.

So, since he already turned it on recently, I thought there was no point in trying to reform the filter caps. I plugged it in and the tubes light up but once I turned it from stand by (with all the pots on zero) all I get is a loud BBBMBMMMM sound, almost like it is shorted somewhere.

The inside of the chassis looks ok with no obvious burned or leaking parts.

With all the tubes out, the amp is silent. With just the power tubes in, I get a hum, but now a loud BBBMBMMMM sound. I noticed putting the pre tubes back in, the sound comes and goes by wiggling the tubes.

Pre tubes are 6EU7 and 12AU7 and 12AX7.

Here is a link to the schematic:

http://www.gibson.com/Files/schematics/GA-35RVT.jpg" onclick="window.open(this.href);return false;

I am gonna try and find some replacement tubes from a TV guy down the road.

Thoughts?

[JimiJames]

Any development's in troubleshooting ?

[rgorke]

Here is an update...I am also attaching a picture of a similar amp so you can see the type of rats nest these amps are and the components.

I noticed a bit of discoloring below one of the filter caps (both large ones are 20uf/20uf. I put in a 16uf/16uf that I had and shazaam, major hum/buzz/screech is gone and it actually sounded pretty good.

The tubes are 6EU7 and 12AU7. I am debating whether to rewire the sockets to take 12AX7s.

There is another big filter cap that feeds the tremolo section. I will probably replace that one as well.

All in all at this point, not a bad sounding amp. It has lots of clean headroom. I turned it all the way up and barely had any distortion.

[rgorke]

Got this amp sounding pretty good. I replaced the electrolytic caps and rewired V1 to use a 12AX7 rather than a 6EU7 just because I didn't have any 6EU7s. Some scratchy pots but once they are set, it sound fine. I am having trouble with figuring out what the extra "on" switch does. I am attaching a link to the schematic to see if anyone can help. Seems to add a 220k resistor and a diode circuit...for more gain?

Tremolo works nicely but having some problems with the reverb which may not be a bad thing, not sure it is a very nice sounding reverb to begin with...

Well, here's another update an hour later....seems like V3 keeps blowing tubes. I had switched the 6EU7 from V1 to V3 and the tremolo worked but got a bad open loop BAAAAAA sound when I switched on the reverb. Now the V3 tube is Kaput. Now would a bad socket or something cause a tube to blow? Not popping or anything, just stopped working. Is there a pop or something when a pre tube goes? I don't have a tube tester to check the tube. The rest of the amp works great. Maybe I''ll rewire that socket for a 12AX7.

Thanks

http://www.gibson.com/Files/schematics/GA-35RVT.jpg" onclick="window.open(this.href);return false;

[Lefty Lou]

Does the 6EU7 have the same pinout as a 12AX7, I'm guessing it doesn't. I refurbished an old Gibson Les Paul amp for a friend years back. I restored the amp using all flying leads, just like it was originally wired (what a bitch of a project!). However, I had an account with ICC (International Components Corporation) and I could get NOS tested dual triodes like the 6EU7 for a song and a dance, and they lasted for quite some time too. I'd use the converters that Groove Tubes used to make for old dual triode tubes to newer dual triodes such as the 12AX7. When you swap out old dual triode tubes for newer ones, other than matching the pinouts, capacitor and resistor values in the preamp circuit might need to be re-calculated as well.

[rgorke]

The pinout for the 6eu7 is different than the 12ax7. I changed the wiring to match the 12ax7. A real pain with short wires and over zealous wire wrapping and then solder. Ugh!! Ain't pretty but sounds good.

[Lefty Lou]

The pinout for the 6eu7 is different than the 12ax7. I changed the wiring to match the 12ax7. A real pain with short wires and over zealous wire wrapping and then solder. Ugh!! Ain't pretty but sounds good.

You should have seen what I had to do to the Gibson Les Paul amp. Since I (couldn't get/didn't know) where to find discrete components with extended lead lengths to approximate the lead length of the Gibson components being replaced, I had to extend the length of EVERY lead and then use wire insulation or shrink wrap (whatever was available) over each lead so when I restored the flying lead wiring, no components shorted out on one another.

Your "On" switch I believe is a polarity switch. Just for shits and grins, see if you have a "relay switch" in the primary circuit of your PT. The very first Vox amp I ever worked on was an AC-50 that had a relay switch in the primary which limited current in the secondary. Being as I had never run across this scenario, I overlooked it initially and it proved to be the culprit.

[rgorke]

Ugh, I am beginning to hate this amp.... Now, after last night getting both channels running and the reverb and trem working. I put it in the case and now the "normal" channel doesn't work. I have resoldered V1 and tested continuity but the channel does nothing. The "Reverb" channel works. What might cause a channel to just stop working. I have switched tubes and done lots of chopstick poking. Argh!!!!

[Lefty Lou]

A partial short in the filament winding of your PT secondary maybe? Did you check for a relay switch on your PT primary?

[rgorke]

Nah, it was a bad tube. Ugh, what is the first thing you check? Not sure why the tube went....

When it actually runs, it sounds good, believe it or not. The reverb works well but starts to hum after about 20 mins. What do you expect from an amp nearly 50 years old?

[Lefty Lou]

I've owned and worked on several, and yes they are sweet little amps. When I get a little ahead I'm going to make a turret board for my Gibson GA-8 Discoverer which is a dual single ended 6V6 amp. To me it's a much better amp for recording and practice than a 5E3 tweed Deluxe amp. BTW, I'm glad you got the Lancer working to your satisfaction.

[rgorke]

Well I did, then I didn't, now I do again. Ugh, when you are dealing with a nearly 50 year old amp, little things go wrong. The RCA cable from the reverb back to the amp had gone bad and was causing a bad shorting noise. I had buttoned the chassis back into the cabinet and then BAAAAAMMMMMPPPP when I clicked on the reverb. Then as I was doing the famous chopstick poke the noise went away, then back. What a minute...what the heck...

So, yeah. works great now. May have done some "extra" work but sounds really good now. I also got some help from a guy on Youtube who did some mods to one of his Gibson GA -35 amps. NIce guy who was very responsive.

[youtube]http://www.youtube.com/watch?v=p2yFgikXrLo[/youtube]

[Lefty Lou]

Nice!!!

[rgorke]

One thing that just sunk in...it still has the two prong power cord, I should probably change that to a three prong. Never done that before on an old amp...time to learn.

[Lefty Lou]

One thing that just sunk in...it still has the two prong power cord, I should probably change that to a three prong. Never done that before on an old amp...time to learn.

Being that your "on/on" is basically a polarity switch, you won't need that anymore. Typically you can just run your earth ground to the chassis but you'll want to do some deeper investigation before you assume anything and wire it that way. I never bothered with the 3 prong cords on Gibson amps, I just restored the particular amp to spec that I was working on.