Gibson Les Paul GA-40 amp

Hi guys, I've been around AK for a while but this is my first time in the musician's department. I spend a lot of time in tube audio and speakers. Any way, I found what is left of a Gibson GA-40, and have two sons that are guitar players, so I thought this would be a great project for us to do together. I have it mostly apart, and can see that all of the pots need to be replaced. Does anyone have any idea what the specs are on those pots? 100K, 500K? Audio taper or linear? I need the two volume pots, depth and frequency pots for the tremolo and the voicing control pot. I have a schematic, but it doesn't really tell me much about the pots. This is a combination of tube audio and DIY, I know, so if you think I might be better off in one of those, let me know. And while I'm here, does anyone know the value of the two metal can capacitors off the power supply? I'm pretty sure they're 10 and 20 uF, but what voltage? Thanks for any help.

Steve

Sorry I can't help you but I will watch this thread closely as I too have an old Gibson GA-?? that I found in the garbage behind some building. It looks like it needs the same kind of rebuild as yours. Good luck.

My schematic shows the values but not the tapers. Freq. 500k. Depth 500k. Volumes both 1M. The remaining pot (I assume "voicing") is in a place in the circuit that I've not seen before, and is 1M.

I'd certainly use audio taper for the volume controls, probably for the voicing control as well. For the tremelo controls I would have thought linear, but I checked a Fender schematic and though it isn't an identical circuit it is similarly an oscillator and they use audio for depth (intensity on a Fender), and reverse audio on frequency (speed on a Fender). I don't know if reverse audio is commonly available, might as well try a linear there. I think I once read about using resistors in parallel with a pot to approximate other tapers, but I don't know how to do so.

Good luck, should be a great amp. I'm not familiar with the 5879 pentode preamp tube or how that will sound, but the PI and output tubes (6V6) can be fantastic sounding.



PS: Power supply caps are 10 and 20, max voltage in the amp is 315V, so I guess a 450V cap would be typically used in that circuit.

Gibson Schematics: http://www.schematicheaven.com/gibson.htm

BTW: I have a sticky in this forum for schematics.

Thanks for the help. I knew I could count on AK'ers. I have the schematic, but am not quite good enough yet to know a pot from a regular resistor. The schematic shows eight different 1M resistors. I'm thinking that the two right after the input jacks are the pots, with the 51k resistors tied to them. Am I right? We are going to build it a new cabinet this weekend while we wait to collect the caps we need. We'll probably put the new pots in too. Here's another basic question - after I get it mostly disassembled and cleaned up, should I bake it to get it nice and dried out before I do too much more on it? I know I've seen that done over at the tube audio thread, and since this thing has been through hell it seems like a good idea to bake the trannies and choke to dry them out. If it is a good idea, what temp and for how long?

I don't have the schematic in front of me anymore, but the symbol for a pot is similar to a resistor except it has another element. Sometimes it's a connection to an arrow pointing at the zig-zag, sometimes it's an arrow shown crossing the zig-zag at an angle. The arrow represents the wiper, or moveable part of the pot. The same physical component could be used in a number of ways, and that will change how the schematic looks. It could have connections to all three terminals, or only to the wiper and one stationary leg.

Okay, the box is done and if I say so it looks great. My son (14) is really fired up about this project. I will post pix as soon as I can. Regarding the amp itself, we took everything apart last night and I'm now assembling the necessary caps and pots for the re-build. One tricky thing - the schematic shows 24 different caps, and my amp only has 21! I see from reading up on this that Gibson often changed schematics during assembly and didn't tell anyone, so I am going to reproduce what I have in hand and hope that works out. There are also a bunch of bumble bee caps with no values on them. Are these little black striped guys likely to be my .005 caps in the schematic? That seems to be right from where they fall in the schematic. Also there are four little cans that the paper has rubbed off of, these seem to be 20uF in the schematic so I'm going with that. If anyone out there knows anything about this amp I would appreciate the help. I can post as many pix as you like if it will help. I'd love to have this done by Christmas, it will make a hell of a present if I can get it to work.

One other thing. I pulled the choke and the OT, left the PT on the chassis, at least for now. Is there a simple, common sense way to check if these work? I may be an old veteran at listening to music, but I'm very much a newbie at re-building 50 year old guitar amps...

Steve

The bumble bee caps have a color code similar to resistors. Google it, I don't have any info on hand. .005 would normally be a ceramic disc cap, but I guess it could be the bumble bees. I see on the schematic that those .005s are in a network with 1M resistors, looks like some sort of tone shaping circuit for channel 2. I'm curious what that sounds like.

Post some pics, please!

Here are some pix. I cut the circuit board out, there was really nothing you could get at with it still in the chassis, and the little white tags remind me which wire connects where. I am going to end up replacing all of the resistors and caps, so this is really going to be a full resto. I'm concerned about the transformers (this thing was out in the rain and snow for many years), but I'm going to take my chances with them for now. They've been drying out indoors for the last six months, hopefully it will work out. Amazingly, the tubes all tested good.

First, a resistor question: some of the resistors are larger (1/2 watt? 1 watt?) and some are smaller (1/4 watt? 1/2 watt?). I have some new replacements for the larger ones, but in the smaller size (I sound like such a newb!) Is it safe to replace larger with smaller? I can be patient and just buy the right ones, but if I already have them...

Next, capacitors: It looks based on the schematic, that those little un-labeled cans are 20 uF, even though they are significantly smaller than the 20uF cap that I have in the power supply. I figure I should just get all caps rated at 450V, since as Jon pointed out the highest voltage is 315. I'm not going to force myself to get bumble bees since they are very expensive, they can be switched in later if we need to. Can I use ceramic discs, or some NOS mica caps I have, as long as the rating is right? I have a whole drawer full of those old domino looking caps, would I be hurting anything if I use them instead of bumble bees? Please be patient, I am new to this. I understand that the tone might not be exactly what the old Gibson had, but would I be hurting anything electrically? BTW, the larger bumble bees are .047, the smaller ones .0047. I'm still trying to work out what the big white Cornell Dubilier cap is on the resistor side, it says its a 25uF but that doesn't show up on the schematic at all. And I'm still missing three caps that I can't quite account for.

Help!

P.S. I like the way the box is coming out. It's going to look great wrapped in vinyl or tweed.

The small 20uF caps are cathode bypass caps. Probably 25V or 35V original ratings. You can get 50V caps to be safe, and they won't be any bigger than what's there now. You won't be able to fit 450V caps in those positions and that voltage rating isn't necessary.

I'd use mica caps for the .005 (or .0047) positions. They'll sound better than ceramic discs. The yellow Astrons are probably leaking by now (leaking DC, not physically leaking like an electrolytic might). I'd replace them all while you're in there. I prefer Mallory 150s for sound, fit, and appearance, but orange drops will be just fine as well. Use 400V caps for a better fit, 600V if you can't get 400s.

What does that big white C-D cap attach to in the circuit?

You can use bigger resistors throughout if you want. Are you using carbon composition resistors? I would recommend using carbon comps in the signal path to help preserve some of the vintage tone.

The big white cap is attached to one of the 5879 sockets. So now I'm thinking that it really is a .25, not a 25. Could a C-D cap of .25 be that big? There is a .25 on the schematic that ties to one of the 5879's (Channel 2) in the same place that a .05 Astron is tying to the 5879 on Channel 1. At least it looks like .25 on the schematic I have, it's a little blurry. Could it be it's supposed to be a .25 and someone put in a 25? Or is my 25 really a .25 with the . washed off? BTW, thanks again for the help. I'd love to get to a point where I can answer other people's questions someday.

I can't make out if it's a 25 or .25 on my schematic. It looks like there may be a decimal point, but it's right on top of a line. I can't tell if it's a point or just the bottom of the "two" running into the line.

I'm not familiar with pentode input stages like this, and they're doing some things very differently on the second channel from the first. I can't help you from a theoretical point of view because I don't understand that part of the circuit. From a practical point of view, I'd say it's probably supposed to be a 25uF, I can't make out the voltage on the cap -- 100V? I make this statement because I think it is unlikely that someone would have removed a small .25uF cap and replaced it with that big sucker. Nothing else in the amp looks like it's been changed, why would that have been?

Good luck, keep posting as you go along.

Here's an update on the Gibson Les Paul. I have most of the caps switched out and all the bad resistors. There is no question that I've lost track of some of the wiring connections, luckily I documented everything with pics before I started. Yesterday the new pots, plugs and chicken heads came (AES, very quick service) and I think I'm almost ready to start putting things back together. I also picked up a new 6SQ7 tube off fleaBay, the old one was rusted away. I should be able to flip the switch in a couple of weeks and see if any sound comes out!