1969 Gibson GA5-T Amp PS Caps

[texstrat]

Before I moved forward replacing the old ps capacitors in my Gibson amp, I wanted to make sure there wasn't anything beyond what is covered in this article that I should pay close attention to, http://www.netads.com/~meo/Guitar/Amps/ ... scaps.html. It seems fairly straight forward, but being new to this, I thought to ask more experienced members.

I am using the Sprague Atoms, 20, 10, and 10 500VDC.

Here is the existing layout. My plan is to locate the new caps where the old cap was. And of course to remove the 22uf cap on V3.

[Phil_S]

Figure out how you are going to secure them. Don't let them float. If you are not gun shy to drill a hole in the chassis, adding a terminal strip to hold the + leads will allow you to make them into a "bank". All the - leads can be tied to one point -- where ever the ground wire from the existing paper cap is located.

If you add a terminal strip as the intermediate tie point for the + leads, then you can clip the wires from the old can cap close to the old cap and bring those to the terminals. IMO, this will make it easier to do the job, as you won't have to disturb the business end of those connections.

If you don't want to drill the hole for mounting a terminal strip, Miles' advice to attach them to the old cap is ingenious. If you have the room, it's a good strategy. Personally, I would drill out the rivet on the retaining band and remove the old cap. The hole that gets left behind is not in a good place for mounting a terminal strip, unfortunately.

[Dr-Joned]

You also need to switch the black and white power wires from the new cord. The way it is wired, you are fusing the neutral wire, which will not protect anything if it shorts to ground. The red from the power switch is ok.

[texstrat]

Figure out how you are going to secure them. Don't let them float. If you are not gun shy to drill a hole in the chassis, adding a terminal strip to hold the + leads will allow you to make them into a "bank". All the - leads can be tied to one point -- where ever the ground wire from the existing paper cap is located.

If you add a terminal strip as the intermediate tie point for the + leads, then you can clip the wires from the old can cap close to the old cap and bring those to the terminals. IMO, this will make it easier to do the job, as you won't have to disturb the business end of those connections.

If you don't want to drill the hole for mounting a terminal strip, Miles' advice to attach them to the old cap is ingenious. If you have the room, it's a good strategy. Personally, I would drill out the rivet on the retaining band and remove the old cap. The hole that gets left behind is not in a good place for mounting a terminal strip, unfortunately.

Yes I was planning to remove the old cap. My thoughts were to cut the leads on the existing cap close so that I could solder the leads to the individual caps and soldering the (-) leads together, connecting to the common ground.

You also need to switch the black and white power wires from the new cord. The way it is wired, you are fusing the neutral wire, which will not protect anything if it shorts to ground. The red from the power switch is ok.

Why would the tech wire the new cord this way?

[Phil_S]

We have no idea what that tech was thinking when he wired to neutral to the fuse, but the Dr is right and it needs to be corrected. Think about this:
plug(line) > fuse > switch, that means when you plug it into the wall the fuse needs to be immediately in the hot leg ready to do it's job. If you fuse the neutral, it is the last thing in the line feed circuit, rendering it ineffective for what you intend to protect with it. You are trying to protect 1st the house and 2nd the amp. fusing the neutral does neither.

As for the caps, maybe I'm not clear on your plan, but I recommend against soldering the negative leads together and then extending that. Just solder all three to the same point - and see if that point can be the ground point. I think it is best to make this as simple as possible circuit-wise. If you must make a junction, please do that correctly and use a terminal strip or something else designed for that function. I would not bundle them up and apply solder, which is what I'm afraid you intending. Solder is not meant to be used as a connector. Good practice is to make a proper mechanical connection and apply solder to make sure it stays together.

If you don't want to drill a hole, you can get a terminal block with screw terminals and a bead of high temperature silicone (not bathroom grade; automobile grade) goo to use as glue to stick it to the chassis. I'm attaching a picture of one kind of terminal block.

[texstrat]

Similar to the way the negative leads are wired to the terminal strip:

[Phil_S]

It's always hard to say when the item isn't in front of me. That looks like a very nice way to do it.

[texstrat]

Unfortunately, mail order is the only way to go to get the terminal strips and shipping will cost far more than the item purchased. So, until I can place a large enough order, I can pick up a terminal block locally. Since I have one negative lead coming off the existing ps cap, would I connect the negative to one connection point, on the terminal block, and then use jumper wire to tie in the other two cap leads?

[Phil_S]

Yes, use jumpers with the white terminal block. Make sure it is high voltage rated.

You should be able to get old style phenolic terminal strips or modern blocks at Radio Shack: http://www.radioshack.com/search/index. ... Strip&sr=1

[boldaslove6789]

I did a cap job on the same amp and just took the existing clamp and drilled the rivet out, took the 3 new caps & soldered the - together and put the new 3 caps together in the clamp zip tied them (just for shts and gigs), bent the + leads away from each other, then secured the clamp back 2gether to the chassis and soldered the + to their places. But that's just me.

[martin manning]

Good tip on the terminal strips from RS, Phil... I didn't know that they carried them.

It would be nice if the three new caps could be bundled and secured with the existing strap, but I'd be surprised if they will fit, particularly if they are Atoms. Regardless of how a bundled arrangement is tied down, doing that will require splicing wire leads onto the individual caps, and I wouldn't be above doing that. A good mechanical connection can be made by twisting the lead(s) and wire together. As far as the ground point goes, I'd definitely take it back to the same place that the multi-cap was grounded. Using terminal strips would be a better solution to mount the new caps, and period correct, even.

What I'm waiting for is to see the ripple voltage drop down to something reasonable, and for the tremolo to start working again.

[Phil_S]

Good tip on the terminal strips from RS, Phil... I didn't know that they carried them.

Stock on old timey stuff in RS is nowhere near what it used to be and things continue to disappear from store inventory. I think I saw some of these terminal strips recently when I went in to buy solder. I like the 4% silver no lead stuff, though I know many are wed to the old 60/40 lead based solder. I agree the silver stuff doesn't wet like the old stuff, but it's still OK.

[texstrat]

Well I corrected the power cord issue, thanks to Dr-Joned for catching that. And I installed the new power supply caps using the terminal strip method to tie the common grounds together as suggested by Phil_S.

Here are the new voltages which did not change much. The values in parenthesis are voltages prior to making the power supply cap change.

V1
Pin 1 - 122VDC (127VDC)
Pin 2 - 0 (0)
Pin 3 - 1.13VDC (1.15VDC)
Pin 6 - 112VDC (110VDC)
Pin 7 - 0V (0.3mV)
Pin 8 - 1.02VDC (1.05VDC)

V2
Pin 2 - 9.8mV (9.8mV)
Pin 3 - 8.10VDC (8.03VDC)
Pin 7 - 264VDC (264VDC)
Pin 9 - 260VDC (258VDC)

V3
Pin 1 - 258VAC (260VAC)
Pin 6 - 258VAC (260VAC)
Pin 7 - 284VDC or 113VAC (284VDC or 129VAC)

V4
Pin 1 - 52VDC (51VDC)
Pin 2 - 11mV (8mV)
Pin 3 - 1.13VDC (1.17VDC)

There is a bit of a hum now and maybe that has to do with lead placement. I tried the tremolo and got nothing. So do I need to replace all of the other capacitors?

Here are some pictures of what I have done so far.

[martin manning]

Try swapping V1 and V4 and see if the 52V you are measuring at V4's plate changes... I can't see why that should be that low.

Do you have any spare tubes to try?

Re the hum, you have grounded the power supply caps twice: once at the terminal strip and again with the black wire. It would be better to not use the ground lug on the terminal strip and just use the black wire, which goes directly to the center tap of the PT secondary. Can you move the cap wire on the ground lug of the terminal strip to one of the other lugs and move the jumper wire so that it does not connect to the ground lug?

[texstrat]

Try swapping V1 and V4 and see if the 52V you are measuring at V4's plate changes... I can't see why that should be that low.

Do you have any spare tubes to try?

Re the hum, you have grounded the power supply caps twice: once at the terminal strip and again with the black wire. It would be better to not use the ground lug on the terminal strip and just use the black wire, which goes directly to the center tap of the PT secondary. Can you move the cap wire on the ground lug of the terminal strip to one of the other lugs and move the jumper wire so that it does not connect to the ground lug?

I do have two new 12AX7's I can use.

Regarding the hum, because I have attached the terminal strip to the chassis, that is considered the first ground. The black lead from the terminal strip to V3 pin 7 is the second ground. To understand what you are suggesting take the terminal strip off, connect the three cap leads with a piece of wire and solder the black lead to that? If that is the case, that is what I was planning to do, but Phil suggested using the terminal strip in lieu of tying the three leads together. Anyway, let me know.

Thanks.