Gibson GA-5T

[Kemmons]

New to the forum and to amp repair so please be patient. I am learning a ton from this forum and am happy to have found it.

I have a 1965 Gibson GA-5T Skylark amp that had a resistor burn up. I will be replacing it. I want to replace the original wax and paper capacitors. The schematic only says 20uF for the capacitors. My question is, how do I determine what voltage to use? Sprague 500V are $8.00 and 450V are $6.00. Can I use the lower voltage capacitor?

Also the schematic shows 4 20uF capacitors and I only have two paper ones. I don't think there was a hack job in it as they are in metal band holders that are riveted to the chassis and nowhere do I see that any bands were removed. There is one white ceramic looking rectangular part which can been seen to the lower right of the large capacitors. What is this? I am not at home so I can't give you the numbers. I can update this post tonight with a better picture of it and the numbers.

The schematic can been seen here: http://music-electronics-forum.com/s...ead.php?t=5465

Thanks for the help. Cool little amp when it is working right. Here are two photos (cell phone camera sorry).

[tubeswell]

450V will do the job (but I'd personally go for the Sprague Atoms because they are more robust) - they typically need to be about 30-50% above what the B+ voltage is at idle.

The little white rectangular box looks like a ceramic resistor to me (but the photo looks kinda blurry).

[Kemmons]

Gibson GA-5T Capacitors

Thanks for the advice. I will get the Sprague Atoms. I will try to get a better picture of this amp. I need to identify the other 20 uF capacitors (if they exist. Someone else posted about this same amp saying they only had three. I am wondering if this was a common mod to pull out one or two of if this was really how it was wired at the factory.

[52 Bill]

I need to identify the other 20 uF capacitors (if they exist.

Welcome to the board.

If you look carefully at the two large cardboard sleeved caps, you'll find that there are two caps in each of the cardboard tubes. They share a common negative connection (black wire) and have two red leads that connect to the positive terminals of the two internal caps.

And yes, the rectangular ceramic part is a power resistor, probably 5 watt.

I'd suggest replacing the burned resisitor and repairing the amp before doing too much else to the amp, or you may make the repair more difficult that it needs to be.

[Kemmons]

Thanks for the information. I noticed the two positive wires from each casing but didn't put two and two together. I just thought since it was 44 years old this was the way these were made. It makes sense to have two in each as it saves space.

So if I do wind up replacing these at some point, I will have to find a way to place four caps on the board. Shouldn't be a problem. Is it bad to have say two Sprague Atoms run side by side and actually touch casings? Obviously the leads shouldn't touch. And yes, I do know about the charges killing you if you ground them with your body. My wife would never forgive me if I got zapped so I am going to make a contraption to drain capacitors using a resistor. I have been reading up on safety (I like living so this is big on my list).

Thank you both for all the advice. I am planning on getting this amp running well. Then fix a SFPR I have and tweak my SFDR. After that I am going to tackle a Two-stroke DIY or a Ceriatone 18W TMB.

[andrew_k]

Thank you both for all the advice. I am planning on getting this amp running well. Then fix a SFPR I have and tweak my SFDR. After that I am going to tackle a Two-stroke DIY or a Ceriatone 18W TMB.

Careful, it's addictive! I started very similarly, replacing the filter caps on my '78 vibrochamp about 6 months ago, now I'm on to my second "from scratch" build and trying to narrow down what build 3 will be

To answer your Q, there is no problem with cap bodies touching. It's also quite common to use silicon to attach them to the board/chassis and each other. That way their weight is not pulling on the solder joints while in transit or if the chassis is a "tubes up" style.

[tubeswell]

FWIW I prefer not to use silicon to glue the big caps in. I'm a more old-fashioned nylon bracket and cable-tie guy. (It stands up to heat better and looks neater too)

[pe001ne]

here's may be a dumb question but, after removing the 4 screws on the back panel of the GA5T, is there anything else preventing the panel from lifting off?
Mine doesn't seem to want to come off.

[Jazz P Bass]

Run a larger screw into the panel hole.
Just enough to catch the full depth of the panel but not the amp enclosure.
Pull on the screw.

[Dave Curtis, dB AudioTech]

"(but I'd personally go for the Sprague Atoms because they are more robust)"

Not so much anymore. I've had some that were microphonic (that drove me nuts until I found it), a few whose + leads came off, and IMHO they're overpriced. IIRC, they're not being made anymore, so the prices are going to go up. Panasonics are much better, and Xicons work well, too.

FWIW, I've used a Xicon 450v cap in a circuit with about 510v on it for a few months and it worked fine until I could get the proper value in it (a 525v CE can cap doing nicely 3 years later)... The tortured Xicon is still kickin' in another amp...

400v caps would be fine in this amp.

I take a dremel with a cutting wheel and cut those metal bands (one cut, near where the loop joins) that are holding the old caps (leaving them riveted), and use them with some ty-raps to hold the new caps. In some cases I've been able to pull the cap out of the cardboard tube and restuff it with a new cap or 2 ("modern" caps allow this; Atoms - not so much). I'm also not a fan of mounting stuff with silicone caulk/glue/RTV, but it will take heat that will turn a nylon cable tie to flaming, black goo (do the blowtorch test...). It has it's uses.