#1
edit: sorry about the crappy pics

Soo...first up, a '62-'64 Gibson GA 5-T, not working. When you turn it on, you get a loud buzzing that changes tone a bit as the tubes warm up(no standby). There's no response from any input signal. I'm guessing this is just a cap job? The tubes look fine to me.

Obligatory gut shot:



Next, an old amp from a long dead company. This looks like it was made in the 70's. It works, at least to the extent that you can play through it, but I've got some problems. First, the power cord is a permanent rubber type, and has worn enough that it slips out of the casing. It's also missing the ground plug. This isn't good, as the amp uses the metal casing as ground in the circuit. (I only touched the strings with a pick..lol) I'm thinking of removing the cord and adding a standard power cable jack on the back of it, like modern amps use. Good idea?

My 2nd problem is that when I turn it on, I get a smell like something electronic might be burning, but the circuit is covered in dust and cobwebs so I'm not sure if it's that stuff burning off or if something is about to catch fire. What should I check for? (The amp had a blown fuse when it came home, so I suspect something is wrong)

My 3rd concern is about the power handling. It says on the back that "for optimum performance, use either two or four (4) ohm speaker systems. The 4 jacks are wired in 2 pairs: 1-3 and 2-4. Could running this with a single 8-ohm cab and leaving the other power amp with no load cause problems?


Also, once I verify that this thing is safe, would it be wise to use that plug on the back? It seems like it'd be pretty convenient...

Nope, no sig here.
Last edited by Mutant Corn at Aug 17, 2009,
#2
Re-cap both amps. If it has ceramic and/or mica caps as well, those are fine. But all the electrolytic caps need replaced and any wax/paper caps should also be replaced (with poly film or something).

as for the no-name, the plug on the back is called a conveinience outlet . It's connected directly to the mains input of the amp.

If you have a can of compressed air, blow it (the amp) out.
#3
Sounds good The Gibby shouldn't be a problem then...it's all point-to-point wiring.

Re-capping the bass amp, however, could be interesting...the "main" caps on it, whatever you call them, are effing huge. Some kind of paper type, I think. 6000MFD, 50V. Where do I find something like that? The most radioshack has is 4700MFD, 35V.

Also, I guess it's ok that there was a blown fuse on it? I wouldn't know how to check for the cause...

I know what a convenience outlet is, btw...I was wondering if it's safe to use...? This thing is really old...
Nope, no sig here.
#4
Quote by Mutant Corn
Sounds good The Gibby shouldn't be a problem then...it's all point-to-point wiring.

Re-capping the bass amp, however, could be interesting...the "main" caps on it, whatever you call them, are effing huge. Some kind of paper type, I think. 6000MFD, 50V. Where do I find something like that? The most radioshack has is 4700MFD, 35V.


6000 uF? That's huge! Here's 6000uf caps on Mouser:

http://www.mouser.com/Search/Refine.aspx?N=11669666+4294620152
#5
Quote by end_citizen
6000 uF? That's huge! Here's 6000uf caps on Mouser:

http://www.mouser.com/Search/Refine.aspx?N=11669666+4294620152

No kidding... Any idea what type they are?

The two 50V ones on Mouser aren't stocked...lol. This could be harder than I thought. One of them says "lead-time 8 weeks", does that mean they can get it?
Nope, no sig here.
Last edited by Mutant Corn at Aug 17, 2009,
#8
Avoid the shack. Mouser, www.smallbearelec.com, www.tubesandmore.com are all good places. Small Bear is mostly pedal stuff, but they have a lot of components that work for amps, not going to find huge caps there though. Tubesandmore is more expensive but is a mostly amp-oriented store and has a more big caps and direct replacement parts for old amps.
Last edited by cedricsmods at Aug 17, 2009,
#9
Anything that huge is going to be electrolytic. If you have to, you can wire caps in parallel to get close to the value. Those a filter caps btw. I have a couple 6800uF 80v caps I'll give you if you want them. We're both in AR so shipping will be cheap.

It's safe to use the outlet.

The fuse blowing could be nothing or it could be somethng. get a new fuse (has to be slow-blow) and if it blows that one too, you have a problem.
#10
Hey, sorry for the delay...and thanks for the offer jim. I've been running around like crazy getting ready to go back to school. I hate transferring colleges...you never know what courses the new one is going to throw out.

So anyway...just by chance, I met a guy that used to do amp repair, and this thing came up in conversation. From what he says, the filter caps in there are not the same as modern aluminum-electrolytic caps. He called them "cans", though he didn't elaborate as to what a "can" was, though he did say that the aluminum shell acts as ground...so I guess it's technically a three terminal cap? I'm even more confused than I already was...

So basically I have to figure out what kind these are before I can even hope to replace them. :/
Nope, no sig here.
Last edited by Mutant Corn at Aug 25, 2009,
#11
they are just like modern lytic caps only bigger. in fact, modern lytic caps are also encased in aluminum shells.

if you want you can prolly take the can apart (if it comes apart. or you could dremel it) and gut it. then solder some new lytic(s) (if its a multi-plate cap) onto the terminals and put the can back together. retains the stock look and still lets you actually fix it. this exact procedure is done alot on old tube radios that are being restored.

The only reason the can itself acts as a shield (you said ground) is because it's mounted to the chassis. the can itself doesn't really serve a purpose, electrically. a bonus is it shileds whatever is inside, but this is pretty much moot anyway.
#12
That certainly makes more sense...I was looking all over the place trying to find what these things were, and coming up with nothing. I'm still not sure, though. On closer inspection, the top part that I originally thought was wax actually appears to be some sort of laminated paper or fabric. I'm not going to touch it to find out though; it's too close to the leads, and I don't know if they've discharged. Also, they're not sealed, or at least not around that part. I can see space between the edges of the paper/fabric/plastic(?) and the wall of the can.

I've also noticed that they're also wired very strangely. I originally thought that there was one cap to each power amp, but when I actually looked at them, I found that they were wired in series. So that's basically the equivalent of a 12,000uF cap? Why on earth would an amp like this need something so huge?
Nope, no sig here.
Last edited by Mutant Corn at Aug 25, 2009,
#13
Then your cap is dead. Beyond dead in fact. and trust me on this stuff.

caps in series do not add. caps in parallel do add (like resistors in series). the formula for series caps is the same as for parallel resistors:

1/C(total)=(1/C1)+(1/C2)+(1/C3) etc...

So if the caps are the same value, all you need to do is take half their value and that's the capacitance.

So if they are both 6800uF and wired in series, the actual value is 3400uF. this is done to *theoretically* increase the voltage rating (take the voltage ratings on each cap and add them).

I say *theoretically* because lytic caps have very poor tolerance and there are some physics things that I won't get into that have to be ideal in order for it to work well.

so basically you can replace those with either 2 6800uF 50v caps in series (that's the value, right?) or you can use a single 3400uF 100v (or higher) cap. if you can find one, id go the second route.