Alamo Capri Model 2560

[olddawg]

Please explain why the heaters would have to be lifted off ground. They are on the secondary side, they are not connected to the mains at all.

I don't see how it would matter. I would get rid of that death cap on the primary side, connect the AC mains high and low directly to the primary, and connect the earth (green) wire of the 3 wire cord to the chassis. Not sure if it would make a difference to use a twisted pair for the heaters since its a series filament arrangement. Probably, it just needs filter caps. As far as changing any wiring I would certainly differ to Enzo.

[Enzo]

I don't think it matters either, I just doubt SGM can rationalize it either.

[Goldhedge]

Thanks for the clarification.

I would like to put a 3 prong plug on this amp for safety reasons.


Anything I need to know before doing it?

[Enzo]

Very simple.

Connect the ground from your power cord to chassis under a screw. it is supposed to be its own screw, but you could get away with a solder lug on one of the transformer mounting screws.

The original schematic is right, the drawn one shows the primary winding wrong. The drawn schematic shows the bottom end of the PT primary terminating in C9 to ground. NO, that is the other wire to the mains plug too, not just the chassis cap. Get rid of that cap. Now you have two wires from the transformer primary. The neutral wire from the mains cord goes directly to one of those primary wires. Splice and insulate. You SHOULD add a fuse clip/holder, this amp is not now fused. The hot wire from the mains cord should go first to the fuse holder, then a wire from the holder to the power switch, and finally the remaining primary wire is wired to the other side of the switch.

Maybe a better way to say all that is this: snip out that cap from mains to chassis, then connect the new cord top the same places as the old one. Just prefer if you added a fuse holder.

[Goldhedge]

Ok, I connected a 3 prong plug. This is a really sweet amp! Sounds great.

I've discovered that the hum 'noise' disappears if I put an RP80 in front of it. It filters the guitar somehow. It also makes the RP80 sound much better, warmer going through tubes.

I also corrected the schematic. I believe it is now correct. I wonder if C9 0.005uF cap is a bleeder cap (bleeds charge from the filter caps)? It's located on the right side of the xformer and taps off of pin 5 of the rectifier and goes to ground. I didn't include it.

I would change/remove all the posts that have that particular error, but for some reason I can no longer edit my posts.

Thanks for all the suggestions and help!


Here's the scheme

[MrG]

I have a Capri 2560 That doesn't work.

I found the 35w4 rectifier tube plugged into the 12au6 socket and vice versa. The power tube is correctly plugged into the center socket.

What would happen if the pre-amp 12AU6 and diode 35W4 tubes were accidentally plugged into each others socket and then powered up?

I got the amp already in this condition and the amp has been turned on previously. There is no audio output at all and the speaker, when checked with an ohm meter, reads open.

What would be the best way to approach this problem? are any of the tubes toast? Would this blow the speaker too?

[Enzo]

Hi, welcome.

really it would be better to start a new thread for your Alamo instead of tacking onto the end of another. That way the two repairs will not get confused.


I can't imagine a way the tubes could burn out the speaker.


The 12AU6 and 35W4 turn out to have not only the same heater pins, but they also use the same pins for plate and cathode.. Guessing the power tube is 50C5?

Plugging the tubes in the wrong holes like that... well, the 12AU6 would try to be a rectifier, but I doubt it would work very well, and so put lieel voltage into the circuit. Hard to say. The 35W4 would certainly not amplify in place of the pentode. it would try to conduct fully, but the typical plate load for the pentode would be like 100k or something so even as a dead short, it ought not hurt anything. In short - no pun intended - well OK I did intend it - I doubt it hurt anything.

SO my approach? I'd put the tubes in right, connect a working speaker, and fire it up and see what it does. How else can you know?

It could be anything, but with the tubes in wrong, it is possible they were removed in an attempt to fix whatever is wrong with it.

What is most likely wrong with it is all the caps need to be replaced. There are a couple electrolytics in the power supply and also the cathode bypass on the 50C5. Then all those 0.005uf and similar are probably leaky electrically. The small one on the inputs is probably OK. Ther is one on one side of the mains wiring. I'd get rid of that and install a three-wire power cord.

[Goldhedge]

What is most likely wrong with it is all the caps need to be replaced. There are a couple electrolytics in the power supply and also the cathode bypass on the 50C5. Then all those 0.005uf and similar are probably leaky electrically. The small one on the inputs is probably OK. Ther is one on one side of the mains wiring. I'd get rid of that and install a three-wire power cord.

That's exactly what my problem turned out to be. Old electrolytics dry out.

I went to the local electronic junkyard and found a replacement.

You can use individual caps as well (they don't have to be in one can).

It's a sweet little practice amp. A very warm sounding 3 watts...

[MrG]

"really it would be better to start a new thread for your Alamo instead of tacking onto the end of another. That way the two repairs will not get confused."

Point taken, new thread started. Thanks.