Old Ampeg Jet questions

[tooold]

I have a '61 Jet that's always seemed to be on the verge of being great but never quite got there. I hadn't looked at the insides in ten years (been learning the baby steps of working on stuff and had too many other amps in worse shape). When I opened it up the other day to pull the speaker, I discovered that, at some point in its life, it had a bad case of Orange Drop-itis, and the worst kind - the shotgun variety...

[img:800:600]http://i115.photobucket.com/albums/n308 ... hassis.jpg[/img]

Orange drops just irritate me, so I looked around in what I had on hand for 150's and had enough of the right ones to do a quick (right?) and easy (right?) re-shotgunning. Everything is fine, until I get down to the end. There's a .1µF ceramic where there shouldn't be one, and there's no 25µF/25V bypassing the 250K 1W resistor where there should be one. Here's the schematic:

[img:800:397]http://i115.photobucket.com/albums/n308 ... 6-crop.jpg[/img]

and the detail:

[img:596:600]http://i115.photobucket.com/albums/n308 ... detail.jpg[/img]

Here's what's there... or not there:

[img:800:600]http://i115.photobucket.com/albums/n308 ... tery-2.jpg[/img]

So, two questions:

My general approach on old amps is to put them back to stock, with a three-prong AC cable and no death cap. I'm assuming I should put the 20µF/25V back in the circuit where it should be? (I'll use a 25µF, but close enough, I hope). What function would that cap have, or not have, depending on whether it's there or not?

The mystery 0.1µF cap goes to ground on one side, and to the back end of the fuse on the other, so it looks like it's the cap indicated in the schematic as a 0.05µF to ground, just stuck in a strange place. Should I pull it? Any other thoughts? Everything else seems to follow the schematic.

Thanks for any help...

[martin manning]

Yes remove the 0.1uF to ground. It's not needed with a polarized AC plug and earth ground, and it is a safety hazard if it fails shorted, putting 120VAC on the chassis and on your guitar strings.

The other cap is an AC bypass for the 250 ohm 10W cathode resistor, and its main purpose is isolating the output tubes from one another, removing a compression effect. Put it back and see if you like the sound better that way. The value is not critical.

[tooold]

Yes remove the 0.1uF to ground. It's not needed with a polarized AC plug and earth ground, and it is a safety hazard if it fails shorted, putting 120VAC on the chassis and on your guitar strings.

The other cap is an AC bypass for the 250 ohm 10W cathode resistor, and its main effect is isolating the output tubes from one another. Put it back and see if you like the sound better that way. The value is not critical.

Thanks, Martin! Just turned the soldering iron back on...

[tooold]

Just discovered (thanks to a friend over at the Plexi forums) that the missing 20µF bypass is in the cap can... doh!

Thanks again!

[ampdoc1]

Not to demean anybody, but I have found on a couple of Ampegs lately, that the Joe Piazza schematics are not correct by a long shot.

a'doc1

[Diablo1]

Not to demean anybody, but I have found on a couple of Ampegs lately, that the Joe Piazza schematics are not correct by a long shot.

a'doc1

I also have found a few mistakes on the Joe Piazza schematics. But to be fair, I have also found some mistakes on some of the original Ampeg schematics too. I print out all the schematics available and compare them to find the mistakes and decipher what should be the correct schematic.

[Firestorm]

Not to demean anybody, but I have found on a couple of Ampegs lately, that the Joe Piazza schematics are not correct by a long shot.

a'doc1

I also have found a few mistakes on the Joe Piazza schematics. But to be fair, I have also found some mistakes on some of the original Ampeg schematics too. I print out all the schematics available and compare them to find the mistakes and decipher what should be the correct schematic.

Ampeg changed their designs more often than even Gibson. Some variations don't seem to be published so you have to hope the schematic mounted in the amp is intact. I think Joe did the best he could with a moving target. (Yes, there seem to a couple of outright errors or omissions, but Ampegs are tough to keep track of).