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My Marshall is sick! HELP

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8/12/1999 3:31 AM

My Marshall is sick! HELP

Hi. I've got a 76 2204 Marshall head. I got it used about 2 weeks ago. It's been a fine amp until recently. I first noticed the problem (which I will get to very soon) when I had the chassis out to replace a few coupling caps. I turned the amp on after replacing the caps, and when I flipped the standby switch, the output tubes started glowing bright blue. I didn't think much of that, since the tubes in my bassman often have a light blue glow in them. After a few seconds, the amp started to hum (with both volumes on 0) and the tubes started glowing bright red. I quickly put the amp on standby and looked for any visible problems. The problem seemed to go away when I repositioned the wires coming from the choke. (This may or may not have been the problem - I don't know what would cause this) The amp seemed fine, so I put the chassis back in the cab. The amp has been fine until tonight.

I recently got a pair of Svetlana EL34s, and I was going to put them in and bias them. Once again, with the chassis out of the cab, the tubes are glowing blue only this time, they crackle VERY loudly (through the speakers) when the standby is flipped off. I don't mean a slight crackle - it is very loud! What could cause this? I don't know very much about the workings of a tube amp, but I'm handy with a soldering iron and dmm. I'm sure I could fix the problem if I only knew what it was. This makes no sense to me as the only things I've done to the amp have been very minor (replacing the can caps, 2 coupling caps and adding bias resistors to the power tubes). I appreciate all of your suggestions. Thanks.

Milan

8/12/1999 3:48 AM

Milan;

It sound like the amp is loosing bias and that's why the power tubes are glowing red and the amp hums. (I just had this happen to me)

The tubes will stop glowing red and will stop humming when you flip the standby switch but as soon as you put the standby switch in the "play" position the tubes will do the same thing.

Check your solder connections where you put the caps in, swap each cap out with a cap you know is good (maybe one is bad...it happens every now and then but it's rare if they are new caps)

What values did you put in? I'd also make sure they are around 600volts to be on the safe side.

Hollar back

Trace

8/12/1999 4:09 AM

I'm sure the connections on the parts I put in are good. I changed the old can cap (BTW - the old one was a shiny blue DALY cap. was this original?) with a new LCR 50uf/50uf/500v. The coupling caps were the ones before the first gain stages (mallory .022uf/400v). I guess I must have just wiggled an old connection loose or something when I opened the amp up. Now I just have to find it.. One more thing I noticed this evening - before I took the amp off standby, I checked to see what the highest neg voltage I could get from the bias supply was (from pin 5 of the power tubes). I couldn't get anything higher and -36v. this seems a little low to me. is it? could this have something to do with my other problem? Thanks

Milan

8/12/1999 1:28 PM
Trace	Hey there; It sounds alot like a bad connections some where provided the caps were all installed correctly of course. Does the amp do it when it's in the box or just when it's out of the box?? Trace

8/12/1999 2:33 PM
Milan	Well, so far it's only done this while out of the cab. I haven't put the chassis back in yet today, and last time it stopped before I put it back in. Milan

8/16/1999 2:34 AM
Milan	Problem solved! It was just a loose connection. One of the wires on the pcb broke it's connection but stayed on the pcb. Thanks for the help! Milan

8/16/1999 11:32 PM
Trace	Milan; Any time! I assume it was in the bias section? Trace

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