Mark III Problems Please Help!

[Goldtop]

To continue on from my post about output tube glowing and hum. It seems like I got the problem fixed for a short time. One of the outer tubes (EL34s) would glow bright red in the center and the amp would hum. I found a broken wire at the tube socket and had it fixed. Well I used my old tubes again and lightly tapped on the one to listen for micro-phonics and all the sudden that tube glowed red in the center and made the to 6L6s glow and then the amp shut off. It blow the fuse and also had a little smoke coming from one of the 6L6s sockets. I waited a few minutes and pulled the chassis to look inside and found nothing burnt but, the one socket from underneath. Is it possible that it only damaged the tubes and tube socket and not the transformer? Please help!!!!!!

 

[Boogiebabies]

The smoke usually comes from the resistors or socket cooking. When the fuse blew, it cut all the high voltage which is necessary to save your OT.

 

[Goldtop]

Thanks for the info Boogiebabies. I hoped that I didn't cook my output transformer. Would it be safe to put a new set of output tubes in and try it again or should I just plan to take it in to the tech?

 

[Boogiebabies]

It could be a bad tube, but the smoke had to cook something. It's not going to kill the amp if it blew the screen grid resistor, but if you want to try another set and a new fuse, it will rule out if any of the bias resistors are bad. I keep a set of good used tubes for testing like this. If it pops again and you are not able to read resistor values and take a few voltage readings, I would take it back to the tech. Hopefully, it's just a bad tube.

 

[Goldtop]

It burnt a small part of the bottom of one of the 6L6 sockets. That is it the best I can tell. I looked inside and didn't see anything else. I think its just the bad EL34 that red plated the first go around. Thanks for your input.

 

[Boogiebabies]

When it burns, yo have to make sure there is not carbon on the tibe pins. Carbon is non-conductive so you may be sending the high voltage to a dead end and causing a backup. You have to scrape it off the pins, or in bad cases replace the socket. I prefer to repair them for originality, but I have seen a few just caked in black from the top of the pin connector down to the inside of the pin housing (connectors). I desolder the component from the carbonized connector, push the stop tab in and slide the pin out of the socket. It usually takes a dental pick, steel wool and contact cleaner, but you can get them clean. If they are seriously caked, I just replace the pin from some old sockets. If it's a phenolic socket that looks like it's been blasted by a cannon, I would replace it. I have not seen too many MK III's without ceramic sockets. They can take a beating.

 

[scottcrud]

I recently bought a MKIII and turned it on and the EL34 to the far right if your looking at the back of the amp blew out. Puff of smoke, turned it off within 5 seconds. Opened it up and it had burnt out 4 little brown resisters. Got thos replaced and now it's back in the shop to get the thermo-couplers replaced.