[PaulS]

A buddy of mine was complaining his amp was blowing fuses. He used it at a jam on Sat night and the next day at home when he turned it on he heard a loud pop and the fuse blew. He said he tried another fuse and the same thing. I told him to bring it over and we would take a look at it for any obvious problems. I checked the tubes at the start and they didn't show any signs of excessive heat so I pulled the chassis and examines all the caps,resistors, diodes and wiring also looking for any traces of arcing or overheating. Now I am by no means an amp tech so everything was unplugged and grounded. No apparent problems with any of the components or wiring. So I turned it over and checked the power filter caps and again all looked good. He had the amp overhauled about two months ago and it was sounding good, by the way this is a real 65 twin reverb he's got here. Putting it all back together I asked him a few questions and he pointed to a tube saying it was really glowing the last time he blew a fuse. This was the first tube in the lineup of 4 6L6 GT's. I took that tube and swapped it with the third one. Then plugged it in and turned on the standby. Everthing seemed good the tubes started to heat up but I noticed the tube moved to the third position was not as bright as the others. This held for about five minutes then I turned on the standby switch and there was a loud pop as he described but also a wicked arc on the third tube. Of course the fuse was blown again and it was the last one. Before we put another one in I'm looking for some help. Could it be a bad tube??

 

[shoretyus]

yup. Real good chance of it. I am about to change the 6v6's in the deeluxe. Need to add to my order?
If you want most of the amps in my house use 6L6's no problem swapping on out. The captain is in town swinging the soldering iron Thursday morning too.

 

[PaulS]

Thanks Pat, I've got a few 6L6's kicking around, I gave him one to try, needs the fuse before he can test it.

 

[nonreverb]

Sounds (looks) like a dead short on that tube.

 

[Wild Bill]

Sounds (looks) like a dead short on that tube.

+1!

Putting in another 6L6 is good for just a test but I would like to see a reasonably well matched quad installed.

If they came from thetubestore.com you could order one with the same number on their label and be close enough, even with tube aging.

The reason I say this is that it's a total crap shoot as to how ambitious the replacement tube is compared to the others. You don't need super close matching like in a hifi amp but you still want all 4 tubes to share the load more or less equally. If one tube is wildly different it can either try to hog the power load or be a slacker and let the other 3 do all the work. Either way, this can dramatically affect tube life.

That's why I've always preferred an idle current test point for each output tube. It lets you spot a lazy or a crazy tube right away. It also lets you try a bunch of tubes to find one close to those already in the amp.

JMHO

:food-smiley-004:

 

[PaulS]

I agree Bill, just gave him the tube for a test, he had Fender GT's in it with the tubestore marking so he should be able to match it up. For the test I gave him a Ampeg GT unfortuneately not from tube store so no marking other than both are 6L6B tubes.

 

[PaulS]

Just got news back from my buddy that the amp works again. I advised him to order a new matched tube from Tube Store to match the sticker.