Help troubleshooting amp

When I turned the system on this morning in anticipation of a long holdiay of music there was no signal in my right channel. I first checked all connections which were fine and then I subbed in a pair of replacement tubes for the right side of my preamp, still only on channel. I then switched my attention to my amp, I switched both pairs of 6550's from left to right. Now no signal in either channel! This leads me to beleive that the trouble rests with the amp. I'm going to try switching 6550's back again but one at a time to see if I at least get the left channel back. Regardless I'm pretty sure it's a tube issue but if anyone else has any ideas I'm listening. Why do these things only happen on weekends and holidays?
Try switching them all back, then move them over again, one at a time, sassuming you were able to get the left channel back.

I don't know what you will learn, except that it might just be a bad tube.
I switched them both back one at a time and now I can't get any sound at all. The really wierd thing is all of the tubes light up and appear fine. I really can't figure this one out.
>>"I switched both pairs of 6550's from left to right. Now no signal in either channel!"<<
>>"This leads me to beleive that the trouble rests with the amp."<<

Just a guess, a bad shorted 6550 tube. The bad tube may have damaged something in the right channel. Installing the bad tube in the left channel may have caused damage there as well.

Post a thread on Audio Asylum on their tube forum and ask your question.
I had the same problem with my VTL MB450's. A bad tube blew the main output fuse on one of the amps but the tubes themselves still lit up because there was a small amount of current going through them. The fact that there was current going through them mislead me to thinking that the problem was not the tubes.

I would agree with Jea48 that a bad tube on the right side when switched to the left side blew a fuse on the left side as well.

I would check the fuses (use a volt meter) on this amp to see if indeed they are blown. If they are, you need to replace the fuses and the tubes before proceeding any farther.

Hi Jond. Sounds to me like a bad tube[shorted] took out the cathode resistors in one channel and when you swapped them to the other channel, the same bad tube or tubes took out the cathode resistors in the other channel as well.If you have a tube tester, It would be a good idea to test all these tubes to ascertain what tube/tubes are shorted before putting them back in. Is your amplifier a self biasing circuit or can you bias each tube idividually?
i'm sure you already have, but check the fuses on your amp. you probably have a bad 6550 as mentioned which caused your fuses to blow. good luck.
Well guys I looked at the fuse and it looks fine. I don't know how to test it. I'm afraid that Ecclectique is right and a bad tube took down some resistors or something like that. I guess tomorrow I'll take it over to Deja Vu Audio and see if Vu's tech can take a look at it. What a bummer! Anyway thanks for the responses everyone, I'll let you know how it turns out.
Ecclectique might be right.
usually the faulted resistor looks bad and acts either as an open ckt(very similar to a fuse i.e. as a part of ckt protection).
ah, the many joys of tube amp ownership. almost makes me want to go back to solid state. almost.
you haven't told us what type of amps you have but...
on my Cary SLAM-100's there are TWO fuses per monoblock - one's just for the tubes and I forget what the other's for - and I have had situations where one of them blew and there was no sound but the tubes still lit up fine.
Sorry my amp is a Graaf GM 5050 pushpull stereo amp using either 6550's or KT88's. It currently is tubed with EH6550's. I thought that you could tell if a fuse was blown by just looking at it, am I wrong?
sometimes, if the break in the fuse is tiny, you need a magnifying glass to see it. assuming you have spare fuses on hand, simply changing them will be a diagnostic if you're stumped. as I said, my monoblocks have two fuses per side, and one of the fuses can be blown (resulting in no sound) while the tubes still light up.

it's usually blatantly obvious when a fuse blows if your using glass fuses. when they blow, the glass usually has some slight to excessive charring.
Member Scifi walked me through testing my fuse with a voltmeter and the fuse is fine. It looks like it's either tube related or worse. I should be able to get my hands on a replacement quad of tubes today and if those don't work it's time for repairs. Wish me luck!
>>"I should be able to get my hands on a replacement quad of tubes today and if those don't work it's time for repairs. Wish me luck!"<<

Jond, imho if I were you I would take the amp to the repair shop first. Without knowing for sure what is wrong with the amp you might cause damage to the new 6550 tubes. The repair shop will test all the tubes including the two 9 pin small tubes.
The repair shop is where the replacement tubes are. I'll do whatever they tell me with the amp, it's in their hands. Thanks for your advice and for posting a link to a pic of my amp, it's a sleek little beastie don't you think?
Jond, if I understand you correctly you are going to take the amp and old tubes with you to the repair shop. They will be able to test the old tubes while you are there. Does the Amp have auto output tube bias adjustment for the power tubes, or do you set the bias mVDC voltage manually for the amp? If the bias mVDC voltage is way off you could damage any new tubes you would install in the the Amp. I still believe something is damaged in the Amp due to a shorted 6550 power tube...
fuses are usually checked for continuity with simple $3 continuity tester. in glass fuses you can see the broken thread of wire(sometimes).
Well everyone, it turned out that all 4 tubes were bad and they took out resistors and caps. So Deja Vu has fixed the amp, I'll be picking it up on Friday and it only is going to cost $180.00. Woo-hoo!
Jond, sounds great. $180.00 thats not bad... Thanks for letting us know what the problem ended up being. I wish more members would follow through on their threads like you did.

I know what you mean! Slight change though, it was $212, still a great deal, but they're still waiting for a new fuse that they had to order over the web, so I won't get the amp until next week sometime. Nonethless I'm excited, especially since the new tubes I picked up were cryoed Valve Art GEC replica KT88's with virtually no hours. Since I've never had an amp that could run KT88's I'm really jazzed to hear them. When I do I'll report back.