Fluctuations of the loudness of speakers???

Re E-Mail, you can just reverse the speakers by reversing cable from left to right speaker and right to left. If the problem is still anchored to the same speaker then its not a bad tube. BTW, I've never experienced fluctuating volume from a bad tube, but that doesn't mean the problem is not in your amp or in a source. If you can, change sources and see if the problem goes away.

Also, are you detecting a shift in the stereo image toward the same speaker when the volume fluctuates, or does it stay centered. If it stays centered then the problem is not likely to be from speakers or, if you have two or more tubes, one for each channel, is it not likely to sourced to a tube. Its to coincidental to have two tubes failing at exctly the same time.

Hope that helps a bit.

What you're describing are the classic symptoms of a bad speaker relay in a power amp. That is:
- Can appear in either or both channels
- Changes if the amplifier is cycled on and off
- Much more pronounced at low volume levels
- Cranking up the volume for a few seconds fixes it at lower volumes for awhile
- tapping or pounding on the unit can cause or alleviate the problem

Of course if you don't have a solid-state amp with a speaker relay . . . then that's not the problem, in which case I'd second others' suggestion of bad cable connections, or loose solder connections inside a component.

It seems like I'm going to have to borrow an amplifier for a day from somebody (nobody I know has any) and see if the problem continues...then I can apply the process of elimination. What a drag...I'll be sure to post results when I figure it out or even as I get closer. Thanks everybody for their suggestions...

Another observation, problem is more common when the system is cold than warmed up...

To change a tube, open the case, just pull out the tube when cold,,,pull straight up - don't twist it... and replace with the same tube type and number. You might have some dirt on the pins of the old tube, so I would remove and replace the same tube to see if the problem goes away.