Bad Tubes in Preamp?

[Richard Warren]

Is there any way to know that a tube is bad without a tube checker? The output of my Mesa Boogie Triaxis pramp begins to fade in and out after playing for 10-15 minutes. Does this sound like a tube? According to the manual, the tubes perform the following functions:

V1 = Lead 1 input
V2 = Input for all other modes
V3 = Lead and Rhythm mixer
V4 = Lead 2
V5 = Effects return stage

It does it across all of the modes so it seems that either more than one tube failed at the same time or either the Lead and Rhythm mixer or Effect return tube may be bad. I assume this since V3 and V5 appear to be the only tubes that are shared by all modes. I turned off the effects loop with the front switch on the Triaxis and it had no effect. Is this pointing to the V3 tube?

Any help would be appreciated.

Richard