ARC LS25 MK1 Trouble-Shooting

I would appreciate any comments on trouble-shooting the following problem. I moved my Audio Research LS25 MK1 pre-amp to a new audio rack and re-started my system. Initially everything sounded fine for a few minutes until I heard a "pop" sound from one speaker with the source material (tuner) still being amplified. I changed my source from tuner to CD and there was initially no problem. After about one minute the only sound was a hissing from the one channel and then my power amp shut down. I replaced the pre-amp function with that from a receiver and everything operates correctly. As for some history on the ARC LS25, I purchased it used here on Audiogon about a year ago and have had no problems. I also powered up the ARC pre-amp and verified that all four vacuum tubes were glowing. Is there anything else that I should check before calling Audio Research on Monday? Thanks for your help.
Sounds like one of the 6DJ8 tubes in the line stage went bad.
I agree with Bobgates.
You can isolate the tube by swithing the tubes from L to R channel.

Thanks for the correct diagnosis. I replaced the existing tubes with a spare set of Sovtek 6922 which I received when purchasing the unit. Some additional questions: when initially turning on the system there was some crackling sound that went away after the first few seconds- is this normal? Also, is there a burn-in period for new tubes and what are the different "musical" attributes of the various tube manufacturers. Thanks again.
The only time I have heard crackling sounds from my LS-25MK1 is when the tubes were shot.... It never does it now.. Make sure the new tubes you buy are fairly well matched or you may get a "ringing" sound when changing the gain level... Burn-in on tubes is usually 48 hours for a fairly decent sound. I would also leave the preamp on the lowest gain setting with no input for 2 hours before listening to new tubes... This lets them start up "slow".