Tweaker 15 sound through fx loop but not input jack

[caufield]

Didn't find anything similar to my issue using search.

I have a Tweaker 15 that's about 5-7 years old. Love the amp, but a few months back it started both cutting out (sound dropping completely out) and fuzzing out (output volume drop about 50%, bass/mids almost 0). It had been many years since I replaced the tubes so I swapped V1/2/3 & power tubes with JJs (ECC83s and 6V6). I thought the problem was fixed but then about 2 weeks ago it started intermittently cutting out but not fuzzing out. Sound would cut out while playing after 5-10 minutes, then as I was twisting dials, flipping tweaker switches, standby/play it would come back - but no discernable cause/effect between me messing with the amp and the sound coming back.

Last week I lost sound completely. Power light is on, power tubes are glowing, fuses look good.

I have a looper pedal in the FX loop and I can play prerecorded patterns through the amp just fine. Master volume works when the looper is playing through the loop.

But no sound from a guitar into the input jack (note I haven't tried plugging the looper into the input jack). I've changed guitars and swapped cables but no joy. Also, there is no noise when plugging a cable into the input or grabbing the cable plug.

Interesting observation: when I toggle the vintage/modern switch I hear a slight pop/crackle, but not when I toggle the other tweaker switches.

What are next steps? I don't want to just blindly start swapping out V1/2/3 if one of those is blowing because of another problem, so I've left them all in place while I seek help.

Thanks in advance!

 

[caufield]

Today I pulled the preamp tubes and took them into a local guitar shop to test. All 3 read as good in their tube tester. So I sprayed the sockets with contact cleaner and reinstalled but left the covers off. Amp was working great for about 20 minutes but suddenly the volume gradually dropped to zero over about 15 seconds.

I noticed that V1 wasn't glowing. Switched to standby for a few seconds and back to play and it started glowing again and sound was back.

Played for about 10 minutes more without issue.

What gives?

 

[9985]

I'd take it to an amp tech asap & stop using until resolved, risk of damaging other tubes, could be any number of failing components. The Chinese made power transformer in early Tweaker 15 models was suspect, mine died & I had an amp tech replace with Classic Tone 40-18035 - they have since gone out of business, Mercury Magnetics replacements are available but more than double the price. Good luck, they are great amps & worth repairing if you can do so for reasonable price.