Lost ch 1 in my Express 5:50!

[Digirat]

Hi

Had my Express 5:50 for a couple of months, and no problems until now. Was working fine two nights ago and had just sat quietly in the corner of my home studio since then....

I just turned it on for some Saturday afternoon fun, but there is basically no sound on channel 1, which also now hisses like crazy ! Still plenty of volume from ch 2, but it now sounds raspy with no sustain or smooth Mesa tone (more fart than Boogie).

I'm still getting volume from ch 2 and the 6L6s look healthy by eye, so is this likely to be a pre-amp tube failure? I just wondered if the loss of all volume on ch1 and loss of normal OD tone on ch2 might be diagnostic for which 12AX7 might have gone?

According to the manual, the V5 tube is exclusively for reverb send/return, so could I try swapping with that with the other pre-amp tubes one-by-one to hunt the guilty sucker down, without affecting anything apart from the reverb if the bad tube goes into V5 :?:

Grateful for any input here from you more experienced gurus

 

[Platypus]

Check V2, sounds like your culprit as one triode of the pair controls the first gain stage and the second triode effects the second gain stage OD.

 

[Digirat]

Check V2, sounds like your culprit as one triode of the pair controls the first gain stage and the second triode effects the second gain stage OD.

Thanks Platypus !

Something really weird happened though. After switching off and leaving it for an hour or so, when I switched back on again, ch1 worked perfectly :?: :shock: Ch2 sounded a bit strange for the first minute or so, but then it sounded fine too. I've "re-started" several times since, and everything seems to be fine. Is it possible for a pre-amp tube to somehow "fail to fire up" on one occasion and then to be OK?

 

[Platypus]

Hmmm no, not really. Sounds like it's going bad but hasn't totally bit the dust yet. I would replace it with a known good tube just to be sure all the symptoms go away.

 

[Newysurfer]

Sounds like an intermittant connection prob to me. Either in a tube or somewhere else

 

[jamme61]

I just posted a possible cause of your problem. on my 5:50 the pot terminals on all the pots of the clean channel were really close to grounding on the chassis. You'll have to take out the chassis to see. Only do it if you know what your doing, or you could get electrocuted.