Help me diagnose tube problem...

[Mush]

Yesterday I was playing channel 3.
Then I switched to channel 1 and no sound ?
No sound either on channel 2 but I still have sound on channel 3 . The no sound state is accompagned with increased hum.
BTW
channel 3 was in 90W mode,
channels 1 and 2 were in 45W mode.


So I figured that V1 is OK if channel 3 is good.
It should be a power tube . Since the problem is in 45W mode
That might be the two inner 6L6 that are broken.

I replaced them , and the problem still persist. Channel 1 and 2 cut off rapidly , randommly. I can play and then nothing.

I am thinking now of a bad rectifier tube.

Am I in the right way ?

Mush

 

[MesaManBoogie]

It's a preamp tube
here the lis of what each preamp tube affects
I would remove one by one and inspect them.

V1A-- Input Stage
V1B-- 2nd Gain stage CH1/CH3,
V4A-- Reverb SendV4B-- 2nd Drive stage CH3
90 Watts = V8, V9, V10, V11 45 Watts = V8, V9
10 Watts = V8, V10
3rd Gain Stage CH2
V2A-- 2nd Gain Stage CH2 V2B-- n/a
V5A-- 1st Drive stage CH3 V5B-- Reverb return
V3A-- 3rd Gain Stage CH1/CH3, 4th Gain Stage CH2
V6A-- 4th Gain stage CH3 V6B-- Effects return stage
V3B-- 4th Gain Stage CH1
V7A&B-- Driver/Phase inverter

 

[SteveO]

It could also be the rectifier tube if channels one and two have tube rectification selected for 45 watt mode. Try flipping the switches to diodes, and see if that cures it. If so, it's the rectifier tube.

 

[ziggys]

MesaManBoogie:
I've seen that list before and thanks for posting it:
Can you clarify the difference between "drive" stage and "gain" stage?

Thanks

 

[Mush]

Yesterday evening I changed each of the pre-amp tubes with a known good one from my F-50. It did nothing.

But this morning while powering on the amp I saw one of the 6L6 (one that I haven't changed) the external one near the recti tube light up good. I changed it for one of my F-50 and all seems good. I haven't had time to push the investigation further.

Thanks for your answers

Mush