DUAL50 problem keeps returning

[Bieworm]

Hi y'all

I have built the Dual 50 last year and it sounds awesome. There is one problem that keeps repeating.. V1B fails every few months. It's the 4th time I had to replace that tube, and when I do.. it sounds perfect again. I tried various tube brands already to rule out a bad batch. What could cause this failure? Under what conditions does a 12AX7 usually fail?

thanks, Bjorn

[JPK]

Interesting. I found this although I have no experience with it:


https://el34world.com/Forum/index.php?topic=12581.0
.

[Williamblake]

The voltage being critical mentioned here is the heater to cathode voltage in a DC coupled cathode follower that can easily be reduced by elevating heaters yielding positive side effects regarding hum. Why wouldn´t you, i wonder.

[Bieworm]

Sorry, I meant V1B.. I  corrected my original post. This is no CF...

The odd thing is that V1B is the 1st preamp triode of the plexi bright channel,...  but V1A (plexi dark) doesn't suffer from failure...

When the amp comes in I will check the voltages, but I can't remember anything odd about them.

[Williamblake]

Maybe the bias is set off by a pedal?

[Bieworm]

Maybe the bias is set off by a pedal?

Is that even possible? Never heard of such thing... that would make V1A go off too, and that one stays ok..

[sluckey]

What are the symptoms when V1B fails? Do you still have one of the failed tubes? If so, plug it into V2 socket. Does it work in V2 socket?

[Bieworm]

What are the symptoms when V1B fails? Do you still have one of the failed tubes? If so, plug it into V2 socket. Does it work in V2 socket?

The amp will be here tomorrow.. i will check that out
Symptoms are power cutting back to 30% or so..

[Jennings]

Normally when a tube, or a section in a dual tube, fails then it’s kind of an all or nothing thing, rather than just a big drop in sound level…I wonder whether it’s something like a dodgy pot or a fry solder joint perhaps intermittently misbehaving…definitely test those “bad” tubes and see what they’re like in a known good amp if you don’t have a tester.