Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on November 11, 2022, 05:46:58 am

Title: DUAL50 problem keeps returning
Post by: Bieworm on November 11, 2022, 05:46:58 am: 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
Title: Re: DUAL50 problem keeps returning
Post by: JPK on November 11, 2022, 10:35:24 am: Interesting. I found this although I have no experience with it:

https://el34world.com/Forum/index.php?topic=12581.0
.
Title: Re: DUAL50 problem keeps returning
Post by: Williamblake on November 11, 2022, 11:10:55 am: 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.
Title: Re: DUAL50 problem keeps returning
Post by: Bieworm on November 11, 2022, 12:14:42 pm: 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.
Title: Re: DUAL50 problem keeps returning
Post by: Williamblake on November 12, 2022, 07:17:23 am: Maybe the bias is set off by a pedal?
Title: Re: DUAL50 problem keeps returning
Post by: Bieworm on November 12, 2022, 08:07:40 am: Quote from: Williamblake on November 12, 2022, 07:17:23 am
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..
Title: Re: DUAL50 problem keeps returning
Post by: sluckey on November 12, 2022, 08:39:57 am: 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?
Title: Re: DUAL50 problem keeps returning
Post by: Bieworm on November 12, 2022, 10:27:12 am: Quote from: sluckey on November 12, 2022, 08:39:57 am
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..
Title: Re: DUAL50 problem keeps returning
Post by: Jennings on November 13, 2022, 01:36:36 pm: 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.