300b question [solved]

[mozarti]

I have a question for the tube experts.  One of my EH300b tubes went defunct after only a month of operation. (its channel went dead, but when I moved the tubes around, the dead channel followed the tube. The failed tube does not light up). Question 1: Can I just buy a single EH300b? In other words, how important it is to have a matched pair? Question 2: what causes tube failure so quickly? I am worried that there might be something wrong with my wiring that led to the failure. I do not have a tube tester, but any advice you may have that would help me understand tube failure will be much appreciated.

[Paul Birkeland]

Yes, you can just buy another 300B.  Matching between channels in a single ended amp is not particularly important.

The tube failure could just be random.  If the voltages in the amplifier were within the parameters specified in the manual, it's unlikely that any of the wiring would have caused the failure.

A lack of glow in the amp would be a broken filament, which isn't super common, but does happen once in a while.

-PB

[Doc B.]

First thing I would do is measure the resistance between the two fat pins on the 300B to verify that the filament is open. If it is, it's time for a new tube. Unless you have an off voltage reading somewhere that indicates a problem in the amp itself, it is not too likely that the amp took the tube out

[mozarti]

Thank you, both, for your helpful responses.  I did measure the fat pins, and they measured 5.9 ohms, which suggests that the filament is not broken. So I am still puzzled.

[mcandmar]

Is the getter material still shiny or has it turned white?   If a tube gets air inside the filament wont glow, but will still be intact electrically.

[mozarti]

Thanks for the suggestion.  The getter is not white and it is still fairly shiny (no different from the getter in the working tube).

[Doc B.]

There may simply be an issue like a dirty pin, or perhaps one of the socket contacts is just stretched enough that the one tube isn't making good contact. Maybe give the pins a bit of cleaning and try again. You might also carefully hold the tube by the base and gently tilt or twist the tube in the socket to see if that makes the filament light. These tubes are quite reliable, that's part of the reason we chose them. So it's worth it to play the odds and see if something else is actually creating the issue besides an open filament.

There is also a slim possibility that the solder inside the tube pin is not in full contact, creating an intermittent connection. Once in a while we (these days we means PB) will heat the pin of a 300B to reflow the solder inside. Try the above stuff first and if it doesn't solve it but you can still measure low ohms across the big pins you might try heating the big pins with your iron.

[mozarti]

Hurray!  You did, Doc!  I reheated the solder and now it works.  Thank you all for your help. 

[Doc B.]

Cool, I scored a point this week! I'm surprised Peebs didn't beat me to that suggestion.