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Mullard flash....or not???

gearhound

gearhound

Lunatic Member
I have two IDENTICAL Amperex branded 12AT7 tubes.
BOTH made in Great Britain at Mullard's Blackburn facility, with identical date codes also.
However, one tube has the famous (or infamous) Mullard flash on start up, while the other does NOT??? :scratch2:
I'm at a loss to understand this, as both tubes also measured identical on my Sencore tube tester.

This wouldn't be an attempt at droll English humor, would it? :D


Steve
 
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anything is possible .sorry i am not educated enough to answer .its a long time since i played with tube stuff so i cant remember one way or the other .i probably wasn't even taking any notice what they were doing .they were more than likely mullards though back in my youth .
 
Take a magnifier of your choosing and look very closely at the wiring in the bottom of the tubes. On the one that flashes, you will see two little white wires protruding from each plate structure. Those wires are actually filaments that extend down a little too far. The other tube may have them as well, try swapping the two tubes position and see if the flash stays with the tube or the socket. What is happening here is when you power up, the filament is cold, and resistance is low. That makes the uncovered portion of the filament glow white hot for just a millisecond. Once the filament heats up (almost instantly) the resistance goes up and the filament glows it's normal orange color. Did I say that right?

Could be something with how the filaments are wired in your amp.
 
The 'flash' stays with the SAME tube Jay after switching them around.
I'll have to look for my magnifying glass, as my wife was using it last for her bead work.
Which of course means, that it wasn't put back?


Steve
 
I have a pair of Siemens 12at7s that do much of the same thing. Using them in Dared SL2000 preamp when I turn it on one tube flashes brightly while the flash on the other tube is barely perceptible. The flashes follow the tubes also.

FWIW it's never been a problem.


...r1
 
It will not matter much, as they are going inside tube shields anyway.
Just curious, more than anything else?


Steve
 
Have had both (flash & no flash)...I would tend to agree with Jaymannnnn
in that perhaps one tube just has a shorter filament than the other.
If they work, use them and enjoy :music:
 
I can't help but think of the flash as a bit of a defect. It can't be a good thing to have those filaments hanging down out of the cathode like that and getting white hot every turn on. That said, I've never personally had one fail, ever.:dunno:
 
I'd say you're fine... I even had a pair of Matsushita 12AX7 tubes that had the Mullard *flash*, and others that didn't!
 
I've seen mullards flash and not flash, I've seen non mullards flash, as long as they test good I write the numbers on the box and put them in the drawer
 
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