Possibly defective Mullard 12AT7? - diyAudio
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Old 19th May 2008, 01:07 AM   #1
Zap is offline Zap  United States
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Default Possibly defective Mullard 12AT7?

I'm curious if some of the experts could comment on the behavior of a NOS tube that I just purchased and began using. It is a Mullard 12AT7 - when the amplifier is first powered on (SimpleSE) there is a bright glow from the bottom of the tube, which then slowly dims as the filaments light up. The tube appears to work fine after this point, but it is somewhat concerning as none of the other tubes (including a Philips JAN 12AT7) exhibits this behavior.

Would this be considered a defect, a normal behavior, or? There is a current inrush limiter installed in the amplifier, so the tubes warm up more slowly.
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Old 19th May 2008, 01:11 AM   #2
SY is offline SY  United States
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Totally normal. If it bothers you, use a slow start on the heater power supply.
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Old 19th May 2008, 01:11 AM   #3
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It's probably normal. Do a search for the difference between a 12AX7 and a 12AX7A. It appears the 12A?7A tubes were designed for "controlled filament warmup", which meant they don't draw a boatload of heater current and flash when you first turn them on.

As long as your 12AT7 isn't wired in series with other tubes, you're probably OK.
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Old 19th May 2008, 01:17 AM   #4
chrish is offline chrish  Australia
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I have one of these mullard valves for my SimpleSE too. What you describe is completely normal.

Chris
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Old 19th May 2008, 05:20 AM   #5
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most new tubes by JJ do the same thing, or at least the ones I have.

that would be, ECC81, ECC83, EL84, EZ81 etc. Weather or not the ECC's are 12xx7A or 12xx7's I don't know..
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Old 19th May 2008, 03:52 PM   #6
Zap is offline Zap  United States
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Thanks for all of the confirmations - I just wanted to be sure I wasn't going to get stuck with a junked tube. It definitely has a good sound to it!
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Old 19th May 2008, 05:54 PM   #7
m6tt is offline m6tt  United States
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I had a mullard 12ax7 in a guitar amp that did the exact same thing–white flash for a millisecond at startup. Never had a problem with it until someone stepped on it accidentally .
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Old 19th May 2008, 06:11 PM   #8
kevinkr is offline kevinkr  United States
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Well, unfortunately I have had a problem, and more than once with Mullard 12AX7(A!) and that was filament burn out. If you are using a very low impedance regulated dc supply to power these the inrush current can be large enough to burn out the filament. I have not seen this happen in small amps with undersized power transformers.. It's rather a pity because these are amongst the best sounding (and measuring) 12AX7A/ECC83 I have used.
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