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.
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.
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.
As long as your 12AT7 isn't wired in series with other tubes, you're probably OK.
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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.
It's rather a pity because these are amongst the best sounding (and measuring) 12AX7A/ECC83 I have used.- Status
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