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    ST70 12AT7/ECC81 tubes

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    vanity

    Posts : 7
    Join date : 2011-01-12

    ST70 12AT7/ECC81 tubes

    Post by vanity on Sat Nov 17, 2012 10:29 am

    Hi. I'm not 100% sure if I actually have a problem or not. Advice please!

    I've just replaced the trio of 12AT7/ECC81 tubes on my ST70 driver board because one failed. The replacements work fine and everything sounds lovely again but........................

    ..................When I swich the amp on all 3 12AT7/ECC81s glow very brightly, like a light bulb instead of a valve, for a second or so affraid then settle down to the normal amber glow I'm used to. I don't remember that happening with the valves I've had to replace. Maybe that's why I had to replace them in the first place.


    So I'm currently scared of using my system at all in case I blow up another £30 quids worth of new valves. cherry Can anyone tell me if I have a fault in the amp upstream of the tubes and, if so, wot it is and how to fix it.

    p.s. cracking forum. I'd be stuck without it! cheers

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 12AT7/ECC81 tubes

    Post by Bob Latino on Sat Nov 17, 2012 11:27 am

    Everything is OK. Mullard and a few other manufacturers have a different type of filament that does glow brightly for a couple of seconds when you turn the amp on and the tubes are COLD. This occurs because the tube's heater resistance is very low when the tube is cold. When the tube is warmed up, this won't happen. After the tubes are fully warmed up and you turn the amp OFF and then ON again with the tubes still warm, you won't get that flash. Note > Although this is referred to in many placed as the "Mullard Flash", it occurs in some other types of small signal tubes that use a similar filament structure as the Mullard small signal tubes. This was mentioned in an earlier post on this form a few years ago. Link below ..

    Older post on the "Mullard flash"

    Bob


    Last edited by Bob Latino on Sat Nov 17, 2012 12:43 pm; edited 1 time in total

    vanity

    Posts : 7
    Join date : 2011-01-12

    Re: ST70 12AT7/ECC81 tubes

    Post by vanity on Sat Nov 17, 2012 12:36 pm

    Yes. You are absolutely right Bob. I didn't think to mention that the new valves were Mullards.
    Thank you very much indeed for that help and reassurance. I'll be making up for lost listening time this weekend.

    ruffian

    Posts : 43
    Join date : 2010-02-26

    Re: ST70 12AT7/ECC81 tubes

    Post by ruffian on Sat Nov 17, 2012 4:42 pm

    The Siemens ECC-801s that I have do the "Mullard Flash" too. Enjoy.

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