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    VTA ST-70 and 12bh7

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    fzapped

    Posts : 3
    Join date : 2018-05-19

    VTA ST-70 and 12bh7

    Post by fzapped on Sat May 19, 2018 12:16 pm

    I have a couple of 12bh7a tubes. They work fine as phase splitters (quite) but are noisy (hum) in the center position. 12au7 is quiet in center spot. 2 bad tubes? Wiring error? Any ideas appreciated.
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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST-70 and 12bh7

    Post by Bob Latino on Sat May 19, 2018 12:24 pm

    fzapped wrote:I have a couple of 12bh7a tubes.  They work fine as phase splitters (quite) but are noisy (hum) in the center position.  12au7 is quiet in center spot.  2 bad tubes?  Wiring error? Any ideas appreciated.

    The center driver tube is the voltage amplifier for BOTH channels. The two side tubes are the phase splitter/inverter tubes for that channel alone. Because their of different function in the amp, the two side tubes are relatively immune to causing any hum/noise in the amp.

    Bob

    fzapped

    Posts : 3
    Join date : 2018-05-19

    Re: VTA ST-70 and 12bh7

    Post by fzapped on Sat May 19, 2018 12:27 pm

    Noisy tubes then?

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    Kentley

    Posts : 497
    Join date : 2015-03-06
    Age : 66
    Location : Worcester, MA

    Re: VTA ST-70 and 12bh7

    Post by Kentley on Sat May 19, 2018 12:36 pm

    Yes, fz. I've often used fine old 12BH7s in that center position (my faves are vintage TungSols and Sylvanias) and they are completely silent.
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    Tubes4ever

    Posts : 119
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: VTA ST-70 and 12bh7

    Post by Tubes4ever on Sat May 19, 2018 12:39 pm

    fzapped wrote:Noisy tubes then?

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    Yes, Noisy tubes. Both of your 12bh7s have 60hz "noise" leaking from the heater to the cathodes.



    fzapped

    Posts : 3
    Join date : 2018-05-19

    Re: VTA ST-70 and 12bh7

    Post by fzapped on Sat May 19, 2018 12:40 pm

    Since they work as phase splitters my money is not totally wasted.

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