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    ST 70 VTA 12BH7s

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    Bugs

    Posts : 91
    Join date : 2009-04-17

    ST 70 VTA 12BH7s

    Post by Bugs on Wed Dec 15, 2010 11:27 pm

    Bob or Roy -

    To use 12BH7s in the low gain VTA board, are there any resistor changes that need to be made? If I remember correctly, Roy use to offer 12BH7s as an option.

    Thanks

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST 70 VTA 12BH7s

    Post by Bob Latino on Thu Dec 16, 2010 7:53 am

    Hi Bugs,

    To change from the standard gain board which uses 12AT7 tubes to the lower gain board which uses 12BH7 or 12AU7 driver tubes you change have to change out 16 resistors - 8 on each channel.

    For the ST-70 to change from the standard gain board to the lower gain board you change ..

    R27 and R28 from 1K (1000 ohms) 1/2 watt to 475 ohms 1/2 watt
    R9 and R10 from 2.2K 1/2 watt to 1 K (1000 ohms) 1/2 watt
    R15 and R16 from 7.5K 1/2 watt to 6.8K 1 watt
    R17 and R18 from 12K 1/2 watt to 12K 2 watt
    R11 and R12 from 68K 1/2 watt to 36K 2 watt
    R13 and R14 from 150K 1/2 watt to 36K 2 watt
    R19, R20, R21 and R22 from 51K 1/2 watt to 33K 2 watt

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST 70 VTA 12BH7s

    Post by Bugs on Fri Dec 17, 2010 2:41 am

    Thanks Bob,

    I'm using the low gain board. It sounds like the 12BH7s are a drop in replacement for the 12AU7s?

    Thanks

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST 70 VTA 12BH7s

    Post by Bob Latino on Fri Dec 17, 2010 6:15 am

    Bugs wrote:Thanks Bob,

    I'm using the low gain board. It sounds like the 12BH7s are a drop in replacement for the 12AU7s?

    Thanks

    Hi Bugs,

    The gain of the 12BH7 and the 12AU7 are almost identical. The 12BH7 is very slightly lower. 12BH7's are a slightly taller tube with larger plates and can swing a little more voltage than a 12AU7. You can use a mixture of the two tubes also. If you mix them I would use a 12AU7 in the CENTER position (initial voltage amplifier) and two 12BH7's in the two side (phase splitter/phase inverter) positions.

    Bob

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST 70 VTA 12BH7s

    Post by Bugs on Sat Dec 18, 2010 2:50 pm

    Thanks Bob, I'll give that a try.

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