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    Low Gain Board Tube Option Question

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    EVGuy

    Posts : 5
    Join date : 2011-03-02
    Location : Earlville, IL

    Low Gain Board Tube Option Question

    Post by EVGuy on Sun Apr 03, 2011 2:39 pm

    I have the VTA70 low gain board with three 12AU7's in there. Which socket(s) has the most effect on the sound quality when tube rolling? I've read that 12BH7's can sometimes give better results and I found one in a drawer. Can I use that one along with the 12AU7's in the other two sockets or not a good idea to mix? I have several RCA black plate 12AU7's from 1957, a pair of Brimar 13D5 and an RFT 12AU7 to choose from and now this GE 12BH7A I found too.

    bigmm79

    Posts : 23
    Join date : 2010-09-15

    Re: Low Gain Board Tube Option Question

    Post by bigmm79 on Sun Apr 03, 2011 3:09 pm

    This is a common question. I think you'll find lots of answers if you poke around on this forum and others.
    I have used both 12BH7As and 12AU7s in various combinations. I think you want to use the same type of tubes in the two outer, phase splitter, spots. My favorite has been a GE 12BH7A in the center position and mullard 12AU7s in the outer positions. That said a 12AU7 in the center position and 12BH7s in the outer also sound real good. I have found that mixing 12BH7s and 12AU7s in this way is an improvement over just using 12AU7s in all positions. I've never tried 12BH7s in all three positions, but probably would be good.

    Bob Latino
    Admin

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

    Re: Low Gain Board Tube Option Question

    Post by Bob Latino on Sun Apr 03, 2011 3:57 pm

    The CENTER 12AU7/12BH7 on the lower gain VTA driver boad has the most influence on the sound of any ST-70 that uses a VTA driver board. This is the initial voltage amplifier for BOTH channels. The two side 12AU7/12BH7's have less influence on the sound of the amp. On the VTA low gain board you can use either 12AU7 or 12BH7 tubes in all three positions. 12BH7 tubes have larger plates, can swing more voltage and may be a better choice than a 12AU7 in the two outer (phase splitter/phase inverter) positions. You can also mix the two tubes on the VTA driver board. The best way is the way bigmm79 has mentioned > a 12AU7 tube in the center and two 12BH7 tubes in the two outer positions.

    Bob

    EVGuy

    Posts : 5
    Join date : 2011-03-02
    Location : Earlville, IL

    Re: Low Gain Board Tube Option Question

    Post by EVGuy on Sun Apr 03, 2011 8:06 pm

    Thanks Bob. I'm going to look for two more 12BH7As and give all three a try.

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