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    tube choices

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    aliveatfive

    Posts : 7
    Join date : 2009-03-21

    tube choices

    Post by aliveatfive on Sat Mar 21, 2009 3:35 pm

    I constructed a new ST70 about 18 months ago using Roy's board and transformers from Dynakitparts.com. The amp sounds really good. As a matter of fact, I sometimes prefer it to my basement system which is worth about 50k! Associated equipment is a Grounded Grid preamp, Sangean HD tuner, Theta Miles CD player and LS3/5a speakers from Stirling.

    I'd like to try output tube rolling in the amp and I have some questions about biasing it. I have a 2 spare sets of new-stock output tubes: Tung-Sol 6550's and Gold Aero KT66's. If I use either of these 2 sets, do I still set the bias to the same value as the EL34's? Do any resistor changes have to be made with the changed output tubes? (I do have a new power xformer with increased amperage rating.) Thanks for any help, in advance.

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: tube choices

    Post by Bob Latino on Sat Mar 21, 2009 8:32 pm

    Hi,

     In your VTA ST-70 with the upgraded power transformer from Dynakitparts you may use 6L6, KT66, EL34, KT77, 6550 or KT88 tubes. There are a lot of thoughts as to the correct bias current in an ST-70. Dynaco ran their tubes pretty hard with higher bias voltages than what might be practical by today's standards. My recommendations are to (and this is for a VTA board driven ST-70 not a stock ST-70 with the 7199 driver borad)

    Bias the 6L6 or El34 tubes at .40 volts DC per tube (40 milliamps per tube)
    Bias the KT66 or KT77 tubes at .45 volts DC per tube (45 milliamps per tube)
    Bias the 6550 or KT88 tubes at .50 volts DC per tube (50 milliamps per tube)

    * A note on the KT88 and 6550 tubes - Don't run these tubes if you have a stock PA-060 power transformer. They draw maybe 30% more current and will put a strain on the stock power transformer which had virtually no reserve. You could damage the stock power transformer.

    You can run the 6L6, EL34, KT66 and KT77 tubes with no resistor changes. You MAY be able to also run the KT88 and 6550 tubes in your amp also with just a rebias change. If the KT88's or 6550 won't bias or are at the very end of the bias pot rotation then you may have to alter R39 on the VTA driver board. In there right now you have a 7.5K (newer CCS board that uses lower gain 12AU7 tubes) or 10K resistor (older higher gain board). You have to LOWER the value of this resistor to 5 K. One trick to do without having to remove the board (R39 is under the lip of the chassis cutout and would be difficult to get out without pulling the driver board) is to solder on the BOTTOM of the board on the two solder connnections of R39 a 27K resistor which will give you 7.3 K total resistance for R39. If that isn't enough then instead solder on ANOTHER 10 K resistor which will give you 5 K total resistance for R39. Don't go any lower than this because if you do then EL34's my not bias now.

    Bob


    Last edited by Bob Latino on Sun Feb 16, 2014 8:23 pm; edited 1 time in total

    deluxmon

    Posts : 17
    Join date : 2013-03-20
    Age : 62
    Location : DENVER

    How about 350B's in a VTA ST-70

    Post by deluxmon on Sun Feb 16, 2014 6:57 pm

    Anyone try the 350B in a VTA ST-70?

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