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    Triode/pentode switching system for the Dynaco ST-70

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    Bob Latino
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    Triode/pentode switching system for the Dynaco ST-70

    Post by Bob Latino on Sat Aug 22, 2009 8:45 pm

    In one of the Sticky's on the first page of the Forum there is a thread about running your Dynaco ST-70, Mark II, Mark III or Mark IV in the triode mode. There is a basic diagram of a switching system that may be used to go back and forth from triode to pentode. The diagram below gives a little more detail. All you really need is a pair of DPDT switches that can handle 400+ VDC for the ST-70 and one DPDT switch for each of the Mark II, III and IV mono amps. You will also need two 100 ohm 1 watt resistors for each switch. The thing you have to decide is where to put the switches? If you have upgraded your stock driver board to the VTA driver board the choice is easy. Pull the original bias pots and you have a nice 3/8" hole to install the switches. I have seen amps that holes have been drilled on the back or top surface of the chassis. It really doesn't matter where you put the switches but if you do, consider easy access to them as one criteria for placement. Use good quality wire to wire them up. In general what you will be doing is pulling the green or green/white wires off pin #4 of each output tube and run them to the switch instead. What I like to do, since the bias diode is on the VTA board, is to install two 2 lug terminal strips to use as junction points for one leg of each 100 ohm 1 watt resistor. If you have the original driver board and have replaced the selenium rectifier with a diode, then just run a wire from lugs 3 and 6 on each switch back to pin 3 of each output tube and put the 100 ohm resistor at the end of the wire and connect the other end of the 100 ohm resistor to pin # 3 of each output tube.

    Bob

    The diagram below is for an ST-70. On a Mark II, III and IV you would just use one switch for each amp.



    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by Bugs on Tue Aug 25, 2009 10:39 pm

    Hi Bob, thanks for providing the more information on triode/pentode switching. I've been looking for switches rated at 400 + VDC and can't seem to locate any. Do you know where these can be sourced?

    Thanks,
    Bugs

    Bob Latino
    Admin

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

    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by Bob Latino on Wed Aug 26, 2009 5:08 am

    Bugs,

    Send me an Email at Bob01605@aol.com and I will give you more info on the switches ...

    Bob

    avi.inc

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    Age : 63
    Location : L.A.

    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by avi.inc on Tue Apr 27, 2010 1:57 pm

    Hi Bob

    Is this the same for the st-120 wiring?

    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by Bob Latino on Tue Apr 27, 2010 3:09 pm

    avi.inc wrote:Hi Bob

    Is this the same for the st-120 wiring?

    Hi,

    Yes - the triode/pentode switch wiring is the same for both the ST-70 and the ST-120.

    Bob

    RichJB603

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    Join date : 2010-05-10

    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by RichJB603 on Thu May 27, 2010 6:28 pm

    When setting bias should the amp be in pentode or triode mode or should it be the same no matter the position of the switch?

    Bob Latino
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    Re: Triode/pentode switching system for the Dynaco ST-70

    Post by Bob Latino on Thu May 27, 2010 6:56 pm

    RichJB603 wrote:When setting bias should the amp be in pentode or triode mode or should it be the same no matter the position of the switch?

    Hi Rich,

    On an ST-120 or an ST-70 that has triode/pentode switches you should set the bias while in pentode mode. The bias will drop about .010 VDC or so when you switch from pentode to triode. On an ST-120 with a bias voltage of .550 VDC (550 millivolts) in pentode - if you then switch to triode you will see the bias drop to about .540 VDC. The reason for the slight drop is that the ultralinear screen tap is disconnected when the amp is switched to the triode mode.

    Bob

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