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    Switching the triode-pentode switches while the amp is running

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    RockyAM

    Posts : 26
    Join date : 2008-12-14

    Switching the triode-pentode switches while the amp is running

    Post by RockyAM on Sun Jun 06, 2010 11:42 am

    Hey Bob, My ST-70 is running great. Thanks for giving us this great kit. I have a question on the triode-pentode switches. In your instructions you say that the switches can be switched while the amp is running. I have done this many times with no ill effects. I have a friend who has this Chinese amp with triode-pentode switches. In the manual that came with his amp they instruct him to be sure and shut the amp off before he switches from triode to pentode or vice versa. He told me that once he didn't turn the amp off and got this wicked *blatttt* noise when he switched. Do you know why he has to turn his amp off and I don't have to turn the ST-70 off before I make the switch? RockyAM


    Last edited by RockyAM on Thu Dec 20, 2012 10:24 am; edited 1 time in total

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Switching the triode-pentode switches while the amp is running

    Post by Bob Latino on Sun Jun 06, 2010 12:49 pm

    Hi,

    The Dynaco amp circuit is set up so that the voltage on the plates (pin 3) of each output tube is virtually the same as the ultralinear screen tap (pin 4). In actual use if the plates are at say 496 volts the screens will be at about 498 volts. (virtually the same voltage but usually a volt or two higher). What happens is that because the voltage gradient (the relative difference in voltage between two points) is very low there is no switching transient when you switch from one mode to another. The switches supplied with the kit are "make before break" switches so that there is always voltage on the plates. In practice what happens on the ST-120 is that in triode mode the ultralinear screen tap is disconnected and the screen is then connected to the plate through a 100 ohm resistor - but there is always voltage on the plate. On your friends Chinese amp the circuit is probably different and my guess is that the ultralinear screen tap is connected through a resistor to the primary side of the output transformer. On his amp the plates may be at say 435 volts but the screens could be at 400 volts. Now you have a 35 volt voltage gradient which can cause a voltage spike if you make the switch while the amp is running. Some McIntosh and Marantz amps were set up like this with a resistor in the ultralinear line.

    Bob

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