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    Bias on the 2 back tubes is way high



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    Join date : 2014-11-04

    Bias on the 2 back tubes is way high

    Post by rilayton on Tue Dec 02, 2014 1:47 am

    Hello everybody, my name is Robin and I'm new here. I built a vta st120 and after a few minor issues got it up and running great. Now today I checked the bias. On the two front tubes it reads 40 or more vdc. the back tubes bias fine but I can't get the front ones to go down. I have swapped tubes and it is still the front tubes that read high. So I am assuming it is the amps problem cause it only happens in front. Any ideas or suggestions would be greatly appreciated. Thanks

    Hi, I figured it out 2 of the 2watt 10ohm bias resistors failed. Anybody know why? Will it be ok to replace them with higher wattage 10 ohm resistors? Thanks

    Last edited by rilayton on Tue Dec 02, 2014 2:43 am; edited 1 time in total (Reason for editing : found the cause of high bias voltage)
    Bob Latino

    Posts : 2644
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias on the 2 back tubes is way high

    Post by Bob Latino on Tue Dec 02, 2014 9:17 am

    Bias resistors fail when too much current is drawn through the resistor. The cause of too much current being drawn is usually a problem with the TUBES or the SOCKET that the tubes are in.

    Center the four bias pots and remove all the tubes. Turn the amp ON and measure the DC voltage on pins 5 and 6 of all output tubes. You should get somewhere between -45 to -65 volts DC as measured to chassis ground. If you do, then the bias system is OK and the problem is with the TUBE or the TUBE SOCKET. The output tube may have a partial short or maybe the tube has a non-contact issue with one of the pins.

    Replace the open 10 ohm 2 watt resistors on those two tube sockets and see if the amp will bias up properly.


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    Join date : 2008-11-30

    Re: Bias on the 2 back tubes is way high

    Post by tubes4hifi on Tue Dec 02, 2014 11:32 am

    better to burn out a 25c resistor than a tube! Also make sure you are actually biasing them correctly, measuring the voltage at the pin1-pin8 junction,
    usually just take a wire from there to the front socket test points.

    Posts : 91
    Join date : 2011-09-20

    Re: Bias on the 2 back tubes is way high

    Post by kaner on Tue Apr 21, 2015 8:42 am

    Sorry to bump an old thread but I wanted to again thank Bob, Roy and all the members for such a useful and informative forum. I lost a rectifier that took out an output tube. No big deal but when I replaced them I could not get the right side of my VTA ST70 to bias below 600mv. I found the above post with a simple search, re-tensioned sockets 5 & 6 and everything was just fine.

    Thanks yet again!

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