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    Checking filament voltage VTA ST120

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    eickmewg

    Posts : 95
    Join date : 2014-08-29

    Checking filament voltage VTA ST120

    Post by eickmewg on Thu Mar 01, 2018 10:51 am

    I was checking tube bias on my ST 120 this morning, after several months since the last check, and found that there was no bias voltage when checking V2 (back left tube) and that the bias on the other power tubes was, not surprisingly, high.  I have been using Black Treasure KT88 tubes.  And the amp seemed to be functioning normally.

    I had previously noted that the filaments seemed brighter on V2.  I chalked this up to differences in tube construction.  I checked the V2 bias resistor and it was fine at 10 ohms.  I put my backup Gold Lion KT88's in the amp and bias voltage returned to  V2, however, again the filaments on V2 seem brighter than the other power tubes.  The left-hand power tubes and the left-most driver use different transformer heater windings from those on the right. Maybe this is simply a coincidence but, I think I need to check the filament voltages.  I have the TDR module installed but I don't think this affects the heater circuitry.  So, with the power tubes removed can I just check the AC voltage on pins 2 and 7 with a multimeter probe on either pin and the second to chassis ground, or would one lead be on each pin?  Then, if there are voltage differences between the two heater windings, is the power transformer faulty?  Thanks in advance for any help the forum can provide.

    Bill
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    Peter W.

    Posts : 1036
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Checking filament voltage VTA ST120

    Post by Peter W. on Thu Mar 01, 2018 11:27 am

    You can check the *UNLOADED* filament voltage as-described. If you do this, be sure to remove the rectifier as well, as the main filter caps do not like being operated unloaded.

    I would suggest that you remove the power-tubes two at a time, checking each socket individually. In this way, you may also trace whether one tube or another has any significant effect on filament voltage. Pull one tube - measure. Pull the second, measure. Put back. Pull the third, measure. Pull the fourth, measure. Put back. Then do 1 & 4, 2 & 3.

    You have not mentioned how many hours you have on these tubes. One-or-more may be starting to fail, have a partial short or other similar problem.

    A good tube-tester would be useful here as a screen for incipient defects or failures. More on this in another post.
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    peterh

    Posts : 1070
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Checking filament voltage VTA ST120

    Post by peterh on Thu Mar 01, 2018 11:37 am

    eickmewg wrote:I was checking tube bias on my ST 120 this morning, after several months since the last check, and found that there was no bias voltage when checking V2 (back left tube) and that the bias on the other power tubes was, not surprisingly, high.  I have been using Black Treasure KT88 tubes.  And the amp seemed to be functioning normally.

    I had previously noted that the filaments seemed brighter on V2.  I chalked this up to differences in tube construction.  I checked the V2 bias resistor and it was fine at 10 ohms.  I put my backup Gold Lion KT88's in the amp and bias voltage returned to  V2, however, again the filaments on V2 seem brighter than the other power tubes.  The left-hand power tubes and the left-most driver use different transformer heater windings from those on the right. Maybe this is simply a coincidence but, I think I need to check the filament voltages.  I have the TDR module installed but I don't think this affects the heater circuitry.  So, with the power tubes removed can I just check the AC voltage on pins 2 and 7 with a multimeter probe on either pin and the second to chassis ground, or would one lead be on each pin?  Then, if there are voltage differences between the two heater windings, is the power transformer faulty?  Thanks in advance for any help the forum can provide.

    Bill

    The "glow" that may be observed is not a reliable indicator of either tube health or filament voltage.
    The visible part of the filament is more or less a random turn from the cathod inside.

    The lack if bias indicator ( voltage measured across the cathode resistor) is a much more
    alarming indicator. If the wiring is complete ( when amp is cold, remove the offending tube,
    measure ohm between tubesocket pin 8 and the corresponding measuring point at the front).
    If there is continuity here the tube is most likley faulty. You can still have decent sound at
    low volume as the remaining tube will function.
    Double check by switching place between the two tubes at the offending side. If lack of
    bias indicator follows tube, tube is broken. If it stays in the same position look for
    faulty soldering or other problems inside the amp.

    eickmewg

    Posts : 95
    Join date : 2014-08-29

    Re: Checking filament voltage VTA ST120

    Post by eickmewg on Thu Mar 01, 2018 12:35 pm

    Thanks for the advice. Indeed, it could just be a bad tube. The tubes are about two years old but I would be surprised if they had over 1000 hours total. I did not move power tubes so I can't say the lack of bias voltage followed the tube and now that I have the Gold Lion's in place I'm not inclined to put the Black Treasures back in for a test. I have marked the offending tube and can check in the future. This all may just be coincidental, but I will check the heater voltages just to be sure. In rereading the assembly instructions, heater voltage is measured as AC voltage ACROSS pins 2 and 7. Peter, I will follow your procedure looking for between 6 and 6.5 volts per Bob's instructions.

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