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    ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

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    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by EugeneHiFi on Sun Jan 19, 2014 6:18 pm

    I completed the upgrade of my ST70 this morning using the VTA70 upgrade kid from Roy Mottram. This morning I was doing the final checkout of the ST70. I follow the instruction and check the filament power and all three driver tube filaments illuminated. Then I remove the 3 driver tubes and insert the rectifier tube and the 2 left channel output tubes and check the bias. Bias voltage set to 0.40 volts on both output tubes. Then do the same thing to the right channel. Bias voltage set to 0.40 volts on both output tubes.

    I rechecked the bias voltages on all 4 tubes and they are all right on at 0.40 volts after 15 minutes. So I assume the circuits are running fine. Now I power down the amp. and install the 3 output driver tubes and power up the amp again and recheck the bias setting on both channels. The bias voltages on the left channel are fine, but the bias voltages on the right channel begin exceed 0.6 volts and raise quick after 30 seconds or so.

    Any thoughts on what I might check next?

    Zimmer64

    Posts : 112
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by Zimmer64 on Mon Jan 20, 2014 2:09 am

    Have you tried to swap the tubes to make sure the problem is on the right channel and not a faulty tube?

    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by EugeneHiFi on Mon Jan 20, 2014 3:40 pm

    The 10 ohm bias resistor was burn out too. I am going to replace the resistor and try it again with another tube and see.

    BTW, I put the tube in the tube tester and it say it is OK with the short testing between all the elements.

    Before the incident happen, I checked the bias voltages on pin 6 on all the output tubes and they are at around -40vdc.

    Any other thought I may check or measure after I replace the resistor?

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by Bob Latino on Mon Jan 20, 2014 5:58 pm

    If the 10 ohm resistor burned then the output tube on that socket was drawing too much current. Most likely cause is a bad output tube on that socket .. The only other things I can think of is a bad coupling cap that feeds that tube socket or a partial contact on the tube socket pins on that socket. If you have the original sockets in your amp, I would clean all the socket pins on that socket with a contact cleaner. You might also try bending in the sides of the socket pins with a small flat bladed jeweler's screwdriver.

    Bob

    EugeneHiFi

    Posts : 12
    Join date : 2014-01-19

    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by EugeneHiFi on Tue Jan 21, 2014 9:34 pm

    One of the Russia pio capacitor was touching the circuit and shorted out somewhere. I didn't know the cases of the capacitors are bare metal.  I put heat shrink tube on all 4 capacitors and replace the 10 ohms bias resistor and the output tube.

    Amp power up without any problem and I am able to set the bias voltages on all 4 tubes. I have been playing music for the last 4 hours. I love my upgrade ST70!!!

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

    Post by arledgsc on Wed Jan 22, 2014 11:48 am

    Good job! Yes the PIOs are conductive and heavy so insulation is mandatory as they may shift if the unit is moved or shipped.

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    Re: ST70 Bias exceed 0.6 volt and raise quickly on one channel after install the 3 12AU7 tubes

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