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    VTA ST-120 Voltage Question

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    geode

    Posts : 6
    Join date : 2015-04-16

    VTA ST-120 Voltage Question

    Post by geode on Sun Apr 26, 2015 6:09 pm

    I just got through repairing my amp. It blew the two a resistors connected to one channel of the Triode/Pentode switch. One tube developed a short and caused the vent. I replaced the resistors and one PIO coupling cap. I turned it on and it biased up okay with 4 KT88 tubes. I also apparently fixed a hum in one channel by replacing the PIO cap. Now no hum in either channel. I took all of the voltage reading in Bob's manual. They all check in spec except one tube which has higher voltage readings on pins 5 and 6. It should read between -50 and -60 volts DC. I get a reading of -69 on pin 5 and -70 on pin 6. Is this too far out of the range. What might be causing the difference. I checked all the resistors and they all are in spec. Right now I am running the amp with new quad of KT120 tubes. They bias up fine and hold their readings. Should I be overly concerned about this one tubes reading be out of spec. Also, I didn't take voltage readings of the three preamp tubes. Any feedback will be most appreciated.

    Bob Latino
    Admin

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

    Re: VTA ST-120 Voltage Question

    Post by Bob Latino on Sun Apr 26, 2015 6:57 pm

    The negative DC voltage that you read on pins 5 and 6 of each output tube is regulated by the position of the bias pot. If one tube reads a little high - that is OK because you could just turn the bias pot a little to get the voltage down to between -50 to -60 VDC. That "between - 50 and -60 VDC" is predicated on a centered bias pot. Unfortunately those bias pots are + or - 10% and sometimes one pot is off just a little from the norm and even when it is centered, the pin 5 and 6 voltages may be off a little.

    As long as the tubes bias up properly, you are OK ...

    Bob

    geode

    Posts : 6
    Join date : 2015-04-16

    Re: VTA ST-120 Voltage Question

    Post by geode on Sun Apr 26, 2015 7:15 pm

    Bob Latino wrote:The negative DC voltage that you read on pins 5 and 6 of each output tube is regulated by the position of the bias pot. If one tube reads a little high - that is OK because you could just turn the bias pot a little to get the voltage down to between -50 to -60 VDC. That "between - 50 and -60 VDC" is predicated on a centered bias pot. Unfortunately those bias pots are + or - 10% and sometimes one pot is off just a little from the norm and even when it is centered, the pin 5 and 6 voltages may be off a little.

    As long as the tubes bias up properly, you are OK ...

    Bob

    Thanks Bob. I have been playing the amp for about 2 hours. I am going to let it settle for a little while and recheck/set the bias to .55. I am running new tubes and one new PIO cap I got from you. Do these tubes need to burn in a while. It seems that the amp plays and sounds good for a while and then the volume appears to drop slowly and the crispness and detail of the music becomes a little muddy sounding. I've never installed brand new tubes before, so I don't really know what the burn in requirement is. I do want to say "Thank You" for getting that PIO cap to me so fast, and also for answering my newbie tube questions.

    George

    Bob Latino
    Admin

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

    Re: VTA ST-120 Voltage Question

    Post by Bob Latino on Sun Apr 26, 2015 7:31 pm

    geode wrote:

    Thanks Bob.  I have been playing the amp for about 2 hours.  I am going to let it settle for a little while and recheck/set the bias to .55.  I am running new tubes and one new PIO cap I got from you.  Do these tubes need to burn in a while.  It seems that the amp plays and sounds good for a while and then the volume appears to drop slowly and the crispness and detail of the music becomes a little muddy sounding.  I've never installed brand new tubes before, so I don't really know what the burn in requirement is.  I do want to say "Thank You" for getting that PIO cap to me so fast, and also for answering my newbie tube questions.

    George

    I am assuming that the issue you are having is the same on both channels ..

    If the volume drops and the music becomes a little muddy, it is possible that the rectifier has an issue. The other tube that could cause something like this is the center driver. Try substitution of the center driver tube and/or the rectifier tube ...

    Bob

    geode

    Posts : 6
    Join date : 2015-04-16

    Re: VTA ST-120 Voltage Question

    Post by geode on Sun Apr 26, 2015 7:59 pm

    Bob Latino wrote:
    geode wrote:

    Thanks Bob.  I have been playing the amp for about 2 hours.  I am going to let it settle for a little while and recheck/set the bias to .55.  I am running new tubes and one new PIO cap I got from you.  Do these tubes need to burn in a while.  It seems that the amp plays and sounds good for a while and then the volume appears to drop slowly and the crispness and detail of the music becomes a little muddy sounding.  I've never installed brand new tubes before, so I don't really know what the burn in requirement is.  I do want to say "Thank You" for getting that PIO cap to me so fast, and also for answering my newbie tube questions.

    George

    I am assuming that the issue you are having is the same on both channels ..

    If the volume drops and the music becomes a little muddy, it is possible that the rectifier has an issue. The other tube that could cause something like this is the center driver. Try substitution of the center driver tube and/or the rectifier tube ...

    Bob

    Yeah it does appear to be both channels. I reset the bias to .55 on all 4 tubes. I have one of the miltary mullards in the center. I use one of the Weber SS rectifiers. I also have the original Ruby rectifier tube. The other two preamp tubes are JAN Phillips 12AT7 tubes. I have a hum in both channels when using one of the JAN tubes in the center, when reinstalling the mullard in the middle position both channels are dead quiet. I'm listening now and it sounds good again. I can swap in the Ruby tube and rebias when/if it happens again. Thanks again, Bob.

    George

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