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    ST70 hum problem

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    GreggW

    Posts : 55
    Join date : 2015-07-15

    ST70 hum problem

    Post by GreggW on Sun Jul 26, 2015 2:07 pm

    New to the forum.... Gifted an ST70 w/KT88s, VTA w/12AT7s. Amp is dead quiet with no input cables. When preamp connected, no hum at minimum volume, moderate hum starting about ⅓ on volume, louder as volume is turned up. Balance control moved to the left, hum on left channel disappears, to the right, hum remains. As soon as it is moved towards the center, hum on both channels. When a record is played, volume is equal from both speakers (AR2a). Rolled 12AT7s and got hum with balance full left or right. Also need instructions on how to bias as no documentation was received....

    Bob Latino
    Admin

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

    Re: ST70 hum problem

    Post by Bob Latino on Sun Jul 26, 2015 2:40 pm

    GreggW wrote:New to the forum.... Gifted an ST70 w/KT88s, VTA w/12AT7s. Amp is dead quiet with no input cables. When preamp connected, no hum at minimum volume, moderate hum starting about ⅓ on volume, louder as volume is turned up. Balance control moved to the left, hum on left channel disappears, to the right, hum remains. As soon as it is moved towards the center, hum on both channels. When a record is played, volume is equal from both speakers (AR2a). Rolled 12AT7s and got hum with balance full left or right. Also need instructions on how to bias as no documentation was received....

    Hi Gregg,

    If your VTA ST-70 is quiet with no input cables OR with the input cables connected to your preamp and the preamp on at minimum volume then the amp is not at fault .. It is your preamp that is at fault.

    To bias an older 12AT7 driver board VTA ST-70 see below .. NOTE - The older VTA amps with the 12AT7 driver tubes have the bias pot's rotation in reverse > clockwise rotation will DECREASE bias level and counterclockwise rotation will INCREASE bias level.

    Get your multitester and set the range for 0 to 2 volts DC. CONNECT THE SPEAKERS. Place the GZ34 rectifier tube and just the LEFT EL34/KT88 output tubes in their socket. Turn on the amp. Allow the amp to warm up for a minute and check to see that the EL34 and GZ34 tubes light up properly. Measure the bias on the LEFT output tubes by placing the BLACK NEGATIVE probe anywhere on the chassis and the RED POSITIVE probe in the hole marked “Biaset 1.56 v”. With a small screwdriver adjust the FRONT LEFT bias adjuster on the driver board closest to the front left tube. Turn the adjuster CLOCKWISE TO REDUCE BIAS and COUNTERCLOCKWISE TO INCREASE BIAS. Set the bias for .400 volts if you are using EL34 or KT77 tubes OR .500 volts if you are using KT88 or 6550 tubes. Now place the positive probe in pin 4 (which is on the opposite side of the power take off socket) and use the BACK LEFT bias adjuster to measure bias on the LEFT REAR output tube. Set it the same as the front tube. Turn off the amp. (Note - on some amps the REAR output tube's bias measuring point may be on a different front tube socket pin) Plug in the two RIGHT channel EL34/KT88 tubes, turn on the amp and repeat the bias procedure as outlined above on the RIGHT channel tubes. Now go back to the left channel tubes and notice that the bias is now a little LOW. ADJUSTING BIAS ON ONE TUBE HAS A SLIGHT AFFECT ON THE OTHERS. Go back and forth between all output tubes until all tubes have the proper bias.

    Bob

    GreggW

    Posts : 55
    Join date : 2015-07-15

    ST70 hum problem

    Post by GreggW on Sun Jul 26, 2015 3:05 pm

    Wow, thanks for the quick response! Reset the bias as it was a touch high; pots are sensitive. Working on the preamp issue: getting a new one.....

    GreggW

    Posts : 55
    Join date : 2015-07-15

    Hum problem resolved

    Post by GreggW on Sat Aug 08, 2015 11:54 pm

    PAS3 basket case project included the power supply, phono and TCLA boards mostly completed. Everything wired up and it passed the smoke test, but no sound - no B+.  Got the latest power supply from Tube Nirvana, passed the smoke test again, good sound but plenty of hum. Sent The PAS3 to Tube Nirvana for a tune-up and oil change. Turns out it had a bad TCLA board. Well, now it doesn't and I have a superb sounding preamp and amp. I'm going to wear out some vinyl and be very happy doing it. Super pleased with Holger and the great service at Tube Nirvana.....

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