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    Help, ST-70 bias issue and single output tube glowing

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    raudio44


    Posts : 45
    Join date : 2014-09-12

    Help, ST-70 bias issue and single output tube glowing Empty Help, ST-70 bias issue and single output tube glowing

    Post by raudio44 Sun Jul 17, 2022 5:23 am

    Hi everyone, OK, have an issue with my ST-70. Had been updated years ago by Troy Madden (recommended tech by Bob L.) so mostly original and using 7199s.
    This amp has not been used much and was working well when I fired up the system to hear my new SVS Prime bookshelves. thanks again to everyone here who helped me find speakers.
     
    So, I did the yellow sheet mod, which went well. Next, I upgraded the speaker posts replacing the originals with a set I got from Roy. Just a single pair for an 8Ω tap.
    Was careful since I never did this before.
    It has been some years since I had cleaned it, so Deoxit on the bias pots, cleaned tube sockets using soft pipe cleaners with a little Deoxit on the ends, cleaned tube pins gently with a fiberglass pen scrapper (or whatever it's called). worked well. then of course cleaned and polished the chassis, which seems to be in decent condition.

    Before I put it back into the system, I looked things over and noticed that the right, rear coupling cap was a little loose. I opened it up, and found when moving the cap, the connection would separate from the board, and there was a small break at the trace. damn. I must have done some bonehead thing to cause this. So, I soldered it back, (really wasn't confident in the way it turned out) and hoped for the best.

    Did have my doubts and, well, put it back in the system, turned on the preamp but did not have a source on and waited a bit. Switched on the amp and did hear a single pop from the right speaker.
    Checked the bias right away since I had cleaned the pots and didn't know where it would be at. damn, on the right channel it read 6VDC (did I misread it, could it be that high?) and adjusting the pot did nothing. Quickly checked the left and was able to set it fine at 1.4VDC just to get close so I could get back to the right channel. Then noticed the right rear output tube was glowing bright.
    Shut it down fast.
    What would cause this condition?

    I am not a tech, and don't know what causes what. (?)
    Now to me, it seems to correlate with right rear coupling cap issue, as I thought might be a problem.


    Before I make things worse, thought I should get some input as to whether I am on the right track.
     
    Thank you for your patience reading this and would greatly appreciate any help.


    Last edited by raudio44 on Sun Jul 17, 2022 9:07 am; edited 1 time in total (Reason for editing : correction)
    Bob Latino
    Bob Latino
    Admin


    Posts : 3172
    Join date : 2008-11-26
    Location : Massachusetts

    Help, ST-70 bias issue and single output tube glowing Empty Re: Help, ST-70 bias issue and single output tube glowing

    Post by Bob Latino Sun Jul 17, 2022 8:07 am

    If the right rear tube glows bright (this is called "red plating") either ..

    1. That tube is bad

    OR

    2. The tube is not making good contact on all the socket pins.

    What I would do is remove the tube, clean the tube pins and the socket pins with a contact cleaner. You should also "retension" all socket pins on that socket that the tube was in. Use pliers and carefully make each socket pin hole a little smaller. Try the same tube again. If the tube still red plates, replace the tube.

    Bob
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    raudio44


    Posts : 45
    Join date : 2014-09-12

    Help, ST-70 bias issue and single output tube glowing Empty Re: Help, ST-70 bias issue and single output tube glowing

    Post by raudio44 Sun Jul 17, 2022 9:01 am

    Thank you, Bob. I had just cleaned all tube pins and sockets were cleaned using Deoxit on soft pipe cleaners. 

    Can try switching tubes from front to rear and see if it is the tube or socket BUT would bad contact of the tube cause the high bias?

    Had thought it may be from the issue with the coupling cap connection on the board.
    Bob Latino
    Bob Latino
    Admin


    Posts : 3172
    Join date : 2008-11-26
    Location : Massachusetts

    Help, ST-70 bias issue and single output tube glowing Empty Re: Help, ST-70 bias issue and single output tube glowing

    Post by Bob Latino Sun Jul 17, 2022 10:02 am

    raudio44 wrote:Thank you, Bob. I had just cleaned all tube pins and sockets were cleaned using Deoxit on soft pipe cleaners. 

    Can try switching tubes from front to rear and see if it is the tube or socket BUT would bad contact of the tube cause the high bias?

    Had thought it may be from the issue with the coupling cap connection on the board.

    Yes - especially if pin # 5 is not making contact .. Pin # 5 is where the bias is "injected" into the tube to control the flow of electrons.

    Again - you may also want to try a new tube in that socket.

    Bob
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    raudio44


    Posts : 45
    Join date : 2014-09-12

    Help, ST-70 bias issue and single output tube glowing Empty Re: Help, ST-70 bias issue and single output tube glowing

    Post by raudio44 Mon Jul 18, 2022 6:20 am

    Thanks again Bob, you gave me something to focus in on. Opened it up and think I may have found something. When I get to check it out, I will update you. Your input is very much appreciated.
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    raudio44


    Posts : 45
    Join date : 2014-09-12

    Help, ST-70 bias issue and single output tube glowing Empty Re: Help, ST-70 bias issue and single output tube glowing

    Post by raudio44 Thu Jul 21, 2022 5:10 am

    Hi Bob, well, since you were so generous in trying to help me, wanted to give you an update. 

    Your mention of pin 5 helped and, as I said previously, thought I spotted the problem. Turned out I was right, and the issue is resolved. 

    Inserting tube V7 pushed the pin socket down where it contacted the grounded leg of the 15.6Ω resistor. Somehow it must have been pushed down where it was close to the pin socket. easy fix and glad I spotted it. Amp seems fine now. No red platting. by the way, I knew this term before, don't know why I typed in 'glowing'. oh well. Thank you again, very much appreciated.

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