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    ST-120 First a buzz, then a tube light show....and then dead.

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    Dogstar

    Posts : 92
    Join date : 2014-06-23

    ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 19, 2014 2:45 pm

    Awhile ago I read about an amp that had a hissing noise and some low frequency rumbling sounds. The solution was to clean the contacts with some WD-40 and perhaps adjust them so the contact pressure was tighter on the tube terminals.

    Well...I sorta did that. I used some DeOxit and also adjusted the contacts. Everything sounded good when I first started up other than that the tubes needed a warmup.

    After 20 minutes or so I heard a buzz from the speakers...my first though was that the CD was trashed...but then I saw the right rear KT-88 glowing like an incandescent bulb...and then the amp went silent.

    I pulled the fuse. The filament looked unbroken but there was a dark spot at the midpoint of the glass tube.

    I opened the case and visually inspected everything and saw no signs of catastrophic failure. I cleaned all the dust inside and out and did a more thorough job of cleaning and adjusting the contacts. I'm running out to get some fuses. I wish Guitar Center had rectifier tubes.

    Is there anything else I should check or look for?

    Thanks in advance....

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by peterh on Sun Oct 19, 2014 3:26 pm

    Dogstar wrote:Awhile ago I read about an amp that had a hissing noise and some low frequency rumbling sounds. The solution was to clean the contacts with some WD-40 and perhaps adjust them so the contact pressure was tighter on the tube terminals.

    Well...I sorta did that. I used some DeOxit and also adjusted the contacts. Everything sounded good when I first started up other than that the tubes needed a warmup.

    After 20 minutes or so I heard a buzz from the speakers...my first though was that the CD was trashed...but then I saw the right rear KT-88 glowing like an incandescent bulb...and then the amp went silent.

    I pulled the fuse. The filament looked unbroken but there was a dark spot at the midpoint of the glass tube.

    I opened the case and visually inspected everything and saw no signs of catastrophic failure. I cleaned all the dust inside and out and did a more thorough job of cleaning and adjusting the contacts. I'm running out to get some fuses. I wish Guitar Center had rectifier tubes.

    Is there anything else I should check or look for?

    Thanks in advance....
    new tubes,and maybe inspect/replace tube socket

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 19, 2014 3:33 pm

    sounds to me like that KT88 went south big time. Do you have spare KT88's?
    Do you have a volt/multi meter? Did you build this amp yourself and have you ever checked BIAS voltages?
    I would pull ALL the tubes and check some voltages, filament and the high voltage, ie the red wires out of the power transformer. If all checks good turn it off and insert the driver tubes only and make sure they 'light up'.
    Then insert rectifier and say left channel KT88's as well as connecting both speakers and see if you can adjust the BIAS to 0.50V per tube, if all there is good, turn it off, insert right channel KT88's, turn it back and adjust the BIAS on the right then re check BIAS on all other tubes, as adjusting BIAS on one tube will affect the others, repeat adjusting the BIAS until all KT88's are at or near 0.50VDC.
    If all that worked out, see if you get any sound now.
    If you do have a spare KT88, I'd use that in place of the one that turned into a light bulb!

    sKiZo

    Posts : 1311
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by sKiZo on Sun Oct 19, 2014 4:55 pm

    Did you plug the tube in correctly? The ridge on the tube base that is supposed to line up with the notch on the socket isn't always all that tight, and it is possible to insert the tube incorrectly.

    (Don't ask me how I know) geek

    I'd also double check the solder connections on the sockets. All the fiddling about could have caused an already weak joint to go south.

    DO check the bias resistors for burns or heat damage. Those are there to help protect and should be the first thing to go on major failures.

    deepee99

    Posts : 1336
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by deepee99 on Sun Oct 19, 2014 5:10 pm

    If it lit up like an incandescent bulb (as opposed to glowing bright red) then I don't guess you're red-plating, which would have been a pin-5 or bias issue. To quote a certain sage, don't ask me how I know. Hope it's just a bum tube and your seller stands by his product.

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 19, 2014 5:42 pm

    I do have another set of KT-88's...but only one rectifier. I had the Latino team build the amp for me. When I opened it up the wiring was impeccable. I checked connections and not being an electrical guy I would know how to check to see if there were bad components. I do however have a DMM and I will perform a startup test procedure as you recommend. Just to be sure when you say I should install just the driver tubes and see if they light up you are referring to the 3 12AU7's on the PCB right? Only those 12AU7's and not the rectifier or the KT-88's?

    The tubes that were in the amp were a few years old and they bounced around in the move. ...so I'm not going to worry if it was just a bad tube.

    Incidentally....when I was removing the tubes from the socket the post with key on it broke off.

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 19, 2014 5:55 pm

    Dogstar wrote:I do have another set of KT-88's...but only one rectifier. I had the Latino team build the amp for me. When I opened it up the wiring was impeccable. I checked connections and not being an electrical guy I would know how to check to see if there were bad components. I do however have a DMM and I will perform a startup test procedure as you recommend. Just to be sure when you say I should install just the driver tubes and see if they light up you are referring to the 3 12AU7's on the PCB right? Only those 12AU7's and not the rectifier or the KT-88's?

    The tubes that were in the amp were a few years old and they bounced around in the move. ...so I'm not going to worry if it was just a bad tube.

    Incidentally....when I was removing the tubes from the socket the post with key on it broke off.

    yes, just the 12AU7's and no rectifier. If you do carry out checking voltages with NO TUBES in and you do measure a good filament voltage, about 7VAC or so, then we can probably assume that the 12AU7's will light up with no other tubes in including the rectifier....but then, we all know that assumption is the mother of the biggest f ups! Rolling Eyes
    If your rectifier has gone south, I would consider and recommend the Weber Copper Top WZ68. Sure, it does not light up and does not look as cool as a tube rectifier, but they are pretty darn solid, and cheaper than most good rectifier tubes at $20 a pop!

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 19, 2014 6:29 pm

    I've been meaning to invest in a Weber.

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 19, 2014 9:14 pm

    Ok....so as it turned out the rectifier went bad and luckily I had a spare Groove Tube guitar rectifier tube that I did get at Guitar Center a long time ago. It's exactly the same shape as the Sovtek.,I put that in and was able to bias all the KT-88's to .550 VDC. The only problem is that the hiss is still there. It does go away after a few minutes of warming up.

    And here's another thing....if I get the hiss when I power up and I shut it down and wiggle the tubes around a little and then power it up again the hiss is gone.

    What next?

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 19, 2014 9:23 pm

    Dogstar wrote:Ok....so as it turned out the rectifier went bad and luckily I had a spare Groove Tube guitar rectifier tube that I did get at Guitar Center a long time ago. It's exactly the same shape as the Sovtek.,I put that in and was able to bias all the KT-88's to .550 VDC. The only problem is that the hiss is still there. It does go away after a few minutes of warming up.

    And here's another thing....if I get the hiss when I power up and I shut it down and wiggle the tubes around a little and then power it up again the hiss is gone.

    What next?

    what model rectifier tube, we are assuming it can handle the current load, better check on that. And, get yourself a Weber Copper Top WZ68!
    If wiggling the tubes makes the hiss go away, I would once again look at the tube socket pins, as well as the pins on the tubes.

    Bob Latino
    Admin

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    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Bob Latino on Sun Oct 19, 2014 9:26 pm

    Hiss is more commonly an issue with the center driver tube especially if it affects BOTH channels. Try another driver tube in the center position .. You can also try removing the tube (after the amp is OFF), cleaning the pins and then reinserting the tube.

    Bob

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 19, 2014 9:28 pm

    Bob Latino wrote:Hiss is more commonly an issue with the center driver tube especially if it affects BOTH channels. Try another driver tube in the center position .. You can also try removing the tube (after the amp is OFF), cleaning the pins and then reinserting the tube.

    Bob

    thats a good point Bob, when I have a hum issue in a new built amp, it is almost always the center driver tube just being too noisy!

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 19, 2014 10:51 pm

    Ok...I did clean all thr KT-88 pins...
    It's time to spring for some new tubes all around anyway.

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Sun Oct 19, 2014 10:55 pm

    Dogstar wrote:Ok...I did clean all thr KT-88 pins...
    It's time to spring for some new tubes all around anyway.

    can't hurt to have some spares and options, such as KT120's as well, make sure you get a matched set, I like Tung Sol. If you are going to try KT120's, set BIAS to 0.60VDC each tube.
    They are bigger too! Smile

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Mon Oct 20, 2014 7:47 pm

    I do not have any extra 12AU7 so I tried switching an outboard tube for the center. There was still some hiss.. I take it that means I should plan on a new set of those as well....which is ok. I don't mind having spares. I take it I might find that the driver tubes may be the source of the hiss and not the KT-88's. As I said I don't mind spares and I am curious about the sound of the KT-120's.

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Mon Oct 20, 2014 10:33 pm

    I'm kinda curious...I removed the driver tubes and cleaned the pins and the socket contacts and put the tubes back in...the amp hissed as usual...I turned it off and then back on 5 minutes later and the hiss is gone. If I shut the amp off and keep it powered down for more than a half hour the hiss is back. Weird huh?

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Mon Oct 20, 2014 10:38 pm

    Dogstar wrote:I'm kinda curious...I removed the driver tubes and cleaned the pins and the socket contacts and put the tubes back in...the amp hissed as usual...I turned it off and then back on 5 minutes later and the hiss is gone. If I shut the amp off and keep it powered down for more than a half hour the hiss is back. Weird huh?

    sounds to me like it is 'heat' related...when cool, there is hiss, once warmed up, no hiss....

    sKiZo

    Posts : 1311
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by sKiZo on Mon Oct 20, 2014 11:29 pm

    I don't have enough spares ...



    ... so I just ordered another set ... ;-}

    Can't go wrong with RCA clear tops ... should set you back maybe $10 a pop on da bay.



    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Mon Oct 20, 2014 11:41 pm

    I ordered Tung-Sols all around...perhaps my next go around I'll go with RCA's and Mullards.

    Guest
    Guest

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Guest on Mon Oct 20, 2014 11:56 pm

    I really like RCA 5963's, I use them in all my ST-70's, but since I use the octal VTA driver in my ST-120's, I use 6SN7's on each channel and 6SL7 in the center, both RCA and GE, all NOS.

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sat Oct 25, 2014 1:15 am

    The Tung-Sol KT-120's and 12AU7's came in yesterday and today the WZ68 Weber rectifier got here. I installed them all, biased the amp for .600 VDC and even with no break in the amp does sound much better. It seems there us a more open dimension to the sound in addition to the instruments sounding much fuller than before. I can only imagine that once broken in the tubes will make the amp sound even better.

    deepee99

    Posts : 1336
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by deepee99 on Sat Oct 25, 2014 10:59 am

    Dogstar wrote:The Tung-Sol KT-120's and 12AU7's came in yesterday and today the WZ68 Weber rectifier got here. I installed them all, biased the amp for .600 VDC and even with no break in the amp does sound much better. It seems there us a more open dimension to the sound in addition to the instruments sounding much fuller than before. I can only imagine that once broken in the tubes will make the amp sound even better.

    The KT-120s age like a fine wine, and they're very robust, last you a long time. You'll be glad you made the switch.
    I have no experience with the RCA 5963s Montana Way mentions, but you can't go wrong with clear-tops. I use 'em everywhere I can in the phono and line preamp stages. The first black-plate iteration of the 6CG7 clear-tops can be very microphonic, however, and are quite spendy. I'm happy with the second-gen grey-plates. Haven't tried 12AU7s in the driver holes yet, it's on my bucket list. RCA 12BH7As in there now, they sound fine.
    You might try .55 or even .5 VDC/tube and see if you can hear a difference. If you can't, leave 'em at the lower voltage.


    10-E-C

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    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by 10-E-C on Sat Oct 25, 2014 1:00 pm

    I use a NOS RCA 5963 mil spec in my center position on a ST 70, It has a much better sound to me than the E-H 12au7. It took a lot of brightness out of my Zu Omen Def's.

    deepee99

    Posts : 1336
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by deepee99 on Sat Oct 25, 2014 1:29 pm

    10-E-C wrote:I use a NOS RCA 5963 mil spec in my center position on a ST 70, It has a much better sound to me than the E-H 12au7. It took a lot of brightness out of my Zu Omen Def's.

    Guess I'm gonna have to try those guys. Know any good sources, anyone?

    Nemmind. I found a quad from a friend.

    Dogstar

    Posts : 92
    Join date : 2014-06-23

    Re: ST-120 First a buzz, then a tube light show....and then dead.

    Post by Dogstar on Sun Oct 26, 2014 1:44 am

    Now I'm concerned...despite installing new tubes and a Weber rectifier and not having any hiss tonight I turned on the amp and without a source playing hiss was clearly audible. Once the amp warms up the hiss goes away. I do have a volume control on the amp so tomorrow I will run a source directly to the amp and bypass my preamp. If there s no hiss then it may be time to replace the tubes in the preamp too. It takes 4 12AU7's so that would be a good place for the RCA's. If the preamp isn't the source of the hiss then what should I check next?

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