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    Blown Bias Resistor - Did I Mess Up?

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    jasn54

    Posts : 30
    Join date : 2015-04-13

    Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Thu Mar 24, 2016 9:57 pm

    Hi all. I sat down tonight to experience my new Genelex Gold Lion KT88 tubes in my ST-120. I'd been running Sovteks KT88 with zero issues since I built the amp a little over a year ago.

    I swapped the tubes and reset the bias, which was only a bit off for each and was able to settle all 4 settings in at .550 Vdc pretty easily.

    Four songs in, I saw, and heard through the speakers, a brief flash on the V7 tube.

    Troubleshooting (if you could call it that), I heard a mild hum and scratchy noise in the right channel (only) and reduced output volume. I did some swapping/removal of tubes to deter kine it wasn't a tube and, after searching this site, determined I have a blown 10 ohm bias resistor for V7. It measures open on my meter and even has a small hole in the case.

    So, replacing this free little guy is easy, but before I do I'd appreciate any advice/thoughts about what I might have done wrong.

    Might this have been an open pin on the new tube Co kd this have caused the bias resistor failure? Is .550 vdc too high? WI of this failure damage the tube? Should test the new tube before reinstalling it to prevent a reoccurence?

    Any help woo kd be much appreciated.

    Thx, jasn.

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Thu Mar 24, 2016 10:00 pm

    Sorry for the typos. Autocorrect on a cellphone is brutal...

    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by Tubes4ever on Thu Mar 24, 2016 11:21 pm

    Sounds like the tube that was in that socket shorted internally.  I would consider it defective and not trust it any further even if it seems to work OK now.  You experienced the other function of that 10 ohm resistor.  It works as a fuse to protect the output transformer in the case of a tube short.

    A bias of .550 is fine.

    A poor socket connection might cause the tube to red plate, but not flash internally.  Replace the tube.


    Last edited by Tubes4ever on Thu Mar 24, 2016 11:35 pm; edited 1 time in total (Reason for editing : More info)

    peterh

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

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by peterh on Fri Mar 25, 2016 2:56 am

    I agree , the tube in question is defective. If bought with warranty then it's time to use it, otherwize
    throw it away and replace the 10 ohm resistor with another one. ( 10 ohm 1/2w flameproof)

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Fri Mar 25, 2016 7:07 am

    Thanks to you both for the help. Complete dummy that I am, I mixed up the tubes when I removed them to open the bottom plate. Will a tube test find the bad one?

    Tubes4ever

    Posts : 61
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by Tubes4ever on Fri Mar 25, 2016 9:18 am

    If the tube has a hard short, the tube test will find it. However, the tube in question may only short after it is good and hot. In that case, keep some spare 10 ohm resistors on hand for when it shorts again.

    Try leaving the tubes on the tester with the heater going for a good while to get the tube hot, maybe that will cause the bad tube to short again. Keep in mind that the tube won't get as hot as it does in normal operation.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by arledgsc on Fri Mar 25, 2016 11:13 am

    Same failure happened to me a few years ago.   Output tube shorted and smoked the 10 ohm cathode resistor.  In my case I heard a loud hum when this occurred.

    The 10 ohm resistor 2 watt resistor should handle 450 mA of current before reaching its power limit.

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by ArlanB on Sat Mar 26, 2016 12:25 am

    I too have experienced the exact same failure of my ST-120 during the first three years after completion of the kit.  I went through 3 sets of  KT88's. First set was JJ, 2nd set was Electroharmonics, and the 3rd set was Gold Lion.  With each failure, the offending tube took out the rectifier as well.  So I would check it as well if you are using tube rectification.

    Kentley

    Posts : 333
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by Kentley on Sat Mar 26, 2016 12:36 am

    Ditto on the experience. In my case it has been not bad tubes but bad socket maintenance. Here's the thing - at least you didn't fry anything really expensive -- or burn down the neighborhood! affraid affraid

    peterh

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

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by peterh on Sat Mar 26, 2016 4:09 am

    junk both tubes, you will ned a matched pair anyway.

    Bob Latino
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    Re: Blown Bias Resistor - Did I Mess Up?

    Post by Bob Latino on Sat Mar 26, 2016 8:54 am

    The 10 ohm 2 watt resistor that runs from pins 1 and 8 to chassis ground on all four output tubes of the VTA ST-70/120 is that tube's only reference to chassis ground. It acts like a fuse. If too much current is drawn through this tube (as in the case of a dead short within the tube) the 10 ohm resistor will blow. In the VTA ST-70/120 we use a 10 ohm 2 watt resistor. I have found that a 2 watt resistor will take a slight amount of abuse (a transient short) and not fail but will fail if there is a constant short within any output tube. The thing that some VTA amp users don't understand is that when this resistor fails, the resistor is not at fault > the TUBE is at fault or one of the SOCKET PINS is at fault. To blame the 10 ohm resistor for its failure would be similar to blaming the fuse in your amp that failed when the fuse failure is caused by some other issue within the amp.

    Why would a tube fail (short). Pins 3 and 4 on each output tube are carrying 440+ VDC. If that 440+ VDC somehow gets to another to another area inside the tube where it is not supposed to be, you have a short and the tube will draw too much current and the 10 ohm resistor will burn up. Sometimes it is the socket itself that is at fault. If, for example, the tube loses contact on pin #5 where the bias (negative control voltage) is "injected" into the tube, the tube will draw too much current and the 10 ohm resistor will fail.

    A blown 10 ohm resistor is a symptom of some issue related to that socket - either the tube that is plugged into that socket or the socket itself.

    Bob

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Sat Mar 26, 2016 3:38 pm

    Thank you everyone for the help and advice.  I have some resistors on order (not terribly easy to find) and am working to get a replacement tube (or two).  I'll be sure to re-tension the socket pins before reinstalling.

    Stand by...

    tubes4hifi
    Admin

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    Re: Blown Bias Resistor - Did I Mess Up?

    Post by tubes4hifi on Sun Mar 27, 2016 7:10 pm

    it's also been recommended by tube guru Jim McShane that when switching to KT120 tubes instead of KT88 tubes,
    it's good practice to change the grid bias reisistors (R29-32) to 47K to prevent bias runaway.
    Bob says this has rarely happened, but when you are talking about $50+ tubes a 10c resistor change is a no-brainer.

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Thu Mar 31, 2016 10:02 am

    An update:

    Turns out I do not have an opportunity to replace a tube under warranty after all. My bad. I was able to test all four tubes using a friends ancient tester, however, and they all looked good. I had mixed up the problem tube one with the three good tubes, so I decided to roll the dice and try to replicate the problem. so I...

    Replaced the 10 Ohm resistor
    Re-tentioned all socket pins
    Reduced bias pots
    Re-installed the GL KT88s
    Reset bias to 0.500V dc
    Sat back, listened to, and watched my amp for almost two hours without blinking
    All is well...so far. Not only that, these GL tubes sound sweet. Smile

    I don't know, maybe it was an open pin ? Will have to give it some more time.

    peterh

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

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by peterh on Thu Mar 31, 2016 10:10 am

    Time will see, but there is a risk that the tube will flash again.
    If it does, make shure you mark it ( and it's twin on that side) and buy a new matched pair.

    Tubetesters won't provoke flash-over ( and generally don't give any usable results)

    jasn54

    Posts : 30
    Join date : 2015-04-13

    Re: Blown Bias Resistor - Did I Mess Up?

    Post by jasn54 on Thu Mar 31, 2016 10:12 am

    Agreed Peter. Glad I bought extra 10 ohm resistors.

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