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Hops
Bob Latino
grosen
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    ST120 click/flash in one tube

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    grosen


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    Post by grosen Sun Mar 21, 2021 10:22 am

    Hi all,

    My ST120 has been trouble free since I built it, but just now I had a click and a flash in one tube (KT120), and now that socket won't bias.  (I get a reading of 20V or so.)  The problem doesn't follow the tube.  Can someone give me a clue about what to check first?  

    Thanks,

    Gideon
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    grosen


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    Post by grosen Sun Mar 21, 2021 12:43 pm

    I just flipped it over and checked the obvious thing: the 10 ohm resistor to ground from pin 8 on that socket is open.  So I assume that's either the problem or a symptom of the problem.  Should I just replace it and see what happens?  Any idea what would cause the resistor to fail?   Thanks in advance!

    Gideon
    Bob Latino
    Bob Latino
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    Post by Bob Latino Sun Mar 21, 2021 12:47 pm

    grosen wrote:Hi all,

    My ST120 has been trouble free since I built it, but just now I had a click and a flash in one tube (KT120), and now that socket won't bias.  (I get a reading of 20V or so.)  The problem doesn't follow the tube.  Can someone give me a clue about what to check first?  

    Thanks,

    Gideon

    Hi Gideon,

    The 10 ohm 2 watt resistor on the tube socket that goes from pin # 8 to chassis ground is probably bad and does not measure 10 ohms anymore. Check to see if it measures 10 ohms. If it does not, you can get a few very cheaply at the Ebay link below.

    https://www.ebay.com/itm/2W-2-Watt-1-Tolerance-Metal-Film-Resistor-5-Pieces-USA-SELLER/183421853100

    Be sure to select 10 ohms from the drop down menu ..

    NOTE - when that 10 ohm resistor is damaged, it usually indicates that the tube that was in that socket and gave a "flash" has an issue. I would replace that tube.

    Bob

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    Bob Latino
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    Post by Bob Latino Sun Mar 21, 2021 12:50 pm

    The tube that was in that socket at the time of the flash is most likely bad. It may bias OK after you replace the resistor but I would not trust the tube that was in there anymore.

    Bob

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    grosen


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    Post by grosen Sun Mar 21, 2021 12:51 pm

    Thanks, Bob. I'll replace the resistor and the tube. — Gideon

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    Hops


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    Post by Hops Sun Mar 21, 2021 2:35 pm

    While I think replacing the resistor and the tube is necessary, I would also re-tension the pins in the bad socket and clean them with deoxit or other good contact cleaner. I would also check the incoming voltage of the power ( I forget what Bob recommends, but 119 V or less should be good, and make sure the bias is at Bob's recommendation or a little less - I think the target is 0.5 Volt. Tubes are expensive, and you want to rule out anything that can make tubes more likely to fail.
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    Chad Spackman


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    Post by Chad Spackman Fri Jan 14, 2022 12:54 pm

    Just had one of my 10 ohms go in a VTA-120. Of note: pins 1 (tube frame) & 8 (cathode) of the KT88 are tied together. With cathode not tied down by 10 ohm, it and the frame wonder up to B+. There is no appreciable source of current there until it drops to under about 50v or a bit less. So it's not gonna hurt too much
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    nmchiefsfan


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    Post by nmchiefsfan Fri Jan 14, 2022 6:12 pm

    I took measurements on some KT-88s and KT-120s that I have on the shelf and NONE of them are connected from pin 1 to pin 8. Some are connected from pin 1 to the base of the tube though. The KT-88s are Golden Dragon and Sov Tec. The KT-120 is Tung Sol. Are all of your tubes like this? Maybe you have a shorted tube.
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    Post by Chad Spackman Fri Jan 14, 2022 7:09 pm

    This is vta-120 specific.  1 and 8 are connected within the amp, by design


    Last edited by Chad Spackman on Fri Jan 14, 2022 7:47 pm; edited 1 time in total
    Bob Latino
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    Post by Bob Latino Fri Jan 14, 2022 9:32 pm

    I consulted with Roy Mottram just to be sure before answering here ... On an EL34 (and also on 6L6GC) pin 1 is tied to the screen grid (pentode) and should NOT be left unconnected. These tubes are almost always connected to cathode on 90% of tube amps out there. On all KT88, 6550 and KT120's pin #1 has no internal connection to any other pin although some brands of these three tubes do connect pin 1 to the metal base of the tube. It is better to connect pin #1 to pin #8 so that both are near 0 volts rather than having them floating above chassis ground. If you connect pin 1 and 8 together, you can then use virtually any type of octal output tube in a Dynaco or VTA amp.

    Bob
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    Chad Spackman


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    Post by Chad Spackman Sat Jan 15, 2022 12:25 am

    Thanks Bob. Just to be clear, I was in no way questioning the wisdom of connecting pins 1 and 8 and your detail here of various tube types makes perfect sense. I just happened to be using the frames to look at the bias as I’d not yet put in the standard test points. And with an open 10 ohm I noted the high voltage, which of course also makes sense
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    Chad Spackman


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    Post by Chad Spackman Sun Jan 16, 2022 10:07 am

    A question related to the flashover. In my case it was a 6550. New tubes. Installed, biased, and idled for about 30 minutes. Started listening and while turning up the volume, one tube flashed. My question: are 6550s more susceptible flashover this than kt88s? With a static plate voltage of mid 500 volts, there is going to be a positive voltage during AC swing on the non-driving tube to well over 600v, which appears to be the limit on 6550s.
    peterh
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    Post by peterh Sun Jan 16, 2022 3:51 pm

    Chad Spackman wrote:A question related to the flashover. In my case it was a 6550. New tubes. Installed, biased, and idled for about 30 minutes. Started listening and while turning up the volume, one tube flashed. My question: are 6550s more susceptible flashover this than kt88s? With a static plate voltage of mid 500 volts, there is going to be a positive voltage during AC swing on the non-driving tube to well over 600v, which appears to be the limit on 6550s.  
    6550 should not flashover. Ask for warranty replacement a tube with same matching parameters .

    corndog71
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    Post by corndog71 Sun Jan 16, 2022 4:04 pm

    This can happen to any tube. Most tubes are not made with white gloves. Debris happens.
    peterh
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    Post by peterh Sun Jan 16, 2022 6:00 pm

    corndog71 wrote:This can happen to any tube.  Most tubes are not made with white gloves.  Debris happens.
    Might be. But tube is junk and should be scrapped.

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