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    Flash under KT-120

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    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Flash under KT-120

    Post by Frank111 on Mon Apr 27, 2015 8:13 am

    This weekend I set up the SP-14 preamp with both M-125 amplifiers. (I also used a Nakamichi CDP 2A CD player and a very small set of test speakers).  I decided to start off with just two KT-120's per amplifier. After a few minutes went by, I heard a slight pop sound and noticed what looked like an arc or yellow flash coming from the area of the bottom of the tube. I immediately shut the amp off. Then replaced the KT-120's with two NOS 6550 tubes and the amp ran for three hours without incident. Next day, I checked the entire amp out. Did resistance checks on the resistors in that area, voltage checks, and all is normal. There is no evidence of any arcing on anything. It's all fresh and clean underneath. The tube socket looks normal too.

    So, I reinstalled both KT-120 tubes and the amp ran fine, sounded fine, nothing apparently out of the norm. I ran it over two hours without incident.

    This leaves me wondering what was this yellow flash underneath the tube all about?
    Any ideas on what could have caused this?

    Frank111

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Flash under KT-120

    Post by sKiZo on Mon Apr 27, 2015 8:52 am

    First thing I'd do is check the socket that flashed and see if there's any debris or such that could short the tube. A blast of canned air is helpful.

    Next step, make sure all the pin fingers are right 'n tight. Not uncommon for those to spread some when swapping tubes, especially if you rock the tubes in or out. (Don't do that.) Insert a small flat bladed jeweler's screwdriver between the socket body and each finger and adjust them with a bit of a twist. Not a bad idea to do that on all the power sockets every now and then in any case.

    I'd also take a real close look at the socket that flashed, looking for cold solder connections or shorts that may only show up when the amp is cold. Heat expands metal so that the amp works "properly", but go open again as the amp cools - shrinkage, doncha know. Use a magnifying glass and also make sure the solder is all shiny with a good spread - no blobs allowed!

    That don't work, time to start poking around with an insulated stick. Power up, and try wiggling the wires to see if anything weird happens - yet another indication of a weak or intermittent connection. If you find ONE, they're all suspect, and I'd consider repointing the entire chassis.

    Oh. Goes without saying - always be ready to cut the power at the first sign of trouble when experimenting.




    corndog71

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    Location : It can get windy here

    Re: Flash under KT-120

    Post by corndog71 on Mon Apr 27, 2015 9:28 am

    Believe it or not tubes aren't made in perfectly clean environments. There could have been some debris inside the tube and it shorted across the pins. This is not uncommon and is usually harmless to the tube. Something like this could potentially take out the tube but more often leads to a flash and it's done. The tube should be fine. Some manufacturers fuse the power tube cathodes to prevent things like this from getting worse.

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Mon Apr 27, 2015 10:02 am

    Corndog71, I wondered about that too. These are Russian Tung-Sol KT-120's that I got from Upscale Audio. They run a burn in test for 72 hours, then do their matching. But who knows? Maybe something loose in shipping could have shifted causing that flash. I'll run them some more and see what happens. Actually, I need to install the other tubes to get the 4 tubes per amp going.

    Skizo, I wondered about the pin placement in the tube socket too, and maybe the 6550's corrected something. Then it was ok when I reinstalled the KT-120's. I don't know. Maybe. The rest of the amp was gone over, and the solder joints are perfect. I always inspect each joint with a magnifying glass as a normal procedure. Better to catch it early than later. Nothing abnormal about the condition of the amp itself as far as indicated in my original post. Also looked at that tube socket with a light and magnifier to see if I could spot any arcing marks and I don't see any.

    I'm wondering about having to have to readjust these tube socket 'fingers'. I hear a lot about that on the forum. That is something usually doesn't have to be done to NOS tube sockets. Are these sockets prone to this problem and would it make sense to at some point replace them with NOS USA tube sockets? I don't recall this being a problem with vintage gear.

    deepee99

    Posts : 1261
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Flash under KT-120

    Post by deepee99 on Mon Apr 27, 2015 12:10 pm

    Even after a burn-in at the seller's end, these tubes get knocked around quite a bit during shipping, especially if the box is marked "fragile," which can loosen extraneous crud off the plates, giving you quite a light show. I have seen this light show.
    sKIzo's advice is gospel. However, I have mic'd the pins on NOS 6550s and the Russian Tung-Sols; and the new Russian pins are ever so slightly fatter and fit tighter. So if your NOS 6550s gave no grief, my guess is you had a one-off event.

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Mon Apr 27, 2015 12:23 pm

    depee99, I'm really hoping that it's something to do with a one off event and things will be fine.
    It has to be something funny about that tube. Maybe some junk in the base, or who knows what.
    But it ran perfectly with the NOS 6550's and then ran perfect for two hours with the very same KT-120's. I'll run it over the next few days and see what happens. Good to know that the pins are a little larger on the Russian tubes. Maybe Putin put in that request!

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Flash under KT-120

    Post by sKiZo on Mon Apr 27, 2015 1:18 pm

    No wimpy little pins for THIS guy, eh ...



    Also give the tube the old shake and rattle test. You might be able to hear anything big enough to cause an intermittent internal short.

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Mon Apr 27, 2015 1:24 pm

    LOL Skizo!!!!!

    deepee99

    Posts : 1261
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Flash under KT-120

    Post by deepee99 on Mon Apr 27, 2015 1:46 pm

    I studied Russian in college. "Putin" translates as "big member." Could explain the pin sizes.

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Flash under KT-120

    Post by sKiZo on Mon Apr 27, 2015 6:40 pm

    PUTIN ON THE RITZZZZZZZZZZZ!!!!


    deepee99

    Posts : 1261
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Just curious

    Post by deepee99 on Sat May 02, 2015 11:47 am

    Frank, did your tube settle down OK by itself?

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Mon May 04, 2015 8:24 am

    So far, so good. I probably put about 25 hours on it since that happened. That was running just two tubes. I just put the other two in each amp and am now running in quad mode. Even with the cheap test speakers, I think running four tubes improves the sound quality. Maybe it's just my imagination, but I think it sounds better. So if that's true, then it's probably best to always run quad tube mode.

    I found that I have a slight continuous hum with the preamp on. If I turn it off, the amps are dead quiet. What I think is going on is that I connected both ground wires to the volume control. Maybe if I break one ground at that point the hum will go away. It's very slight and continuous, and not impacted by volume control position. If you shut the preamp off, it goes away.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Flash under KT-120

    Post by Bob Latino on Mon May 04, 2015 10:22 am

    Frank111 wrote:So far, so good. I probably put about 25 hours on it since that happened. That was running just two tubes. I just put the other two in each amp and am now running in quad mode. Even with the cheap test speakers, I think running four tubes improves the sound quality. Maybe it's just my imagination, but I think it sounds better. So if that's true, then it's probably best to always run quad tube mode.

    I found that I have a slight continuous hum with the preamp on. If I turn it off, the amps are dead quiet. What I think is going on is that I connected both ground wires to the volume control. Maybe if I break one ground at that point the hum will go away. It's very slight and continuous, and not impacted by volume control position. If you shut the preamp off, it goes away.

    Hi Frank,

    1. The amp may sound better with FOUR output tubes because with the 4 tubes in there you now have about 3 dB more "headroom" on musical peaks.

    2. If you have a slight hum, see if the hum could be a ground loop. If your preamp also has a 3 wire cord (like the M-125's) you could try floating the ground wire on the preamp (if it also has a 3 wire cord) with a 3 to 2 wire "cheater plug". You could also try floating the ground on both M-125's with cheater plugs. Also - if you have your TV cable box on the same AC line - try just disconnecting the cable wire from the cable box just for a moment to see if the hum is reduced or goes away. I had a bad TV cable ground at another house that caused hum in my stereo.

    Bob

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Mon May 04, 2015 10:32 am

    Thanks for the follow up, Bob. Before I cut that 'extra' ground connection at the volume control, I'll try you're suggestions. Your amps are very nice, and you really did a great job with your build instructions. Once the hum is resolved, I'm going to move everything up to their permanent area. For now, I'll be using Klipsch Quartet speakers that I bought new. Though I think that these amps would benefit from speakers that are more refined. I'll need to spend some time researching that at a later date.

    Frank111

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Flash under KT-120

    Post by tubes4hifi on Thu May 07, 2015 2:34 pm

    single wire ground from the PCB to both grounds on the volume control

    Frank111

    Posts : 83
    Join date : 2015-02-23
    Location : Minneapolis/StPaul Area

    Re: Flash under KT-120

    Post by Frank111 on Thu May 07, 2015 2:39 pm

    Thank you for this clarification Ray.  Will make the correction.

    Frank111

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