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KT-88 Red plating question

ConchBoy66

ConchBoy66

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So, I cleaned my VTA ST120 (it was really dusty) that I built probably 8 years ago, and, in the process, I removed the tubes for a moment making sure to reinstall them in the same order. I then played the full Meddle album at a good volume of course and it sounded great and then on to some classical for an hour or so then shut it off. Next day I turned it on again and was doing some light listening when I heard a pop. When I looked over, I couldn't believe my eyes, I had never seen this before but thanks to this site I knew what had happened. I immediately shut it off, so it was doing this for 15-20 seconds or so. The KT-88's are Sovteks and original to the system. I have removed the bottom and the resistors look fine. I did have an issue with the right rca twisting on me when I cleaned it and I see now that the soldered wire came loose. Perhaps that was the cause? Anyhoo, I have 2 back up tubes so I will not try and reuse that offending tube. I will need to rebias it and then keep a good eye on it for awhile.....Any thoughts?

Just to clarify it was only one tube and it was in the v2 socket as pictured

IMG_0098.jpegIMG_0100.jpegIMG_0099.jpeg
 
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ConchBoy66 said:
So, I cleaned my VTA ST120 (it was really dusty) that I built probably 8 years ago, and, in the process, I removed the tubes for a moment making sure to reinstall them in the same order. I then played the full Meddle album at a good volume of course and it sounded great and then on to some classical for an hour or so then shut it off. Next day I turned it on again and was doing some light listening when I heard a pop. When I looked over, I couldn't believe my eyes, I had never seen this before but thanks to this site I knew what had happened. I immediately shut it off, so it was doing this for 15-20 seconds or so. The KT-88's are Sovteks and original to the system. I have removed the bottom and the resistors look fine. I did have an issue with the right rca twisting on me when I cleaned it and I see now that the soldered wire came loose. Perhaps that was the cause? Anyhoo, I have 2 back up tubes so I will not try and reuse that offending tube. I will need to rebias it and then keep a good eye on it for awhile.....Any thoughts?

Just to clarify it was only one tube and it was in the v2 socket as pictured
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Clean the sockets and tube pins with contact cleaner. I like Deoxit D100. Work the tube into and out of the sockets a few times, to clean the oxidation off the metal.

Does the amp have a provision to check the output tube's idle bias? After cleaning the pins and sockets, I would check the bias before any long listening sessions.

Deoxit D100.
 
Mr Boochie said:
Clean the sockets and tube pins with contact cleaner. I like Deoxit D100. Work the tube into and out of the sockets a few times, to clean the oxidation off the metal.

Does the amp have a provision to check the output tube's idle bias? After cleaning the pins and sockets, I would check the bias before any long listening sessions.

Deoxit D100.
Click to expand...
Yes, it does and yes, I will;)
 
It was a new Bob Latino kit that I purchased maybe 9 years ago. The amp has seen every other day use for maybe an hour or so each time.
 
ConchBoy66 said:
It was a new Bob Latino kit that I purchased maybe 9 years ago. The amp has seen every other day use for maybe an hour or so each time.
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If there is no hum, hiss, or other odd noises --- the main filter caps are probably OK.

Be sure the idle bias is steady. I lightly touch the tubes with the eraser end of a pencil and see if the idle bias moves. That can mean socket tensioning is needed.

:thumbsup:
 
Regarding re-tensioning of the base contacts: I've had different results over the years; sometimes it helped for a long time while other attempts only lasted a short time. I've read about this for other metal items that develop weak grips as they age. Fairly often it seems that changes in the metal prevent long-term benefits from re-tensioning.

I'm not real knowledgeable about metallurgy but I'll venture a guess that some alloys, annealing, or other production processes help to make a high-quality contact, which may benefit from tweaking after some years of use.
 
Best source for a quad of matched gold lions? I checked out Jim McShane's site, but I don't see any listed.
 
oldflame said:
Regarding re-tensioning of the base contacts: I've had different results over the years; sometimes it helped for a long time while other attempts only lasted a short time. I've read about this for other metal items that develop weak grips as they age. Fairly often it seems that changes in the metal prevent long-term benefits from re-tensioning.

I'm not real knowledgeable about metallurgy but I'll venture a guess that some alloys, annealing, or other production processes help to make a high-quality contact, which may benefit from tweaking after some years of use.
Click to expand...
Yes, it depends on the maker of those sockets. Old-school US-made ones are pretty good at holding tension until the bitter end.

Many brand new, off-shore modern ones are flimsy as all get out.
 
Mr Boochie said:
Clean the sockets and tube pins with contact cleaner. I like Deoxit D100. Work the tube into and out of the sockets a few times, to clean the oxidation off the metal.

Does the amp have a provision to check the output tube's idle bias? After cleaning the pins and sockets, I would check the bias before any long listening sessions.

Deoxit D100.
Click to expand...
It would be good to check that the octal socket pins are gripping the tubes' pins securely. Re-tensioning the pins to ensure a good grip on tube pins could prevent a runaway tube problem caused by a poor or intermittent connection. Inexpensive (doesn't sound as pejorative as cheap) 7 and 9 pin sockets that are made with two phenolic wafers riveted together in the center are also fair game for this sort of a fix, too.
 
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I might swap the tube to the other socket in that channel and see if the problem follows the tube or not. If it follows the tube, its the tube. If it does not, you've got other problems.

Tubes coming out of Russia are currently a little difficult and expensive to buy. I don't know how the JJ or Shugang KT88 or 6550 compare but they might be worth consideration. I do like the sound of the GL KT88 though.
 
gadget73 said:
I might swap the tube to the other socket in that channel and see if the problem follows the tube or not. If it follows the tube, its the tube. If it does not, you've got other problems.

Tubes coming out of Russia are currently a little difficult and expensive to buy. I don't know how the JJ or Shugang KT88 or 6550 compare but they might be worth consideration. I do like the sound of the GL KT88 though.
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Gadget73,I was hoping you’d chime in! swapping the tube to another socket is a good idea to diagnose my issue as are all of the previous suggestions. Thank you everyone! The only tubes I would consider would be the GL’s. Looks to be about 400.00 for a matched set on that auction site
 
gadget73 said:
I might swap the tube to the other socket in that channel and see if the problem follows the tube or not. If it follows the tube, its the tube. If it does not, you've got other problems.

Tubes coming out of Russia are currently a little difficult and expensive to buy. I don't know how the JJ or Shugang KT88 or 6550 compare but they might be worth consideration. I do like the sound of the GL KT88 though.
Click to expand...
Both JJ 6550 and JJ KT-88 works fine in Dynaco MkIII , stable bias.
 
ConchBoy66 said:
So, I cleaned my VTA ST120 (it was really dusty) that I built probably 8 years ago, and, in the process, I removed the tubes for a moment making sure to reinstall them in the same order. I then played the full Meddle album at a good volume of course and it sounded great and then on to some classical for an hour or so then shut it off. Next day I turned it on again and was doing some light listening when I heard a pop. When I looked over, I couldn't believe my eyes, I had never seen this before but thanks to this site I knew what had happened. I immediately shut it off, so it was doing this for 15-20 seconds or so. The KT-88's are Sovteks and original to the system. I have removed the bottom and the resistors look fine. I did have an issue with the right rca twisting on me when I cleaned it and I see now that the soldered wire came loose. Perhaps that was the cause? Anyhoo, I have 2 back up tubes so I will not try and reuse that offending tube. I will need to rebias it and then keep a good eye on it for awhile.....Any thoughts?

Just to clarify it was only one tube and it was in the v2 socket as pictured

View attachment 3127903View attachment 3127905View attachment 3127904
Click to expand...

you may want to look for a leaky coupling cap and loose pins on that V2 socket. If any of the pins are loose, replace the socket. If you have a DC voltmeter you can measure for the presence of DC on output side of the cap. if the cap is passing DC replace it.
 
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