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-   -   JJ KT88 tubes glowing bright yellow. (http://www.diyaudio.com/forums/tubes-valves/181044-jj-kt88-tubes-glowing-bright-yellow.html)

taj 13th January 2011 08:57 PM

Quote:

Originally Posted by bottletop (http://www.diyaudio.com/forums/tubes-valves/181044-jj-kt88-tubes-glowing-bright-yellow-post2432190.html#post2432190)
Taj, do you know what the score is on using GE Jan 6550a tubes in place of kt88? My gold lions test at Ip:71, Gm:8,300. The JJs test at PC:70, TC8. The 6550a are both different at PC: 49.8, TC: 5.9 and PC:42.6, TC:5.6. The chap at the shop with the tester said the 6550a's test as new but are not matched. So how can 6550a tubes be a direct drop in replacement for KT88 with such different PC and TC ratings? I did have a bad 12au7 tube that had a short on the same channel as the puffed resistor. Heard a buzz/hum through the speaker as well. Perhaps it was that.

The GLs had their plate current (Ip) and transconductance (Gm) measured at some operating point (voltages/currents applied to the various tube pins). The others were measured with some other system that I'm not familiar with, so I have no idea how to interpret those PC or TC numbers. I suspect PC is also "plate current", but I don't know what TC is (TransConductance using different units?), and we don't know what operating point it was measured at so, it's difficult to really derive meaning from the numbers or compare the GL's to the others.

Yes, that bad 12AU7 may have caused some downstream 'collateral damage''. It depends how bad 'bad' was ,what was shorted, and what the circuit looks like. But I'm no expert there either.

..Todd

tomchr 13th January 2011 09:01 PM

Quote:

Originally Posted by bottletop (http://www.diyaudio.com/forums/tubes-valves/181044-jj-kt88-tubes-glowing-bright-yellow-post2432249.html#post2432249)
Tom, yes you have described exactly what I am seeing. The grey anode plates are not changing colour at all. It is the part in the top top of the tube that is in the middle of the mica. Do you have any light to throw on the tube values above?

I don't think you have issues then. Sit back and enjoy the music.

If you post a picture, it would probably eliminate some of the guesswork, though.

I'm not intimately familiar with the KT88, but one indirectly heated tube is not that different from another in terms of basic stuff like heaters, cathode sleeves, etc. Not on an intuitive level anyway. I have some Sovtek 5AR4 rectifier tubes where the heater pokes out and glows bright yellow/orange like you describe. A google image search for "KT88 glow" turns up this: tube glow 2 | Flickr - Photo Sharing!. Does that look like what you're seeing?

~Tom

bottletop 13th January 2011 09:19 PM

Yes the JJ's look like that, if anything not as red through the holes. I have spent about 1/2 an hour now trying to upload a couple of images by going advanced. No joy as yet but i will get there. The GL tubes I have glow less on the top than on the tubes in the link you have sent.

taj 13th January 2011 09:28 PM

1 Attachment(s)
After you "Go Advanced", use the paperclip icon to the right of the font selector and happyface icon, which opens the attachment window. Browse to your image and select it, then click the upload button and close that little window and ...

..Todd

bottletop 13th January 2011 10:03 PM

2 Attachment(s)
Attachment 205381

Attachment 205382

bottletop 13th January 2011 10:06 PM

Nice one Todd! Well after all that I hope the images show the what I am on about.

bottletop 13th January 2011 10:10 PM

2 Attachment(s)
Attachment 205383

Attachment 205384

Thought you might want to see my modest set up. The phonograph does work but is not connected to the amp yet.

tomchr 13th January 2011 11:05 PM

The glow you have on the top of the KT88's is the heater glow. Nothing to worry about. Nice phonograph too. Now you just need a jack russell terrier to stare into it... :)

~Tom

richwalters 14th January 2011 02:11 AM

1 Attachment(s)
If youv'e got a tube behaving like in pic.....electron beam is concentrated on one side..it's a gonner. Came out of an inverted Fender amp and due to vibrations the cathode slipped in the mica causing misalignment. Manufacturer should have known better.

richy

tomchr 14th January 2011 02:20 AM

Quote:

Originally Posted by richwalters (http://www.diyaudio.com/forums/tubes-valves/181044-jj-kt88-tubes-glowing-bright-yellow-post2432522.html#post2432522)
If youv'e got a tube behaving like in pic.....electron beam is concentrated on one side..it's a gonner. Came out of an inverted Fender amp and due to vibrations the cathode slipped in the mica causing misalignment. Manufacturer should have known better.

That looks like an unhappy anode to me. With the damage you describe, I'm not surprised.

~Tom


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