JJ KT88 tubes glowing bright yellow. - diyAudio
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Old 13th January 2011, 10:45 AM   #1
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Default JJ KT88 tubes glowing bright yellow.

I bought a matched pair of JJ KT88 tubes to use in my SET amp. It has no bias adjustment. I was using a matched pair of reissue GL kt88, but wanted to try the JJ tubes as I was impressed with the look of the build quality and weight. The glass must be near on twice as think as other modern tubes I have seen. I have tried the GL and GE6550a tubes recently, they have always glowed a nice orange colour as regards to the grid filaments (if that is the correct term). The thing is these JJ filaments are glowing a bright yellow even after a few minuets. IE it is not start up flash. This concerns me, should I be worried?
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Old 13th January 2011, 12:21 PM   #2
Arnulf is offline Arnulf  Europe
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Measure voltage across heaters (FWIW, indirectly heated tubes have "heaters" versus "filaments" of the directly heated tubes). Is it within acceptable range ?
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Old 13th January 2011, 02:26 PM   #3
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Excessive grid currents........JJ KT88's are known to bias differently to other KT88's.

Perhaps other members elaborate more.

richy
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Old 13th January 2011, 02:36 PM   #4
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Hi Richwalter, Thank you for pointing the finger at the issue. Think I will take them back to the shop as my amp has no bias pots. I have found another issue, not sure what has caused this but one of my Mills resistors is swollen.
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Old 13th January 2011, 03:07 PM   #5
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by bottletop View Post
Hi Richwalter, Thank you for pointing the finger at the issue. Think I will take them back to the shop as my amp has no bias pots. I have found another issue, not sure what has caused this but one of my Mills resistors is swollen.

Replace the resistor and if it is part of one channel "Left or Right" then change the same resistor in the other channel. If the resistor is in the cathode of the output tube then check the voltage across it to give you the current and wattage dissipation! You need to know what the voltage drop across the cathode resistors should be under "normal bias" condition and compare if you change output tubes!

All tubes tend to bias differently and should be checked. Do not assume that the same type of tube will run within correct limits.

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M. Gregg
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Last edited by M Gregg; 13th January 2011 at 03:11 PM.
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Old 13th January 2011, 04:01 PM   #6
taj is offline taj
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What is that resistor connected to? Which pin on which tube (or whatever)? Got a schematic of your amp?

If you can do it safely, measure the voltage across that resistor (one meter probe on each of its leads) and the one in the opposite channel to see if there is a substantial difference. Do you know the value of that resistor?

..Todd

Last edited by taj; 13th January 2011 at 04:15 PM.
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Old 13th January 2011, 04:45 PM   #7
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Yes I have a schematic, but it is different from what is under the cover. The amp is a separo se88i. The mills are 12w 470r. The original resistors are 10w 470r. By the way I know less than a little about electronics. Just upgrading the coupling/decoupling and power caps has been a learning curve. I think M Gregg is correct from what I remember. The resistors are on the output tubes, thought to be sure if it is the cathode pin I will need to work out what pin does what. I am totally confused about what has caused this problem. I think it will be to do with tube rolling. I bought what was thought to be a matched pair of 6550a valves. I had them tested yesterday and they are not matched, same dates of production and test like new but not matched. My amp has no bias pots and what is even more confusing for me is that in the owner's manual it says there is no need to bias this design of amp, just used matched pairs of output tubes. The values of the GL KT88 tubes and JJ tubes are very similar/close. Yet the JJ do not seem happy at all. So the JJ tubes must "run on" less current. 6550a are supposed to be a drop in replacement for a KT88 but how can they be when the PC and TC values are totally different from a KT88. I am confused!
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Old 13th January 2011, 04:47 PM   #8
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A camera closeup pic of tubes ? The only thing one should see is the glowing cathode and the slightly brighter heater within it. There shouldn't be any grids' glowin' perhaps with an anode with a dull red.

richy
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Old 13th January 2011, 04:53 PM   #9
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Ok, I will take a picture of the GL tubes at work and them the JJ. I have replaced the swollen mills resistors with the originals. I am not sure of the bias load under normal conditions as there is no spec of this in the manual. Indeed I do not know what the normal bias condition should be.
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Old 13th January 2011, 05:42 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by bottletop View Post
Ok, I will take a picture of the GL tubes at work and them the JJ. I have replaced the swollen mills resistors with the originals. I am not sure of the bias load under normal conditions as there is no spec of this in the manual. Indeed I do not know what the normal bias condition should be.
Just for interest,

Link for kt88 pin 8 is cathode. Dissipation is shown in part2.

With Ohms law current is (Resistance of resistor divided into Volt drop across it = current).

KT88 and 6550 Tube Directory

Regards
M. Gregg
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