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Old 18th September 2009, 01:12 PM   #1
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Default Red Plate,Tube runaway, glowing screen grids

I've read that a little plate glow is ok as long as it doesn't "runaway"
I just purchased two new sets of tubes (6l6GC coke bottle chinese) and JJKT77) Both exhibit plate glow that is almost the entire crease in the winged part of the plate (very narrow but vertically long).
How long do you wait to see if it runs away? If it's going to happen does it happen within an hour (hasn't )
I have a simple se with the largest selectable cathode resistor being 680 ohms. (no problems highest setting-460ohms with JJel34 or JJKT88)

I can see the heater glowing thru the plate holes no problem But where do I look for the screen to see if its glowing too.
I've read many posts about people worried about and checking for this.
Thanks Paul
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Old 18th September 2009, 02:17 PM   #2
bigwill is offline bigwill  United Kingdom
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I don't know what you read, but I'm quite sure that red plate is NOT something you want. Only some valves are made to be operated at that temperature (big transmitting tubes), but 6L6 types are not. The life of the tube will be considerably shorter.

Last edited by bigwill; 18th September 2009 at 02:19 PM. Reason: Clarification
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Old 18th September 2009, 02:34 PM   #3
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Glowing plates = buy more chinese tubes

You need to check the voltages. ie Bias and VP. Sounds like your over max dissipation for that tube.
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Old 18th September 2009, 02:37 PM   #4
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You don't want any amount of red glow on the plates. If it is hot enough to emit visible light, then it is hot enough to boil out the trace gases dissolved in the metal. It will slowly contaminate the vacuum and ruin the tube.

The screens can only be inspected if there are holes in the plate somewhere. I've seen Chinese tubes with solid plates, so there is no easy way to see the screens. Here's a photo of a Russian tube with plate holes. You might be able to see two spots of glowing screen grid through the upper hole, and one spot through the lower hole.

Click the image to open in full size.
http://i69.photobucket.com/albums/i4.../hotscreen.jpg
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Old 18th September 2009, 02:48 PM   #5
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Attached is a Generic 6L6 "triode" curve. Plot out the Bias voltage and plate voltage at idle to see what the dissipation is.
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File Type: jpg 6l6trio.jpg (43.7 KB, 380 views)
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Old 18th September 2009, 03:45 PM   #6
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Those Chinese 6L6's only have a dissipation rating of 19 watts. For the JJ KT77, I wouldn't go higher than 25 watts. Do you know ohm's law? How much voltage is across that 680 resistor? Divide that measurement by 680. That's the cathode current. What's the plate voltage? Multiply that number by the cathode current and you will get a quick and dirty way to find the tube dissipation.
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Old 18th September 2009, 03:50 PM   #7
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Yeah, My thoughts exactly! I made a quick guesstimate of where it is biased (-40V, 62mA 420 VP) THATS 26 WATTS!! The chinese tubes are probably BEGGING FOR MERCY!
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Old 18th September 2009, 03:59 PM   #8
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Here's a real quick 18W dissipation line in red on that curve.

You are WAY over it I am sure.
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File Type: jpg 6l6trio.jpg (51.5 KB, 368 views)
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Old 18th September 2009, 04:01 PM   #9
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Most likely going to have to lower the B+ to get those Chinese tubes not to burn up.

You are probably "pushing it" with the KT77's also.
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Old 19th September 2009, 01:38 AM   #10
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Measured voltage across R 13 &23 = 3 V rechecked this 3V again.
This is where to measure plate current right?
Measured voltage across R17 &27 = 36 V Cathode current assuming 680 ohms is 36 V.

I have actually loooooked! cant find anywhere where it actually tells how to measure B+ voltage (except some guitar amps sites that sounds different enough from my simple se that I was fraid of doin' it.

To measure B+ with the tubes in and one probe clipped to the ground could I connect the other probe to the piece of the tube socket sticking thru the bottom of the board?

Plate current should be measured across R13 or R23 right?

How can B+ be reduced?
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