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Old 27th May 2009, 02:12 AM   #1
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Default Red Hot 6L6GC

After reading some discussions on "red plate" baddness on output tubes - I thought I would post a picture.

This tube is running on my Tubelab Simple SE with the stock 560 ohm resistor.

I also took a temp reading - the plate where it is glowing is reading 341 deg F.

Thoughts?
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Old 27th May 2009, 03:23 AM   #2
rknize is offline rknize  United States
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Yeah, that's about what my 6V6GTs look like. If you are after longevity, you should probably go to a colder bias resistor on those Chinese 6L6's.

It's difficult to get a good reading of the plates through the glass and know what to do with it. Many beam tubes direct the beam right at the seam of the two plate halves. The seam forms a wing that helps dissipate the heat. I've measured over 400^C on a big beam tube in the SimpleSE with no plate glow. My 6V6GTs don't have wings and the beam lands right in the middle of the plates. So those spots are hot, but the average reading of the plate is well below 400^C.

Here is an old pic:

Click the image to open in full size.

Where are the red lines on those tubes?
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Old 27th May 2009, 04:14 AM   #3
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I have been having the very same issues. I have a switch that I can change the value of r17 and r27. 560 ohm, 660 ohm, and 820 ohm. Unfortunately after some testing today even 820 has the biased too hot. I have purchased some kt88s. Problem solved lol. I will eventually build a 6 position switch and have even higher resistor values so that I can run my 6l6gcs also. 560ohms really had them glowing, 660 not as bad but still too much, 820 is pretty faint but still is visible. My b+ after the transformer, so testing on the blue wire of my opt I get a b+ of 450volts. Check my thread, Russ has helped me out a lot with some of the formula for figuring plate dissipation etc.
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Old 27th May 2009, 05:24 AM   #4
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I'm listening to some 5992's in my Simple SE that have good stripes on them.

I tend to worry more about the current, than the overall dissipation, so as long as the current is not excessive, minor striping does not bother me. But I have a lot of tubes, so I have the luxury of being cavalier.

The worry seems to be that the newer tubes will outgas and degrade the vaccuum when the plate heats to incandescance. I don't know that there is any actual evidence of that.

If it bothers you, you can reduce the dissipation by increasing the value of the cathode bias resistor, or selecting a rectifier tube with a larger voltage drop (e.g. 5R4), or both.

Win W5JAG
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Old 27th May 2009, 05:50 AM   #5
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6L6 were not designed to run with red plates, it kill them. Check a voltage on the 1'st grid after it's plate got red.
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Old 27th May 2009, 06:41 AM   #6
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So all this talk about red plates and getters had me take a very close look at the quad of Philips/Sylvania JAN tubes I've been mentioning. One of the four tubes is particularly hotter than the rest and I have to bias its companion a bit hotter to balance the pair (matched quad my foot). So looking carefully at the getters, the hotter tubes' getters are slightly more transparent that the other pair. The hot tube in particular I can just see through the getter down to the mica if I shine the light just right. No rings in the getter, just a general fading.

There's nothing I can do about it...the bias supply has nothing more to give. I have many 100s of hours on these tubes, so I guess I can't complain. The hot tube is still really hot, so it appears that these tubes will get gassy before the cathodes give out. I have a quad of NOS "Mazda" tubes for this amp too that are much more sane as far as tube current. Maybe I'll set these JAN tubes aside since I like how they sound so much and the current price on them is getting a bit high....
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Old 27th May 2009, 12:10 PM   #7
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Russ - The hot spot is where the two plate halves meet. Also the temp measurement was 340 F or 171 C!! I'm not even sure why it is glowing at that temp.

Nic6 - voltage & calculations are next on my list. I'm not sure that there is anything wrong the way it is.

Wavebourn - I plan to take a few measurements this evening.

Although killing these tubes would only put me out $18
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Old 27th May 2009, 02:25 PM   #8
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Quote:
Originally posted by oldmanStrat
the temp measurement was 340 F or 171 C!! I'm not even sure why it is glowing at that temp.
You are measuring the temperature of the glass, not the plate itself.
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Old 27th May 2009, 02:47 PM   #9
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Quote:
Originally posted by Ty_Bower


You are measuring the temperature of the glass, not the plate itself.

Ack - you are right ! Didn't even dawn on me that IR reflects off of glass. Time to take out the meter.
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Old 28th May 2009, 01:55 AM   #10
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Ok here are the readings:

B+ 437v
cathode bias, 37v

That makes the cathode current 66ma. 400 volts drop across the tube - 26W tube dissipation. All these numbers meet spec (for an old RCA 6L6GC that is... god knows what the real numbers are on this Chinese version)

And boy it took a lot longer than I thought it would to ramp up B+. Thought for a minute there I broke something.

I'm just thinking that these tubes just glow... Wouldn't be good for an original RCA, but I don't have any of those now do I?
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