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red plating with low anode dissipation?

hendrixgr

Member
2018-04-20 10:05 pm
Hi
I want to ask anyone who might have encountered this how is it possible for a tube to red plate at low current.
Specifically i have a new 829b that i just bought which draws 20ma in each section (0,2v measured across a 10 ohm resistor) at 650v.
Only one half is red plating, the other one works fine with even higher plate dissipation and of course i replaced the tube with another one which works fine.
In a tube tester everything shows perfect....
Chris
 
Measuring tubes in this way, doesn't work. You would need to test it hot.
I remember several decades ago, a 6DQ6 that was a true headache. It was from my father's B&W 17" TV, the set starts ok, and when it wants, the image suddenly dissapear. No HV, no horizontal, no EHT. Changing the 6DQ6 cure the apparatus. When looking inside with a magnificient glass, the cathode lead from the socket to the cathode itself was broken. So, when cold it was touching, and it worked OK, but at some moment temperature dilates the wire, and then shut down.

Mysteriously, this tube was which I used as final stage in my first AM 80mts ham band. As it was open frame, it worked at room temperature, and do its job until I dismantled it. Tubes have their own spirit, undoubtedly.
 
20 mA × 650 V → 13,000 mW …

So, its all a question of whether it really is 13 W dissipation causing the red-plate or something different.

According to ( http:\\tdsl.duncanamps.com\show.php?des=829B (repl \ with slash) ), 40 W is the PA rating of the valve.

So… “dunno”.
Just Saying,
GoatGuy ✓
 

hendrixgr

Member
2018-04-20 10:05 pm
Of course, i didn't expect to measure anything besides a short maybe.
The problem i have is that i have never ever seen this behavior before.
It red plates slowly at the middle section of the anode and the adjacent part of the glass is getting hot while the other half is relatively cold although it draws the same current.
Btw i did tried to re bias it and the tube ''listens'' to the grid bias adjustment but i will red plate even at low plate current.
One logical explanation is that it oscillates but this is not evident on the measurement which shows a steady current and all the other tubes i tried in it's place work fine.
The dissipation (12W) is not enough to create a hot plate in the first place....
Chris
 
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hendrixgr

Member
2018-04-20 10:05 pm
GoatGuy you are correct, this is a 20w per anode tube and should not red plate at 12w.
I did measured the current many times and i even re adjusted it to less than 10W but in vain., it took her a little longer but slowly she red plated again.
After all all the other tubes work fine for months, the reason i am asking is because i can not explain it although it is evident that there is something wrong with this particular tube which i just received as it as bought from Ebay.
Did i mentioned that the tube works fine in the amplifier despite the mild red plating?
One has to be in a dark room to see the glow.
Chris
 

hendrixgr

Member
2018-04-20 10:05 pm
The tube likely has an alignment problem. Either it was built wrong (common) or something has moved in the 50 years since it was made (also common). This forces all of the dissipation to occur in a small spot, as opposed to being evenly spread over the entire plate.
This might be the case as the red plating starts at one spot at the bottom and then it spreads up until the middle of the anode.
I don't know enough to visualize what is wrong and that's the reason i asked.
Chris.
 
Sounds like QC or poor storage maybe? Like it was built with parts misaligned, or that it maybe was dropped enough to shift the internals but not damage the glass?

A buddy dropped a 5u4 years back, it bounced (hardwood floor) and the anodes shifted internally. Out of curiosity we fired it up and the off-kilter side redplated when used at a moderate to high (200mA~) draw. I'm assuming the cathode was too close at that point and made a hot spot.
 
Its a transmitting tube, the anode is supposed to get very hot, the envelope is much bigger as a result. Big transmitting tubes orange-plate routinely, I wouldn't worry about one this size glowing red a bit per se.


(Note the 235C max bulb temperature rating, way hotter than any receiver/signal valve - forced air cooling is assumed with this class of device).
 
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hendrixgr

Member
2018-04-20 10:05 pm
Hi
The datasheet says that it shouldn't red plate even at max dissipation and all other 829B's i have work fine so probably it has to do with the construction of the tube, most probably i got a lemon.
Also only one tetrode is red plating, the other is normal, did i say that i use both halves in parallel ?
The amplifier is a PP with 4 pieces of 829b and each one is used as one tetrode.
Chris
 
Big transmitting tubes orange-plate routinely.....Its a transmitting tube, the anode is supposed to get very hot

NO, most transmitting tubes are NOT designed for red plate operation. True some are, and they will state this in the data sheet. These typically need forced air or a chimney to keep the glass to metal seals from failing.

The 829 type tubes are not supposed to glow red in normal use. The fact that it "hot spots" says that it's misaligned.

When I'm selecting tubes for a high powered amp, I purposely run them into the red zone and choose the tubes that show dim red in a dark room evenly distributed on both sides of the plate with no hot spots.

These are conducting roughly equal current across a large area. Any tube that hot spots has it's plate current concentrated in a small area. The cathode and screen grid will be overstressed in this area and lead to early tube failure.
 

hendrixgr

Member
2018-04-20 10:05 pm
When I'm selecting tubes for a high powered amp, I purposely run them into the red zone and choose the tubes that show dim red in a dark room evenly distributed on both sides of the plate with no hot spots.
Excellent information thank you!
That is what i do with unknown or new tubes, i close all windows and lights and watch them for red plating at nominal bias point, then i re bias them at 1/2 of this usually...
829B should take at least 40W of dissipation (2x20W) but i use them at 20W (2x10W).
Chris