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Studdering mercury vapor diodes
Studdering mercury vapor diodes
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Old 4th August 2020, 07:48 PM   #11
HollowState is offline HollowState  United States
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Originally Posted by jdg123 View Post
I read about this delamination on RCA 866A tubes somewhere and now I have experienced it.
That may have been me as I cautioned about it in this post to Brian28 on 6/26/20 "Tube rectified PSU"

"Depending on just what tubes you have there is a caution. The older tubes by makers like RCA, Sylvania and a couple of others have a coating on the elements that when heated can flake off. These will be the larger (full size) size 866's. The slightly smaller style, like the GE's, seems to be free of this condition. Their plates will be shiny metal compared to the dark (sometimes orange peel) coating on the older versions. When this coating comes off it will short the tube. If in doubt of what you have, power them up (2.5V) and run in series with an incandescent lamp to get them hot."
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Old 5th August 2020, 02:35 AM   #12
trobbins is offline trobbins  Australia
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I guess the outcome could be a persistent short, and hence damage to power transformer or protection by fusing, or a short-duration high current that melts the shorting section of 'flake'.

It does highlight the need for over-current protection that suits the application, both on the primary, and also preferably on the secondary windings.
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Old 5th August 2020, 03:25 PM   #13
xenofonn is offline xenofonn  Greece
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Hello from Greece! See this video! 1000W Tube Amplifier Interference Pattern of the MV-Rectifiers - YouTube
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Old 5th August 2020, 05:46 PM   #14
50AE is offline 50AE  Bulgaria
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Studdering mercury vapor diodes
My 866s flickered too. It seemed as if the glowing gas cloud loved to change positions. I think I posted somewhere about it a long time ago, but didn't get a convincing answer. Mine were connected in a full bridge CT configuration. The interesting thing is, some tubes would stop flickering, while others would start. It was so random I couldn't find a logical pattern for years.

P.S. IIRC, making a video of the flickering couldn't register it. Was a high FPS video needed, I don't know. The flickering was some kind of glow fluctuation between the plate and the rectangular hole above the spiral cathode.

Last edited by 50AE; 5th August 2020 at 05:52 PM.
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Old 6th August 2020, 03:10 AM   #15
jdg123 is offline jdg123  United States
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Studdering mercury vapor diodes
Very interesting video Xenophon. It is another example of good tubes behaving with a flicker. It implies that the current draw is changing causing the tubes to pass more and less current and thus change the amount of glow in both a slow, whole tube change and a flickering partial tube glow change. There is quite a variety of glowing mv vapor according to multiple conditions within the tube. Gads! So much to learn!
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Old 6th August 2020, 12:45 PM   #16
Kay Pirinha is offline Kay Pirinha  Germany
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Originally Posted by xenofonn View Post
Well, as the video title says, the flickering shown here is an interference between the mains frequency and the power stage's load current, modulated by twice the signal frequency, hence is quite expectable. This Philips EL 6471 amplifier features a three phase main power transformer and six DCG4/1000G-01 mercury rectifiers (equivalents to 866A's) in full wave configuration that feed a pair of QB3,5/750 power tetrodes in class AB. The filter capacitor is just 2 F.

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