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Lessons/questions from the deaths of the 811As

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LAN_403

U4R1A

Tue Jan 11 2011, 01:42AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Well much like I expected when I bought the 811a's from the great country of china they are dead. Now I'd be lying if I said they weren't rode hard and put to bed wet but how and when they died is what causes me to ask questions. The night before I had been trying out different secondary coils but I hadn't pushed the tubes to the point of seeing any visible color change on the plates. However the next morning I noticed the plates had sagged into the grid. Here is the weird thing, due to the orientation of my setup the tubes are not facing the same direction and both sides that have sagging was the side facing the MOT. This causes me to assume that the magnetic field from the MOT must affect the plates to some degree since I can feel the field from the MOT with a screw driver. I'm open to ideas and I'm hoping you seasoned tubers have some insight for me. I'll post a pic of their sweet demise.

Herr Zapp

Tue Jan 11 2011, 10:48PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

U4R1A -

The original US-made 811A's were rated for two orientations:

1. Vertical (base down)
2. Horizontal, with pins #1 and #4 (filament) in vertical alignment

Questions:

1. Do you know what the maximum plate current was?
2. If you weren't monitoring plate current (bad practice!), what was the plate color, if any was visible?
3. What was the minimum distance from the tube envelope to the MOT?
4. What part of the MOT was closest to the tube (core, or windings)?

If the tubes were closest to the MOT windings, and the distance was less that 3" or so, you may have had some additional "induction heating" of the plates going on.

I wouldn't necessarily blame the tubes yet .......

Herr Zapp

Steve Conner

Wed Jan 12 2011, 09:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You said you never saw any visible colour change, but there is a huge oval burnt spot in your photo, that suggests the plate got very red.

The sides facing the MOT would have got hotter, because the MOT core is kind of shiny and would reflect infrared back into the tube.

Also, some tubes heat more on one side than the other, because of tolerances in grid alignment.

Herr Zapp

Thu Jan 13 2011, 01:12AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Steve, U4R1A -

The odd thing is that while the plate clearly shows the central, oval discolored area (where it got red or even orange hot), the discolored area does not extend all the way to the bottom of the plate structure, where the greatest structural deformation is.

U4R1A - Can you look carefully at the central vertical "rib" on the plate, and see if there are any broken spot welds there?

Digging through boxes of well-used transmitting tubes at ham swap meets, I've seen a fair number with similar discolored areas and even holes burned completely through the plate, but I don't remember seeing this type of structural deformation.

Herr Zapp

U4R1A

Thu Jan 13 2011, 07:02AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

@Herr I don't know how much current was going through them and I know that's bad but the reason is I have a CT(60:5) that I picked up for the purpose of monitoring the current flow however when I use it my fluke flops from overload(10amps) to some figure of ma's nonstop and it wasn't providing any real data for me. That could be that I'm missing something with the operation of a CT but that's one of the many probs I plan to work out. While I was using the tubes this particular time the plates didn't get hot or have any visible color change. But the bit of missing info I didn't have in my original post was that they were working fine and I left them with the filament voltage applied for ~2hours then turned them off. Then the next day flipped them on and they were all crazy melted. The color change wasn't there before because I was watching for that and its only on the side towards the mot. There is also zero structural damage to them in the form of broken welds or any thing. To finish up your questions the mot is about ~8 inches from the tubes and the mot windings face the tubes.

Dr. Drone

Thu Jan 13 2011, 06:16PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

ubuntupokemoninc

Thu Jan 13 2011, 10:38PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
Registered Member #3299 Joined: Sat Oct 09 2010, 08:11PM
Location: Bantown, USA
Posts: 220

Look at the origanal 811a

,r:14,s:0&biw=1152&bih=641

the plate is like a virtual heat sink, and i imagine those ridges reinforce the plate

I tuned my 811a (NOS 811a) vttc running on 940vdc, then i switched to a mot and haven't had any trouble

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