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National Electronics 811A




 
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W4AMV
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« on: November 19, 2018, 07:44:34 AM »

Good day...

Picked up a set of 3 new in box with original paper work. Decent price. Made in Russia, so the base says. Two of the 3, had open filament. After a bit of head scratching and dis belief, I lightly sanded the filament pins. Tubes lit up for about a second or two and then died again! Placing a hemostat at the pins I applied solder and re heated the filament pins. Good as new! What's the story with these tubes??? Will I see open grids and open plates? Quality?? I hate to use these in the amplifier if there are hidden issues. Thanks! Alan
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Patrick J. / KD5OEI
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« Reply #1 on: November 19, 2018, 09:04:21 PM »

Just try resoldering the pins, or, maybe if it is not helping, remove the base (may not be possible!!) and clean the wires and resolder. But I think you just got some bas solder connections, that's all.
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W4AMV
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« Reply #2 on: November 20, 2018, 12:29:03 AM »

Hi Pat, thanks for the reply. YES. That is what I did on both tubes.

Just try resoldering the pins, or, maybe if it is not helping, remove the base (may not be possible!!) and clean the wires and resolder. But I think you just got some bas solder connections, that's all.

I guess I was just surprised to see this issue which prompted the query. Is this typical for these brands? Tnx, agn.
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Patrick J. / KD5OEI
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« Reply #3 on: November 20, 2018, 09:55:11 PM »

I've never had it on a new 811. Maybe they were made on Friday afternoon?
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« Reply #4 on: November 21, 2018, 12:43:56 PM »

Hi Alan,

This is old info, but it may be of value here...

The operating desoldering of filament pins is a common problem with 3-500Zs that do not have adequate cooling across the fil pins. I've had it happen in commercial amps that use just blade fans on top. Unless there is a chimney, no air gets circulated below on the pins and they get very hot.

That said, these 811As may have the same problem due to a manufacturing flaw or more likely a simple soldering mistake -  right from the factory.  Maybe factory burn-in testing did it.  Perhaps the soldering job you did will be OK now - if not, then consider a small fan below chassis to move some air.  That would be a band-aid fix because normally, 811As are designed to operate without forced air - just need some natural ventilation/ radiation cooling.


T
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« Reply #5 on: November 21, 2018, 02:21:25 PM »


 I have seen this issue with my CE 600L linear amplifier. It uses an 812 for the screen regulator to the 813 final. The RCA 812's I had all did this. No ventilation either like Tom mentions.

In desperation I popped in an 808 (wire added grid cap to base pin). That 808 running at 6.x volts instead of 7.5v seems to work fine.

Last year I picked up ten NOS Russia 811A's from a Eastern European seller. I made an offer, and took the chance. They came in perfect, and I made a test fixture to screen and match them. They all tested identical with grid tied to plate with a variable DC source. All had constant emission at 150ma cathode current when varying the filament voltage from 5.5 to 6.5 vac. These tubes also don't have the ring type Getter, but instead have two special washers riveted to the plate structure. No issues with the first 4 I put into my Gonset GSB-201. The Ruskies could build tubes.

Jim
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W4AMV
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« Reply #6 on: November 21, 2018, 05:56:18 PM »

Thanks Tom and Jim for further info.

These tubes also don't have the ring type Getter, but instead have two special washers riveted to the plate structure.

Yes Jim,  these 811A's use the same plate structure with the rivet washers.

I suspect the fellow who originally purchased these, put them into their sockets, had no filament and moved on! These tubes as I received them had all the original paper work and protective wrapping and look like they have had zero hours of operation.
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KK4YY
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« Reply #7 on: November 21, 2018, 06:07:06 PM »

I suspect the fellow who originally purchased these, put them into their sockets, had no filament and moved on!
One wonders how many hands they've been through before yours - the hands that FIXED them!
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