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Author Topic: 811A  (Read 988 times)
G3RZP
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Posts: 1316




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« on: November 18, 2018, 02:45:58 PM »

I think this is aimed mainly at KM1H and W1QJ and maybe W8JX......

I have four Russian 'National' brand 811As. For use in a 30L1 they are dubious, because none of them have plates aligned such that with the filament pins in the horizontal plane, the plates and thus filaments are not in the vertical plan, by as much as 30 degrees. But I have a friend who wants them to use vertically mounted as AM modulators at 1kV on the plates. So far no problem....

So I put them on my ionisation tester to see where the plate - grid breakdown occurred. Bear in mind that an ionisation tester detects the start of breakdown long before a Hi Pot microammeter will. I got results from those age unknown tubes varying between 2500 and over 4000 volts before signs of ionisation occurred, which suggests to me that all of them should work perfectly well at 1kV plate supply in Class B audio at under 100 watt output.

The question I was asked is "What voltage would new 811As made in the 1960s have shown ionisation at in the years when they were new?"

Any ideas or comments?
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KM1H
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Posts: 5513




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« Reply #1 on: November 18, 2018, 05:43:42 PM »

I remember when Svetlana was selling them and they were a disaster in any 811A amp, they were from the Ukraine if I remember. Svetlana sold many with their logo and then stopped entirely; I was told by Svetlana USA the real one run by George Badger, W6TC, formerly from Eimac, they couldnt get any decent quality and it was hurting their reputation.

I did put a pair in my Clegg Zeus modulator which runs at 900V and they were fine at voltage but seemed low on audio. I put my original RCA's in to confirm and didnt bother playing with the bias. When those got tired I replaced with the under sized anodes Svetlana 572B's which would randomly spit in a SB-200 and are still there.

Ive never used an ionisation tester and only hi pot testers (is this just a terminology issue) which I built with parts help from Rich Measures. Good tubes would test beyond peaks seen in an AM TX which would be 5000V in ICAS and typically 9000V for NOS RCA without any outgassing.

Carl
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WA5VGO
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Posts: 75




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« Reply #2 on: November 18, 2018, 06:25:21 PM »

Been running a pair of these at 2.2KV as modulators in my Johnson Viking 500 for 25 years. Never had a problem. They also still have excellent emission.

Darrell
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G3RZP
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Posts: 1316




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« Reply #3 on: November 19, 2018, 12:39:24 AM »

Carl,

the high pot tester actually requires a measurable current. The ionisation tester applies the test voltage and at the low potential end , there is a series resistor, around 100K. This is coupled to a high gain audio amplifier - in my case, a pair of cascaded 6SJ7s and 6J5 A/F o/p feeding a loudspeaker or headphones. Ionisation starts as a 'fizzing' noise when there's no current showing on a 50 microamp meter. Mine uses a 6F6 as an oscillator at about 200 kHz and a tube EHT rectifier as used in very old TV sets. It had to be rebuilt but cost me under $10.....

A 15kV PIV solid state replacement for the tube, bearing in mind the frequency, would take some finding and more than $10........

Incidentally, have you seen the current QST? There's an antenna switch design there that detects the RF from the transmitter and actuates a relay to protect another rx using the same antenna. Whether it's really  fast enough is one question, but the RF is 'detected' by either a pair of 1N4004 or 1N4007. Bearing in mind the trrs of those are measured in microseconds, I have doubts about their efficiency....
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