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Old 26th Sep 2012, 12:52 pm   #1
kalee20
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Default Filament polarity - battery valves

I'm wondering, is there a reason behind the specified filament polarity for battery valves? I'm interested in the DAF96 diode-pentode, which is specified with Pin1 negative, Pin 7 positive. If I reverse this, will there be any effect? I'm thinking, with the voltage gradient along the filament, is the pentode grid wound with variable-pitch wire spacing as a cunning means of compensation? (Obviously there is the matter of the effective HT changing by 1.4V, but I'm considering this as negligible).

The reason I'm asking, is that the 'A' diode is located at the Pin 1 end. I'd like to bias this 'off' a bit to use as a delayed AVC diode, which I can do by just reversing the voltage between pins 1 and 7. I'd make the necessary reconnection of the pentode grid leak resistor at the same time. But the pentode will still be operating with its cathode potential gradient reversed, and as the valve cuts off at about 3.5V, a reversal of 1.4V along the active length could well be critical.
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Old 26th Sep 2012, 1:51 pm   #2
mickjjo
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Default Re: Filament polarity - battery valves

On the DAF96 pin 1 is internally connected to g3, Reversing the heater connections would raise the potential on g3 from 0v to +1.4v, I'm not sure what effect that would have... .

http://www.radiomuseum.org/tubes/tube_daf96.html

Regards, Mick.
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Old 26th Sep 2012, 5:15 pm   #3
kalee20
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Default Re: Filament polarity - battery valves

That's a good point, probably it'll make almost no difference to current (if it was strongly negative, it could repel electrons back to the screen, thus increasing Ig2 and decreasing Ia).

It could attract and divert a few µA of its own, being positive, I suppose...
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Old 26th Sep 2012, 7:55 pm   #4
ukcol
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Default Re: Filament polarity - battery valves

It would perhaps be interesting (if a little involved) to build a test circuit and plot curves for the valve; one set for normal filament polarity and one for reverse polarity.

Colin M
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Old 29th Sep 2012, 8:52 pm   #5
kalee20
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Default Re: Filament polarity - battery valves

That's what I was hoping to avoid having to do!!
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Old 29th Sep 2012, 11:08 pm   #6
WME_bill
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Default Re: Filament polarity - battery valves

Filament polarity. Good point about the suppressor g3 connection. A positive suppressor might liven up the anode current flow nicely. But there is also the point of the effect on the diode in the DAF96. My Mullard data book of 1956 says that "the diode anode is located at the negative end of the filament." If at the positive end, it would perhaps give a useful AVC delay. wme-bill.
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