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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Actually i think that would be great for a mot powered vttc. Use the same basic schematic for the 811a, and just tweak the values. It would be better than the 811a, because its absolute max plate current is 400ma, whereas the 811a is only 175ma!! You can have higher plate voltage than what it is rated for though, just not too much more. The 811a is also made for 1500volts max for the plate, whereas the tube you have is rated for 2000volts max. You could probably run your tube with 2500 volt or so, because the 811a's are run with 500 volts over there max ratings(cause we all like to abuse our tubes with higher voltage and current :P) so that will allow for a higher voltage mot to be used.
I suppose just tie all of the grids together and use all of them as one grid, and use steve wards 811a vttc schematic.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need to be careful when tying all the grids of a tetrode together. The screen grid can end up starved of power while the control grid gets too much and melts. I personally would use these tubes with a separate screen supply, and save the tying-grids-together trick for bigger tubes like the 4-400 or whatever.
As for abusing tubes, if you're running level shifted AC rather than smooth DC, you can use peak voltages twice the values given for "Class-C Plate Modulated" service, before you even start abusing anything. (Level shifted AC is just like 100% AM.)
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Steve said it about the grids.
Another thing, be very careful with the feedback coil voltage, I can at overvoltage get sparks between controlgrid leg and filament legs in the socket of the tube, just where the legs go through the glass. It have now left some marks and is not quite enhancing the lifetime :)
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Hmmm... I asked the same (almost) question on diyaudio and one of the guys there came up with a version of the 7094 datasheet that I hadn't seen - foolishly (!), I was using the RCA one from 1958, but there is a 1962 one here that discusses triode operation with g1(control) & g2(screen) grids connected together.
I really don't want these rather rare tubes to sulk in a box in the loft... equally, I'd rather they didn't crash and burn... Do you think the g1/g2 linked mode could damage the tube - I also note that the beam shield is connected to g2 - would it matter which side of g2 I connect to g1, or should I connect all the g2 pins (3,5,7) to g1 (6)?
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VTTC question with RCA 7094...
