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Registered Member #515
Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94
I had two QB 3/300 laying around for a long time. And now I'm wondering if these tubes would be suitable for a VTTC. And if they are, which topology would be best to use.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Andrew L. wrote ...
The ratings for the tube are great. But I didn't get any success yet, I probably didn't have enough voltage on the grid to drive it.
I see from the Philips data sheet that 'typical' operation at 3kV would have screen grid 350V, and control grid negative bias of -150V, with positive grid swings to +300V, control grid current 6.5mA, and dissipation of 2W.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Why did you decide to connect g1 and g2 in parallel, rather than tying g2 to the anode or to HT+ via a dropping resistor in the usual tetrode configuration?
Registered Member #1153
Joined: Mon Dec 03 2007, 07:27PM
Location: Croatia
Posts: 213
Harry wrote ...
Why did you decide to connect g1 and g2 in parallel, rather than tying g2 to the anode or to HT+ via a dropping resistor in the usual tetrode configuration?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Tying g2 to the anode will overheat the g2 and melt it in tubes like these. The reason is that they're designed to work with an anode voltage much higher than the screen voltage. A dropper resistor would help, but it'll dissipate a lot of power.
Connecting the two grids together makes it work reasonably well as a triode, the risk is now of overheating g1 with the level of drive needed for g2. But people seem to use these kinds of tubes as grounded-grid RF amps with both grids grounded (which amounts to the same thing) without any trouble that I've heard of.
One other possibility that seems good (though I've never tried it) is to connect g1 to the cathode and use g2 as the control grid.
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Is this tube sutable?
