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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
From what I've seen, people usually tie the screen and control grids together on larger pentodes and use it as a triode. Is this OK to do? I'd be afraid the control grid overheats because it is at a higher potential than it should be and rated just for little dissipation. Or should I rather bias the screen grid from a few hundred volt supply through a large resistor and drive just the control grid?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
I think it would be a good idea to drive the pentode the way it should be. The suppressor grid should be at cathode (or filiment if directly heated) potential. The screen grid should be a couple hundred volts above the cathode potential but it depends on what the datasheet says it needs. The control grid should be what you use to control the current through the tube. The screen grid put at a couple hundred volts affects the current through the tube more than tying the grids together. Tying the grids together and driving them as if it was a triode is fine. It shouldn't melt the control grid as long as you stay within the ratings of the control grid. It's best to drive the tube the way it was meant to though.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I agree with Myke, it is a good idea to run it as it was intended. It is simple, but it could require an extra small transformer, or a resistive divider. Most pentodes seem to have an internally cathode-tied suppressor. With pentodes, running with enough voltage on the screen, it is higher gain than a triode so feedback takes less power. Like with these very high frequency coils, you might be able to use capacitively coupled feedback, like an antenna.
I have tied the grids together and used it as a control grid. It works, but it may not let the tube turn fully on...? It sure did not work as good as giving the grid the voltage suggested on the datasheet. Unlike the filament who's voltage is non-negotiable, you have some leeway with grid (and plate, but that is beside the point) so i have never killed a grid that way... But, it would probably not be too hard.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the replies I had another idea- connect the control grid to ground and drive just the screen grid, this should bring the grids to potentials close to what they should be during the conduction phase and still without any extra supplies. Anyone thinks this is ok?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
It would vary the tube's current a little but I would think it wouldn't be able to vary the current in the tube as well as the control grid. This is because the spacing between the screen grid's wires is a lot larger than the control grid's. I think the distance from the cathode/filament also has an effect on how much a grid can vary the tube's current but I don't remember why. It might have to do with the space charge. There are some designs out there that modulate the screen grid but they aren't too common. I think that setup would most likely be used to modulate the output of an oscillator. The screen grid's distances from the plate and cathode will be different from the distances for the control grid so the capacitances are different but I don't think it would matter in this case.
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Another VTTC thread: pentode?
