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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I was playing with my VTTC which uses a Russian GU-81 transmitting pentode. Originally I had it connected as a pentode, with the screen supplied from auxiliary supply and the suppressor to the cathode. I wondered what might happen when I connect the screen and the suppressor grids together. The screen+suppressor current did increase a bit, but not much (~130mA vs 110mA previously), the plate input increased and the sparks are longer.
I wonder if there are any problems with doing this? Or should I go another way about increasing the power? Why do they use pentodes in power transmitters?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I think the dissipations are within limits, AFAIK the plate never got red, at least before the modification, but I too haven't seen it getting red now. The cathode peak current however might be exceeding 6 amps, but I think a directly heated cathode doesn't mind much.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
They used pentodes because the beam tetrode wasn't invented at that point.
The suppressor grid makes the pentode more linear than a plain tetrode, which is important in a RF amplifier to avoid interference to other channels. This doesn't matter in a VTTC, so you can do whatever you like with it.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
If you did connect the two grids together, wouldn't you have to worry about melting your suppressor grid since they aren't really built to dissipate any power unless I'm misinformed?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the replies
Myke, the suppresor grid in this tube is pretty robust. The g2+g3 current increase was just some 20 mA, so I suppose this is the current which flows to the suppresor grid. This is a 12 watt dissipation, while the screen is designed to dissipate up to 120 watts, so I believe this is not a problem.
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Pentode into tetrode
