I built a grounded grid amp a few years ago....um..I guess 20..and used a
pair of 4-1000PRs. I built it with RTTY contesting in mind. I used a
separate filament transformer and B&W filament choke for each tube and had a
second blower that could be brought on line for RTTY. A pair of Hammond 795
transformers in the P/S. It loafed at the limit on the low bands.
Unfortunately, even though I built it in a big box and was very careful
about stray capacitance in the output, I saw greatly reduced output on 10
meters and also on 15. The output capacitance of these tubes is far beyond
what is necessary to resonate on the upper HF bands and still see the proper
plate load impedance. 7.6pf per tube for a total of 15.2 on the pair. I
assume this is because the screen grid is in very close proximity to the
anode.
But boy did it like 40M! It is currently sitting idle at VE7ZZZ.
Get a triode if you want a GG amp. The 3-1000Z has 0.12pf per tube.
If you are stuck on using the tetrodes, and want 10 and 15 meters I guess
you need to run them in another configuration.
I often wondered what would have happened if I had let the screens float.
Or tied them to the anode. Something bad probably.
-Robby
VY2SS
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