[Amps] grounded anode RF

[John T. M. Lyles]

I own several commercial RF systems that are configured this way, to a degree.
1) WT Larose Thermall dielectric preheater, that 
i test plastics, hoses, and insulators in before 
designing them into high power RF amplifiers. It 
is 90 MHz, using the Amperex 500 Watt triode. The 
anode is connected to the output plate on top, 
where you do your cooking. The filament is 
floated at -DC hV.

2) Continental 314R1 powerrock AM tranmitter, and 
for that matter, various PDM AM transmitters that 
used tubes in the 1980s. It has the RF tube (pair 
3-500Z) floated on an insulated, heavily bypassed 
G10 deck. The filaments are operating at -3kV, 
supplied from a 'totem pole' 3-500Z operating as 
a class S PDM switch tube. Filament of that tube 
is conected to -8.5 kV DC supply. The anode of 
the RF tube is connected to the output network, 
without a blocker cap, or HV choke.
This system is DC coupled, there is no blocker cap on the cathode side either.

73
John
K5PRO

>Years ago I recall reading about an amplifier where the B+ supply was
>grounded and the cathode was "hot" with the B- supply.  I'm wondering if
>that might have some advantages for high power amplifiers.
>
>Specifically, the pi-network output could be simplified by eliminating
>the plate choke.  Here's how the output would look:
>
>http://www.dslextreme.com/users/teeaye/B+.jpg
>
>Believe it or not, this is a pi-network output circuit, just redrawn.
>The same component values would be used as with a conventional circuit.
>
>I see some drawbacks: 
>
>1.  The filament transformer would have to be insulated for the full
>plate supply voltage.
>
>2.  Likewise, the grid current meter would be at the full plate supply
>and would have to be carefully insulated from the chassis.  Metal case
>meters would not be a good idea.  In fact, everything in the
>cathode/grid circuit would have to be well insulated, such as the zener.
>
>3.  Switching between standby and operate modes would require a
>well-insulated relay, perhaps a vacuum type or reed relay. 
>
>4.  The load capacitor would have it's frame grounded as always, but the
>tune cap would have to be insulated on both sides.
>
>And there may be others, but eliminating the plate choke with it's nasty
>resonances would be a big plus.  While it appears at first glance that
>the coupling capacitor is gone, it's function is actually shifted to the
>B- connection at the cathode.
>
>Comments are welcome.
>
>--
>Bill, W6WRT