[Amps] Using 813 tubes on 17 meters and above...

[Peter Bertini]

I'm getting serious about building an 813 amp as winter project and need
some input from other OT's who've played with these tubes on 17 meters and
above.

Plans are to go with three or four tubes in parallel.  Power transformer is
rated 1500va and I'll be near 2.8 kV plate voltage, in grounded grid.

Plate load for two is around 3500 ohms, so I'll be around 1750 or so with
four of the beasts in parallel... which is doable using a Pi L on 80 and
160 to keep the load capacitor size reasonable, and then going over to a Pi
Net for the upper bands.  I was more concerned with the combined plate
capacitance, which is going to be close to 60 pF  ???  That is value is
over the design value for a Pi Net with a reasonable Q at that load
impedance.  I am still very curious how others have made out using two or
three tubes in parallel on 17 meters and above. I've heard, anecdotally,
that tapping the plate tune cap down on the tank coil helps, but I've not
seen that in any published literature.  Any problems with using Amidon
T255A-2 core for a combined shorted-turn inductor for 80 and 160 meters on
the low Z side of a Pi-L,  and another core for the 160 meter inductor on
the plate side of the Pi L?

Any advice?  By the way, I am going this way because I have plenty of NOS
RCA tubes and sockets, and I can always change over to a more modern tube
in the future.

73

Pete

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