[Amps] 813 efficiency

[Vic K2VCO]

The 813 has a relatively high output capacity (14 pf) so it can be difficult to get a 
reasonable Q on 28 mHz with practical components. It helps to use a vacuum capacitor with 
a lower minimum as the pi input capacitor. You can also add a little inductance between 
the tube and the pi network to make it an "L-PI" network. This will solve the Q problem, 
but increases the inductance in the parasitic tank circuit formed by the path from the 
plate, through the capacitor and back to the cathode (or grid, in the case of a gg circuit).

A certain amount of power will be wasted in the parasitic suppressor. If you can produce a 
stable amplifier without a suppressor in the plate circuit (suppressors can be placed in 
grid and screen circuits) then you could improve performance.

My 2x813 grid-driven amplifier produces about 100 watts less on 28 mHz than lower bands.

On 2/20/2013 12:18 PM, Mike Lord wrote:
> Some time ago (maybe 2 years) there were comments filed on this site regarding increasing the efficiency of the 813 tube on 21 - 28 mhz.  I've looked through my history of this group but failed to find a reference.  I also checked some GE Ham News articles that I've saved.  I realize the tube wasn't designed for this spectrum.  However, I do seem to recall some references being made on this site based upon personal experiences.  I really don't intend to waste the groups time, but I sure would appreciate any circuit suggestions that would lead to increased efficiency of this tube.
>
> 73, Mike K4ELV
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/