[Amps] 813 efficiency

[Eddy Swynar]

On 2013-02-20, at 3:18 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.
> 


Hi Mike,

There was an article in an issue of QST (when it was still of the small-format, I believe), as well reference made in the large-format HANDBOOKS, regarding inserting a small coil in series with the output of tubes like the 813 that, in effect, acted like an "L" network, thus raising the efficiency of the tube tremendously on the higher bands...

I went through the mathematical gyrations (and they are considerable!) when I built my 2x813 linear here some 22 years ago: I ended-up inserting a switch-selectable 0.3 uh. coil in the plate leads right at the 813 plate caps...a mercury switch (glued atop a popsicle stick!) allows to coil to be "in", or "out". When it's "in", the amplifier seems to perform every bit as efficiently on 10-meters, as it does on 160...

Mind you, I incorporated a switch in the plate tuning capacitor, too, allowing me to switch it out entirely, & use the high interelectrode "C" of the tubes to resonate on the higher bands, ALL AIDED & ABETTED BY A VARIABLE PI-TANK COIL that I use here (26 uh. of 1.4" edge-wound copper ribbon).

There's considerable work involved, but it CAN be done.

Barring this, take a look at the scheme used for 10-meters only in the 2x813 amplifier that's featured in the 1968 ARRL HANDBOOK...

~73~ de Eddy VE3CUI - VE3XZ