40m class E CW transmitter kit NS-40 with an audio modulator

[kb3ouk]

The biggest problem with linears on AM is the carrier acts like a bias source and actually pushes the amp more towards class A even though it may be set up to run class AB (this has been discussed on here many times). A string of zener diodes coming off the center tap of the filament transformer will bias the tube into class C, so that when the amp is keyed up, the carrier causes the bias to drop into class AB or even class B. It takes a little bit more drive, but your tubes will run more efficiently.

[ka1tdq]

I seemed to notice that when I drive the tubes with a little more power, they seem to run more efficiently and don't get red in the face too early.  I noticed that they seemed to get red quicker at 15 watts than 25.  My amp has a diode string of about 7 volts total.

Anyway I did notice that my loading capacitor on the amp is fully meshed right now, so I'm going to add a doorknob capacitor to see if things run better.  

While I'm doing all these tests I'm improving my receive setup.  The antenna is basically just a long wire run to the receive SO-239.  I'm welding up a pi tuner to peak the receive signal.  The base is a 10 inch square of 1/4" steel.  It's a beast of a tuner!

Jon
KA1TDQ

[Steve - K4HX]

How can RF drive act like DC bias on a grounded-grid linear?