[Amps] Tuned Input - IMD and efficiency

[Steve Thompson]

Bill Turner wrote:
> ORIGINAL MESSAGE:
> 
> On Tue, 8 Aug 2006 11:14:52 -0500, you wrote:
>>Are there other tuned input
>>circuits that work as well for matching cathode driven tubes and reducing
>>IMD products better than the pi and uses commonly available components?
> 
> 
> ------------ REPLY SEPARATOR ------------
> 
> Since most modern transceivers have a built in antenna tuner, I wonder
> how well that works in place of a dedicated tuned input?
Tom mentioned a point that's been observed before when the topic comes 
up - a key aspect of the input circuit appears to be the impedance at 
harmonic frequencies at the cathode. If you have a pi network close to 
the tube you're in control of that, if it's down to the ATU in a rig 
somewhere down the end of some coax, things can be all over the place.

Here's my take on why harmonics are important. The current pulse the 
tube passes every half cycle is not a pure sinewave (unless you run 
class A). It's a spike of current so it contains harmonics and the 
harder you drive the tube towards saturation/flat topping, the higher 
the harmonic content goes. If the impedance in the cathode circuit won't 
pass the harmonics then it's going to mess up the way the tube works.

It's an effect that will be pronounced when the drive is high - and it 
could show as poor efficiency as Tom observed, or as poor linearity if 
the shift into gain compression happens earlier. At drive levels/loading 
conditions where you stay clear of saturation, you might see only small 
effects, so it will be very dependant on individual amplifier setups.

As an aside, adding some capacitance with low inductance leads from 
cathode to grid won't only help linearity/efficiency, it might improve 
vhf stability too.

Steve