[Amps] SB-220 bias question

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sun, 30 Aug 2009 00:17:43 -0400, "Joe Subich, W4TV" <lists at subich.com> wrote:

>No ... with SSB the plate load impedance is dynamic as 
>opposed to static (a single value) with CW or RTTY. 

REPLY:

I get the feeling we're talking about two different things here. 

Just to be clear, the pi-network, properly tuned, "looks" like a pure resistance
at a certain value to the tube. That is the load resistance the tube delivers
energy into, and that value of resistance does not change with changes in drive
level to the tube. 

To be strictly correct, the resonant elements in the pi-net are in parallel with
that load resistance, but as long as they are properly tuned they are a very
high resistance themselves and do not add either resistance or reactance to the
system except for very small ohmic losses. They do add the famous "flywheel
effect" but all that does is average out the energy over the RF cycle. 

The result of all this is the tube "sees" a constant load impedance, regardless
of drive level, and reducing the drive reduces the circulating current, just as
one would expect.  Any measurement you care to make within the tank circuit will
verify this. The obvious and easy measurement is the output power. If the
circulating current were increasing somehow, the output power would increase,
but it doesn't. 

And to get back to the original question, tuning up at 1500 watts and then
reducing the drive to some lower value is somewhat less efficient but  will not
harm anything. 

73, Bill W6WRT