Steven Cook wrote:
>Thanks to all for your collective wisdom.
>
>I assume manufacturers added the CW position on older amps to meet FCC
>limits on input power for that mode.
>
>Did this serve to maintain circuit Q, or were there other considerations;
>e.g., power supply limitation, et al.
>
>
>
The tank circuit is an impedance transformer. It converts the load
impdance (typ. 50 ohms) to the high resistance presented to the tube
plate - typ. 2-4k. If you don't touch the tuning controls, the transform
is fixed, in which case the maximum power you can generate is related to
the plate voltage (tube maximum current might also be a factor, but let
it pass for the sake of simplicity). A lower voltage means lower maximum
output, which was a way to keep on the right side of the rules at the time.
>Just seems weird to operate an output stage that is not fully "dipped and
>peaked." Old habits are hard to break!
>
>When I tune up my Kenwood TL-922 in the CW mode, and then decrease my
>exciter output to around 50W, I notice a "HUGE" residual "dip and peak"
>remaining in C1 and C2 respectively and a 200+ watt increase in output
>power. What's up with that???
>
>
>
When you drive the amp with 100W, the peak drive sets the peak current
that the tubes can deliver (each rf cycle). In tuning for max output,
you adjust the plate load resistance to give you the maximum possible
voltage swing to go with that peak current. If you don't touch the
tuning and back off the drive, then the peak current is reduced but the
resistance is the same, so the peak voltage is also reduced - to use an
audio analogy, you're no longer swinging 'rail to rail'. If you retune
at lower drive, you will find a higher plate resistance where the lower
peak current results in higher ('rail to rail') peak voltage, and thus
more power out.
You won't go wrong following Phil's recommendation, but I disagree that
nothing else should be used. Most amps will happily work with a 1.5 or 2
VSWR antenna using the tank tuning to compensate, so why not use it to
change the plate resistance by a factor of 1.5? Without doing maths or
meaurements I think it's unlikely that the Q or harmonic output are
going to go way out of bounds.
Steve
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|