[Amps] CW and High Voltage

Steve Thompson g8gsq at ic24.net
Sun Sep 4 03:07:10 EDT 2005


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


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