>From: "Manfred Mornhinweg"
> I prefer not using any such "fudge factors", but instead calculate the
> real situation, based on the actual tube and operating condition.
Thanks Manfred, that makes sense to me. But then I wonder why fudge factors
are used at all? Your example calculation doesn't appear any more
complicated than the "fudged" handbook equations. As a newby amp builder, I
am trying to understand the theory and calculations behind it, which sends
me to the handbooks. I ran through your example calculation and came up
with a K factor of 2.5. The only mathematical difference between what you
displayed and what is in the handbooks is that you use RMS RF voltage, and
the handbook uses DC plate voltage. So perhaps the K-factor is used since
DC plate volts is more easily known than the RMS voltage swing and
saturation voltage?
> In class C you would typically overdrive the tube to bring it deeply into
> saturation.
So, can I infer that I should "design" for Class B SSB operation, and then
if I need to use the amp for Class C CW operation, just use more drive?
>The unknown factors in the tube's characteristics and its operating
>conditions are much more
> important! And anyway the PI circuit fortunately provides enough
> adjustment range to render exact calculation a platonic exercise!
My intended 'operating condition' and design is:
- Three 813's in grounded-grid.
- CW and SSB on 160,80,40,20m
- Around 1000 watts output.
- PI-EL output network.
- 2700v supply.
- TS-870 for exciter.
> From: "Tom W8JI":
>Is K 1.8 or 2? Why worry? It might actually be 1.5 or 2.5.
>If you get the Q too low the loading control will work
>backwards. If you get it too high the tuning will be
>needlessly sharp and there will be a little more tank heat.
Yes I admittedly have been too caught up in the calculations - but only in a
effort to understand what's going on. I was struggling with the PI-EL
design in order to figure out what capacitors I have to obtain (lower values
are much easier to obtain than the higher values, and wanting 160m ability
in the amp doesn't help), and what kind of coils I need to start winding up.
I suppose with capacitors I can just find the largest values I can (say
250pF tune, and 1500pF load) and pad or trim as needed. And with coils,
wind them for the larger calculated values and cut them down as needed
during the on-board fine-tuning effort.
Steve, KK7UV
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