> In the Pi-L network, C1 and C2 (vacuum variables) are
> coupled by a
> toothed belt - motor driven by a servo amp. The manual
> describes
> C1 and C2 maintaining a capacitance ratio of 2.5:1, L1 has
> taps
> selected by the bandswitch while L2 is 17uH variable to
> some
> minimum inductance - the continuously variable part of L2
> is a
> 'pancake' inductor, with minimal lead length. L2 is
> motor driven by
> another servo amp.
How do you know Sunair intends it to behave like a Pi-L and
not a double L? By the capacitance ratio?
>>From Bill Orr's handbook (23rd Edition), the Pi-L table C1
>>and C2
> values show a ratio of between 4.5 and 5:1 for an anode
> impedance
> of 2000 ohms - close enough for 2500v @ 0.65A.
All that is is just an approximation for some value of tank
Q and transition impedance between the Pi and L that Orr or
someone picked. There isn't anything cast in stone that says
those values have to be used.
> Test conditions :-
> Amplifier running - no HV. 1900 ohm resistor from anode
> to ground.
> MFJ analyser connected to output side of L2.
> C1/C2 and L2 adjusted manually band by band for minimum
> SWR
> reading - all bands less than 1.2:1 except for 10 metres.
> The highest
> frequency that will yield less than 1.5:1 is 25.5 mHz.
My guess, and it's only a guess, is the stray C from the
anode is preventing the network from tracking above 25 MHz.
> Question - Did anyone on the the list ever use the
> GSL-1900A amp on
> 10 metres?
Not me.
> Also, I may have missed something along the way, but why
> do the
> amp manufacturers use 2.5:1 ratio when Bill Orr's
> (Eimac's) tables
> show an average of 4.75:1?
Because any value can be used so long as a minimum Q
requirement is met. There is also nothing cast in stone that
says the network must behave like a pi-L, or the first
section must have a phase shift more than ~100 degrees. It
could be in double L mode. Many commercial amps do that to
obtain more bandwidth before retuning is necessary.
Many of us tend to think in terms of "my amp won't work
unless I follow this published tank formula". In reality the
"formulas" we follow exactly are just approximations someone
decided to use. They aren't rules.
> Question - Observing the serious decrease in power output
> with
> increase in frequency, should I be looking at the C1/C2
> ratio as a
> likely problem?
I'd look at stray C on the plate tuning cap as being a
contributor. One would think they compensated with some
fixed C at another appropriate spot in the tank, like across
the "loading" capacitor.
> Disregarding the difference in recommended ratios, it's
> possible that
> some previous owner has "played around" with the toothed
> belt.
> The manual calls for a Vector Impedance Meter to set the
> capacitors -
> I don't have access to one. Maybe there's a
> work-around?
The method you are using with the 259 should work, although
it may take a bit more fooling around. I assume you are
connecting it so stray impedances are minimized.
73 Tom
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