As several informative replies were received, some of them raising
other questions, I'll try to summarise as briefly as possible.
W8JI asked - <How do you know Sunair intends it to
behave like a Pi-L and
not a double L? By the capacitance ratio?>
The Sunair manual states "The tuning and loading
capacitors, C20 and C25 are mechanically ganged
and motor driven ... the ratio of C20 to C25 should
be approx 2.5.
>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.>
Yes - there is a 50 pFd cap in parallel with the
loading cap.
"The nominal desired plate impedance is 1900 ohms
with zero phase angle..... the junction is the midpoint
and is near 100 ohms. The midpoint impedance
can be measured and is an indication that the ratio
of C25 to C20 is the proper value."
Bob W6TR wrote - < I would also take into
consideration the output capacitance of the
tube as well as the stray capacitance of the socket,
chimney(if metallic) and C1 mounting hardware.>
Output capacitance is 11.8 pF. In this amp the tube
screen is grounded, -250vdc is applied to the
cathode, bringing the net anode-cathode voltage to
2750 - I neglected to include this in the impedance
calculation which becomes 2350 ohms. I think.<G>
The chimney is non-metallic.
Tom said >I'd bet money that ratio is optimum for
their goals
which would be bandwidth and harmonic
suppression requirements.>
Agreed, and as the amp is spec'd to 30 mHz I
assume that it will tune at least to 28 mHz which
leads me to assume that (having eliminated every
thing else), the 2.5:1 load/tune capacitor ratio has
become maladjusted by whatever means, causing
an increasing mis-match with increasing frequency.
However, my assumptions are not always valid.
The MFJ analyser has a 9" coax cable to the output
terminal of the Pi-L network - the 1900 ohm metal
film resistor leads are just long enough to reach from
the anode connecting strap to chassis.
Thanks again for your guidance - it's much
appreciated.
Ken ZL1AIH
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