Hello,
Just finished(?) a homemade GS35-B amp. for 80, 40, 20, 15 and 10
meters. All the control, metering, input and filament circuits are
working the way they should. Unfortunately, the PiL output network
isn't working as assembled and tested on the bench. A description of
PiL network's design, assembly, bench and in the amp testing
techniques are below. The question to the group is what are the
variables I'm not taking in to account that could be throwing off my
measurement from the bench to the amp? Also, what are the proven
technique(s) for adjusting (finding the taps on the L1 coil) a PiL
output network?
The PiL was designed using the Pi-EL Designer by Jim Tonne, WB6BLD,
and checked using Ian's, GW3SEK, PiL spreadsheet. L2 is made from
two, T200-6 toroids with the taps on the #12 wire established as
close as possible along one side of the coil to the calculated
values. The L2 coil taps were found using the L output from an Autek
set at ~4 MHz. The L1 taps were found using the info. from the Pi-EL
Designer and the L output of the Autek. The PiL was assembled using
a vacuum variables for C1 and C2, respectively, 250 pF and 1000 pF
max. A double pole, eight position, progressive shorting, ceramic
switch is used for L1 and L2.
The PiL circuit was assembled on the vertical aluminum plate to be
used in the amp. The L1 taps and capacitors were adjusted for a 1:1
SWR, 50 Ohms on a MFJ 259 with 2700 Ohm resistor across C1. Good
values were obtained for L1, C1 and C2 for all bands with sufficient
(so I thought) leeway for eventual tube and stray capacitance to be
accommodated. Unfortunately, when the assembled tank circuit was
installed in the amp. cabinet and a 2700 Ohm resistor connected
across C1, the values weren't close to what they had been. The tube,
RFC, etc. are installed in the amp. cabinet, but not attached to the
PiL output network. The PiL compartment is large compared to
commercial amps.
Shorting turns on L1 has not shown any large change in the C1 and C2
settings. For example on 14.0 MHz, the best that can be obtained
with the L1 and L2 in the 14 MHz position is a SWR of 1:1.25 at 40
Ohms when C1 and C2 are at minimum capacitance.
Whatever help, ideas, tips, things to try, etc. will be appreciated.
73,
Mike, K4GMH
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