Took this new (to me) beastly amplifier apart yesterday to document what it
has. It was dated from the 1960s and is in great physical condition. I'll put
something together online sometime with some photos. Meanwhile, my first
observations are documented. There are deviations from the OEM schematic that I
got from Bill, W6FF. While it is a big amplifier on wheels, it is not as manly
as most in this package. Some of the changes noted are that the grids on the
3-400Zs are hard grounded (instead of through 0.01 uF and a grid current
resistor). Also, there is a separate relay that energizes the filament
transformer, from the toggle switch. Both of these appear to be factory changes
or well-done field mods. Other undocumented mods aren't so pretty but are
interesting ideas. A big power resistor is in series with the HV transformer
220 VAC primary, with a switch to bypass it. The switch is labeled START and
RUN - apparently a form of 'manual' step-start. A short time delay relay wo
uld
serve nicely here. What I don't understand is why this is needed, as the HV
power supply uses a swinging choke input section, followed by a measly 6 uF
capacitor with another choke. There appears to be a second cap but not on
schematic. This power supply will undoubtedly have inherent low frequency
resonances and may have sloppy regulation with some keying or voice waveforms.
Another good addition, is a pair of UHF connectors on the rear apron that
break-out the RF input to the cathode circuit of the triodes. This was
connected to a homemade L network outside of the amplifier, tacked to a piece
of pine 1x4 wood on the back. The RF101 originally used untuned-input and the
manual warns of selecting the proper input coax length with the Harris-supplied
SSB exciter (using tubes). These days, this wouldn't be appropriate for a
"Rice" box or other modern solid-state exciter as discontinuous input impedance
is inherent with class B triodes as they draw grid current for part of the RF
cycle. I have some ideas to add some Q with small parallel resonant networks
across this point.
The original HV rectifiers use some early 600 Volt 1/2 amp diodes (1N2071), 32
in all, 8 per leg of a FW bridge. That is barely a safety factor of 2 per leg,
for the fully loaded voltage of 2400 VDC, but when the power supply is
unloaded, when drive is removed, the voltage can soar as high as 3300. While
they were compensated with RC networks, I measured them all, and the forward
drop as measured on the diode mode of my Fluke DMM show significant variations
from diode to diode. One diode was dead shorted, and i noted that the capacitor
in parallel had the top blown off. It will be easy to just remove this board
and replace with four potted HV modules as used in microwave ovens (8 kV 1
amp).
The plate blocking capacitor was originally a 2000 pF 6 kV ceramic disk. It had
been replaced with a 20 kV television HV-type epoxy ceramic cap, not a good
idea for high power. I intend to replace this with a ceramic doorknob capacitor
rated for RF current.
The amplifier has individual output pi-networks, 6 of them, that are switched
with a motorized large ceramic rotary switch. Each has a variable inductor
tuned via a hole (with supplied insulated tool in the back). The capacitors are
all fixed, no tuning capacitors. It is channelized in three ranges, 1.6 - 3.5,
3.5-7.5 and 7.5-16 MHz. Each range has only two channels, 2 for 160 meters, 2
for 80 and 40, and 2 for 20 meters. Not a lot of possibilities here. A small
remote box has a 24 VDC supply and selector switch that is used to remotely
select which channel the PA is tuned to.
My plan is to fix the HV rectifiers with four modules, and build a small
cathode tuning board with (4) LC networks that are either relay or PIN diode
selected for the input channels, one for the two 160 meter channels, one for
80, one for 40 and two for 20 meters. Then will give her a test on air.
73
John
K5PRO
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