That PS sounds like it will have much better regulation than your
standard fat 20-50 mF cap only PS.
As long as the input choke meets the minimum inductance standard (see
any handbook from the 60-70's back) it will hold the output very close
to 90% of the xfmr AC rating. The second choke and cap provide
additional filtering.
I use a 5-25 H swinging choke and 10 mF filter and the voltage holds
under 200V from 150 ma to 1A at 3500V in SSB/CW service. Id add the
second filter section if it was for a Class AB/B AM modulator which is
exactly the way I run the PP 810's and 304TL's.
Carl
KM1H
----- Original Message -----
From: "John Lyles" <jtml@losalamos.com>
To: <amps@contesting.com>
Sent: Sunday, August 31, 2008 11:02 AM
Subject: [Amps] RF Communications RF101 power amplifier
> 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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