432MHz GS-23B Cavity PA
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|PA PERFORMANCE PARAMETERS:|
This PA is cathode-driven, with control grid grounded to chassis and screen grid bypassed
to ground for RF. Performance Tests were run at five different drive levels to characterize
power output and plate efficiency. The amplifier was tuned for maximum power output at each
drive level due to the drastically different optimum plate load impedances associated with
the various drive levels.
These tests suggest that optimum parameters for 1500W output are plate voltage on the order of 2500 - 3000V, plate current of 1000 - 830mA (respectively), and drive level of 33 - 40W. At this plate voltage, optimum screen grid voltage is 500 - 540V. The GS23B tube should last indefinitely at these levels, as the tube in this PA has!
|Click on pictures to enlarge|
432 MHz PA RF deck
Complete PA - RF deck w/top off
|RF DECK (CAVITY) ENCLOSURE:|
The cavity enclosure is made of 1/8 inch thick aluminum and measures 8x8x12 inches.
The front and sides are a single sheet of aluminum bent into a U shape. The rear
wall is a flat plate measuring 8x12 inches.
Top View w/o tube & resonator
Top view w/tube & resonator
Bottom w/o cath. resonator
Bottom w/o cath. resonator
|SCREEN & GRID CONTACT PLATES:|
The screen bypass plate is mounted to and insulated from the top surface of the
enclosure divider plate by 10 mil Ultem or 5-8 mil Kapton, and teflon mounting
washers as seen in the pix. The grid grounding plate is mounted directly to the
underside of the enclosure divider plate. Both are made of 6x6x0.093 inch copper
The anode resonator is 5 inches long, made of 4.125 inch O.D. copper pipe, 0.1
inch wall thickness. This material has been seen from time to time on ebay for model
steam engine construction. The length of the resonator is influenced by the location
of the tuning and loading flapper capacitors. Higher in the cavity causes the resonator
to be shorter. This design has the flappers at the lower end of the resonator to make
their adjustment less critical.
|ANODE HV CIRCUIT:|
The plate bypass is made of two pcs 0.125 inch aluminmum sheet, 2x3.75 inches. There are
actually two plates to the bypass; one on each side of the cavity wall. Both bypass plates
are secured by a single 8-32 screw through a 0.75 inch O.D., 0.125 inch thick teflon washer
insulator and two 10 mil Ultem sheet insulators; 5 - 8 mil Kapton would work as well. Two
bypasses are probably not needed but the space was available and it was a convenient way to
mount the capacitor to the cavity.
The anode cooling chimney is made of 40 mil teflon sheet measuring 3.5x15 inches. The sheet
is formed into a 4.125 inches I.D. cylinder with an overlap of about 2 inches. The chimney is
attached to the cavity top plate by means of a 6x6 inch copper plate and 1/2 inch length of
the 4.125 inch copper pipe. The square plate has a 4.125 inch hole in the center, allowing
the short copper pipe to be soldered in place. Before soldering. the copper pipe was
drilled and tapped 6-32 in eight places for attachment of the chimney.
The cathode resonator is a 2x6 inch piece of 0.093 inch copper plate. The flat plate
is supported by two pillars made of 3/4 inch O.D. teflon rod, 1.063 inches long. The
pillars are 2.875 inches apart. This plate sits over the input tuning and matching
flappers which are located near each end of the plate. See any bottom view.
The copper pipe fitting mounted in the center of the flat plate, the cathode-filament
connection, is a 7/8 inch O.D. end cap with a 1 inch piece of 3/4 inch diameter copper
pipe soldered inside, total length of 1.125 inches. Approx 20 slots, 1/4 inch long,
5/32 of an inch apart, are sawed into the end of the copper pipe. This leaves “fingers”
that are shaped with long nose pliers to fit snugly inside the GS-23B cathode connector
ring; this is the cathode-filament connection.
The 3/4 inch diameter copper pipe encloses the filament connector pin. The threaded part of
the filament pin is used to hold the two parts of the resonator together but is electrically
isolated from the resonator by a teflon insulator and an insulating washer. The center pin
from an old Cushcraft 2m antenna coax connector was a perfect fit for the job. This contact
can be made from the center pin of an SO239 connector soldered to an 8-32 brass screw.
The 8-32 screw assembly seen in the
picture below is detailed in the "cross-sefction" sketch at right. The visibile metal washer
is 3/4 - 7/8 inches O.D. Make sure all burrs from the manufacturing process are removed so
that the insulating washer isn't damaged by them. The insulating "washer" is 3 (or more) mil
Kapton. The hole through the resonator plate and tubing end cap must be large enough to accept
the shoulder of the teflon washer which insulates the 8-32 threaded portion of the center
filament contact from the resonator and cathode-filament connection. The O.D. of the top
portion of the teflon shoulder washer should fit snugly inside the 3/4 inch tubing. A metal
washer of no more than 1/2 inch O.D. is next, and a nut above this washer secures the entire
stack. It is this nut which sets the center filament contact to the proper height, before the
components of "the stack" are put on the #8-32 brass screw below the resonator plate and
tightened to hold all in place.
Filament and cathode rf chokes are air wound #14 tinned copper wire, 3/8 inch I.D., 6 turns, 3/4 inches long with 1/2 inch leads. Ceramic standoff insulators mounted to the grid plate provide convenient support for the rf chokes. The filament and filament-cathode voltage enters the input cavity via 1KV .001mF feedthru capacitors mounted in a side wall of the cathode cavity portion of the RF deck enclosure.
Bottom w/cath. resonator: front
Bottom w/cath. resonator: rear
|TUNING & LOADING "FLAPPERS":|
See pictures of top and bottom of RF deck (above) for the details of the descriptions below.