432MHz GS-23B Cavity PA
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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.

All power output measurements were made into a dummy load through a length of RG213/U coax cable exhibiting 10db loss at 432MHz. This method attenuates second harmonic level approx 3db and third harmonic level approx 10db with respect to the fundamental.

   Drive power - W3035607590
   Output power - W13901500220024002525
   Gain - dB16.616.315.615.114.5
   Ep (idle) - V39503950395039503950
   Ep (max Ip) - V36003550338033253300
   Ip (idle) - mA120120120120120
   Ip (max) - mA642690105111551215
   Pinp - W23112460355238404010
   Pdiss - W9211028125213401485
   Pl eff - %
   Eg1 - V5656565656
   Ig1 - mA-3-212040
   Eg2 - V580579578573575
   Ig2 - mA2382015

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


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.

Aluminum 1/2 inch angle stock, 1/8 inch thick, is used to fasten the back wall, top and enclosure divider plates to the main enclosure U piece. It is also used around the bottom edge of the enclosure to secure the RF deck to the PA cabinet mounting surface.

The divider plate in which the tube is mounted is 7.75 inches square to fit inside the enclosure, and is located 2.75 inches from the lower end, leaving 9.25 inches above for the plate cavity. There is no cavity bottom plate as the aluminum cabinet housing the RF deck and power supply provides this function.

The RF deck top plate plus the aluminum of the PA cabinet, which fits snugly on the top of the RF deck when fully assembled, gives a total of 1/4 inch thickness at the air exhaust. The air exhaust for anode cooling air is comprised of 185 holes 0.22 inches in diameter drilled through both top plates. A scrap piece of perforated aluminum was used as a drill guide for these holes. The extra thickness of the two combined 1/8 inch thick plates gives greater attenuation of rf leakage through the top air exhaust, acting as multiple waveguides beyond cutoff.

Top View w/o tube & resonator

Top view w/tube & resonator

Bottom w/o cath. resonator

Bottom w/o cath. resonator


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 sheet.

Type 97-135 fingerstock is used for tube contacts on both plates. The center hole in each plate is sized 0.2 inches larger than the tube connection rings to accomodate the finger stock compressed thickness. Note that manufacturing tolerances for the GS-23B are +1 / -0.5mm for the "50mm" diameter screen contact ring, and +/- 0.3mm for the "35mm" diameter grid contact ring. Measuring tubes on hand would be prudent, rather than merely going with 50mm and 35mm diameters.

From the above description, it should be clear that the grid is grounded in this amplifier. The 6x6 inch grid grounding plate provides a convenient means to mount the input tuning and matching flappers. Twelve small holes were drilled in the grid plate around the fingerstock area to allow some cooling air to pass near the GS23B grid and screen tube connector rings. The screen voltage is brought into the RF deck enclosure via a 1KV .001mF feedthru capacitor mounted in a side wall of the cathode cavity. The connection to the screen bypass plate is a brass screw that passes through the RF deck divider plate, insulated by teflon washers, to the cavity divider plate. See either RF deck bottom view, rear.


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.

The I.D. of the copper pipe resonator is too large to fit the GS-23B cooler. The next smaller size copper pipe is too small. The larger size was made to fit properly by using a short length of the pipe as an internal spacer to reduce the O.D. of the tube end of the resonator. In the PA shown, a 1 inch length of the 4.125x0.125 inch pipe, with a 0.65 inch slice of the circumference removed, is used as a spacer. A large muffler clamp was used to squeeze the spacer O.D. enough to fit inside the resonator. Type 97-135 fingerstock and the internal spacer were soldered inside the resonator and make a perfect fit over the GS-23B anode cooler.


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 plate RFC is the same construction as the filament and cathode chokes, i.e. air wound #14 tinned copper wire, 3/8 inch I.D., 6 turns, 3/4 inches long with 1/2 inch leads. One end of the plate choke is connected to the lower end of the plate bypass with a 8-32 flathead countersunk screw. The other end is connected to a thin brass clip mounted on the upper end of one of the four resonator support pillars. The brass clip is shaped to apply firm contact pressure to the resonator. The support pillars are 0.75 inches O.D. teflon rod, 1.375 inches long, mounted to the screen bypass plate with 8-32 flathead countersunk screws. The edge of the GS-23B anode cooler rests on these pillars as well as the heavy copper resonator. The length of these pillars were carefully trimmed to allow the GS-23B to sit squarely in the socket and the anode resonator centered in the cavity enclosure.


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.

Cooling is provided by a Dayton 4C442 blower mounted to the rear wall of the cavity enclosure. The blower air outlet measures 2.5x2.5 inches and this area of the cavity wall is drilled with more 0.22 inch O.D. air holes. There are seven more holes drilled in the enclosure divider plate, in a line near the front wall of the cavity. These holes allow about 10% of the anode cooling air to bleed into the cathode cavity. A group of 14 more holes are drilled into the rear wall of the cathode cavity to allow the cathode cavity cooling air to exhaust.


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


See pictures of top and bottom of RF deck (above) for the details of the descriptions below.

The plate tuning flapper is 1x4 inch thin hobby brass (0.020??). One end is fastened to a side cavity wall near the front corner by two screws. The other end is fitted with a teflon button the prevents contact between the flapper and plate resonator and provides a means to attach the fishing string actuator line to the flapper. The fishing string will immediately burn through without this button. This teflon button is located approx 1 inch from the end of the flapper. The flapper is shaped into a shallow z to allow the end to rest near the resonator (maximum C) when the actuator string is loosened.

The plate loading flapper is 0.75x4 inches, mounted to the end of 0.75x1.375 inch Teflon pillar, near the rear cavity wall and corner. There is another teflon button on the end of this flapper used in the same manner as the tuning flapper. A short, heavy connection is made from the cold end of the loading flapper to the rf output connector.

The fish line actuator strings pass through the cavity sidewalls and around three teflon guides to the front panel control shafts.

It is important to note that the tuning and loading flappers must be mounted 90 degrees from the axis of the plate resonator. They will not function at all when mounted parallel to the resonator axis, due to h field interaction.

The cathode tuning flapper 0.75x 3.5 inch hobby brass. The cold end is fastened to the copper grid plate and the hot end has a fishing string actuator attached for front panel adjustment. More teflon guides are used to route the fishing string.

The cathode matching flapper is also 0.75x3.5 inches. The cold end is suported by a 0.75 inch O.D., 1.5 inch high teflon pillar. A short, heavy connection was made from the support end of the flapper and the input BNC connector. Another fishing string is used for front panel adjustment of the matching flapper.

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