GS-23B 2M PA

Construction Details

The pictures below show details of various parts of the construction project.

Top view
Above:  View of the inside of the anode compartment of the PA.
Below:  Click on pictures to enlarge.

Loading and Tuning Capacitor construction
Load Cap Load Cap 2
Tune Cap Tune Cap 2

The load and tune capacitors, along with the mechanical apparatus that change the plate spacing, are shown in the color diagrams. The tune capacitor is built from 0.064" copper plates, 4" X 2.5" in size. The load capacitor uses square plates 1.5" on a side. The mechanical screw adjustment facility is designed to vary the spacing between the plates from about a minimum of 1/4" to a maximum of 3/4", with the spacing varied by turning the tune knob on the front panel. The plate nearest the front is fixed to ground using a thin copper strap, the width of the plate. This overall design is not very critical, as long as it provides sufficient minimum and maximum capacitance to handle the amplifier in normal operation.

The screw-based mechanical mechanism uses a 3/4" copper pipe (Online Metals #1876 Copper 122 tube, 0.875x0.065x0.745) approx 3" long, soldered to a square mounting flange. A 3/4" teflon plug (Online Metals #6516 plastic teflon virgin round 0.75" dia), approx 3" long, is cut such that it will slide freely inside the copper pipe. The moveable plate is fixed to this plug by drilling and tapping a hole in the plug such that a nylon machine screw will hold the plate firmly to the plug. The head of the nylon screw serves as a stop to prevent the gap on the capacitor from being adjusted too small and shorting the anode to ground. The other end of the plug is threaded such that a shaft made of common "all-thread" will fit and serve as the adjusting screw. The all-thread is held stationary by a McMaster-Carr 6383K213 ball bearing. A nut is used on each side of the bearing to hold the shaft stationary in the bearing. The bearing is held right at the end of the copper pipe by splitting the pipe lengthwise 1/2" by 4 hacksaw cuts, the end of the copper pipe is then compressed onto the bearing by a hose clamp. See drawing, below.

Finally, a Dremel tool is used to cut a slot lengthwise in the copper pipe, to allow a small screw (labeled "pin" in the drawing) to be threaded into the plug to prevent the plug from rotating as the tune knob is rotated.

The plate load capacitor is constructed similarly. However, the movable plate is attached to the "N" output connector (instead of ground) with a thin copper strap.

After assembly, check the operation of the capacitors. If the plates of the tune capacitor are not parallel, or they are adjusted too close, it is possible to have an HV arc between the plates. This is usually not damaging, but quite unnerving for the operator. However, if an HV arc appears on the plate load capacitor side, the center of the coax will rise to the HV potential. This is to be avoided at all costs for safety, typically by employing a shorted stub harmonic filter right at the outlet "N" connector. It is possible to have RF arcs between the movable load plate and the chassis, and right at the outlet "N" connector if the circuitry comes too close to ground. These are very hard to find, as they do not leave the black mark that appears at the site of an HV DC arc. To prevent such, the nuts on the all-thread shaft are adjusted such that, when the shaft is turned, the teflon plug slides in and out of the copper pipe to place the capacitor plate the correct distance away from the corresponding anode plate. The moveable plate should not approach the anode plate any closer than 1/4" to prevent arcs - this separation is enforced both by the setting of the nuts and by the head of the nylon screw that attaches the movable plate to the teflon plug. On the opposite extreme, the movable plate should not come any closer than 1/4" to the amplifier chassis. Thus the moveable plate has approximately 1/2" to 3/4" of useable travel which is more than adequate to load and tune the amplifier.

Tube Socket Construction

The GS-23B socket is constructed by hand using copper sheeting and fingerstock. It is designed to ground the G1 ring to the copper base flange for both RF and DC voltages. The screen grid G2 ring floats above this G1 surface on a teflon ring which isolates the grid 2 ring from DC ground. This thin teflon ring does provide a few pF of capacitive coupling of RF to ground. It is necessary, however, to have a minimum of (approximately) 2000 pF of low-inductance coupling to ground. This was provided by carefully soldering six 470 pF 1 kV low-inductance chip capacitors (Mouser 5982-22-1000V470) between the G2 ring and ground. These are soldered to a copper flashing that is bolted to the ground plane to allow for easier soldering of the chips to ground. However, it is still very difficult to solder the chips to this flashing, as heat is quickly dissipated by the flashing. Resistance soldering was used to mount these chips to the flashing.

An SO-239 coax connector is used to connect to the heater pin and cathode rings below the G1 surface. The outer portion of the connector is mounted on a brass flange which is isolated from the G1 surface by nylon bolts (this flange must float at 47 VDC with respect to G1, and it will be driven by the tuned input circuit. A shortened 3/8" brass pipe nipple was used to bush the outside of the SO-239; small fingerstock was used to form the connection between this nipple and the cathode ring on the tube.

Lastly, a copper wire was resistance-soldered to the G2 ring to allow soldering RFC1 to the ring.

Anode Resonator Construction
resonators top clamp bottom clamp

Above see the pieces of the resonator assembly; at left the top assembly, at center the top resonator clamp and at right the bottom resonator clamp. Below see (left) a resonator installed on the podest with the bottom clamp and (right) the completed resonator assembly with top clamp, DC blocking capacitors, etc.

resonators   resonators

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