Single GS-15B 23cm PA
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Construction Details

Click on pictures to enlarge
NOTE: The reddish "stuff" seen in some pictures is the Kapton insulation.
1xgs15b PA dwg
Single GS-15B PA
1xgs15b PA dwg
Single GS-15B PA insulators

Anode Cavity

    OD   =   94mm (Not critical)
    ID   =   78mm
    Height   =   21mm
    Material   =   Brass, Copper or aluminum.

    Drill and tap 8 holes per side for attaching top and bottom plates. I used M3 X 10mm screws. The sleeve for the output coupler is a slip coupling for 5/8 OD copper pipe. Actual OD of the coupler is 0.6985. The cavity ring was heated to 350-400 deg F and the coupler sleeve pressed into the cavity. Cut 2 slots in the sleeve to allow claming of the output coupler.

Anode Cavity
Top Plate

    OD   =   94mm - (3.700) - (Not critical)
    Thickness   =   3.2mm - (0.125)
    Center hole   =   38mm - (1.500)
    Plate ring holes circle   =   51mm - (2.000)
    Mounting holes   =   3.2mm - (0.125)
    Plate ring threads   =   M3 - (Not critical)

Anode Cavity

    OD   =   65mm - (2.56) - (Not critical)
    ID   =   26.4mm - (1.040) - (To match finger-stock)
    Thickness   =   3.2mm - (0.125)
    Mounting holes   =   8mm - (0.3125) - (For Teflon insulators)

    Top picture is of anode decoupling capacitor plate. Second picture shows kapton dielectric and teflon hardware insulation. Third picture shows anode decoupling capacitor assembled on anode cavity top plate. Bottom picture shows actual measurement of the value of the anode decoupling capacitor in pF.

Anode Cavity
Bottom Plate

    OD   =   94mm - (3.700) - (Not critical)
    Thickness   =   3.2mm - (0.125)
    Center hole   =   42mm - (1.654)
    Grid ring hole circle   =   51mm - (2.000)
    Mounting holes   =   3.2mm - (0.125)
    Grid ring threads   =   M3 - (Not critical)

    The tuner sleeve is made from a long - 3/8 diameter brass rod that is drilled and tapped - 20. Solder the sleeve in place and keep it flush with the inside surface of the cavity.

Screen Ring
Anode Cavity
Bottom Plate

    Height   =   13.8mm - (0.543)
    OD   =   42mm - (1.654)
    ID   =   39.5mm - (1.556)

    Solder the finger-stock in the top of the sleeve. Solder the sleeve to the bottom plate. The ring should be flush with the bottom side and extend upward into the anode cavity.

Grid Ring

    OD   =   65mm - (2.56)
    ID   =   32mm - (1.260) - (To match grid sleeve)
    Thickness   =   3.2mm - (0.125)
    Mounting hole circle   =   51mm - (2.000)
    Mounting holes   =   6mm - (0.250) - (For Teflon insulators)

    The grid ring has a 14mm (5/8) cut-out to clear the anode cavity tuner sleeve.

Grid Sleeve

    OD   =   32mm - (1.260)
    ID top 4mm  =   31mm - (1.220)
    ID bottom 4mm  =   27mm - (1.063)
    Height   =   8mm - (0.316)

    The sleeve has a step 4mm from the bottom to act as a mechanical stop for the tube socket. Solder the finger-stock in the top (larger end) of the sleeve and solder the sleeve to the grid de-coupling ring, flush with the bottom of the ring.

Grid decoupling
capacitor Insulators

    The kapton dielectric and teflon hardware insulation used to install the grid ring/sleeve & input resonator assembly to the bottom of the anode cavity.

Input Resonator

    OD   =   37.5mm - (1.478)
    ID   =   35mm - (1.380)
    Length   =   46mm - (1.810)

    The resonator is made from a standard 1-1/4 copper pipe coupler, available in any hardware store.

Input Resonator

    Center the input resonator on bottom side of the grid ring and solder in place. Drill a few holes in the resonator, close to the grid ring, to allow for air circulation.

Input Cavity

    OD   =   75mm - (3.000)
    ID   =   70mm - (2.750)
    Length   =   60mm - (2.360)

  • The input cavity is standard 3 brass tubing with 1/8 wall.
  • Drill 8-12 holes, 1/4" - 5/16" diameter, close to the plate cavity end to promote air-flow.
  • Input coupler and tuner sleeves are the same as the plate cavity output coupler.
  • The centers of the sleeves are located 40mm (1.575) from the plate cavity end.
  • Push these through the wall and solder in place.
  • Drill and tap 8 - M2 holes for mounting of the bottom plate.
  • Drill a hole 10mm (0.400) from the plate cavity end. This hole is for a grid voltage feed through capacitor.

Input Cavity Top


    Center the input cavity on the plate cavity bottom plate and solder in place.


    Install the Kapton insulator, Teflon shoulder insulators and the grid ring with input resonator to the bottom plate. The Kapton insulator should be >2mm oversize in all directions to prevent and grid - screen arc. Voltage differential between the grid and screen ring will be 350-400V.
    Install a >400V rated 22-470pF feed-through capacitor for the grid voltage and connect to the grid ring
    The bottom picture shows the value of the grid capacitor in pF.

Input Coupler

    The coupler is fabricated from a piece of copper pipe (0.625 OD) with a brass rod as center conductor. The disc in the end is a 15mm (0.600) brass washer. Glue a piece of Kapton or Teflon sheet to the end of the disc to prevent accidental short between ground on grid.

Input Tuner

    The input tuner is fabricated from a 2-1/2 long -20 brass screw with a 15mm (0.600) brass washer in the end. The holder is fabricated from a piece of 5/8 copper or brass rod.
    The holder is threaded in the disc end and hollow in the other end to allow space for a spring. The cap is threaded in the outer end and hollow in the opposite end to slip over the holder. This will pre-load the tuner threads and give a smooth tuning. Glue a piece of Kapton or Teflon sheet to the end of the disc to prevent any short between ground on grid.

Input Cavity
Tuner & Coupler

    Install the input coupler and tuner and secure with #6 hose clamps. Initial setting is about 4mm from the input resonator for both the coupler and tuner.

Output Coupler

    The output coupler line section in made the same way as the input coupler section except for the disc. The loop is made from a 6mm (1/4) wide 1mm (0.040) thick copper strip. Bend the strip to the shape on the drawing and solder in place. Insert the coupler in the cavity until there is 2mm clearance between the screen ring and loop. It is a good idea to solder a short piece of tubing to the coupler to act as a stop. See picture. Secure with a #6 hose clamp.
    The output cavity tuner is made from a 5 long -20 brass screw with a 15mm (0.600) disk in the end. I recommend gluing a piece of Kapton to the disc to prevent any arcing. Install the output tuner screw.

Cathode Resonator

    The cathode resonator is made from a 55mm (2.170) length of 1 brass tubing machined down to 22.65mm (0.8915). Cut 6 - 20mm (0.75) long slots from the tube end to allow for compression. Drill 6 holes, 6mm (1/4) diameter to promote air-flow. See Cathode decoupling explaination, below.

    Install an 8mm wide brass strip across the bottom of the resonator, outside surface flush with end of tubing. Solder in place. Drill a 4.5mm hole in the center for heater connector mounting.

Cathode Resonator
& Decoupler

    Cathode de-coupling ring dimensions.
    OD   =   45mm - (1.775)
    ID   =   22.65mm - (0.8915)
    Thickness   =   3.2mm - (0.125)
    Mounting hole circle   =   35mm - (1.375)
    Mounting holes   =   M3

    Solder the cathode resonator into the de-coupling ring. Make sure the bottom side is flush.

Heater Connector

    The heater connector is made from a 50mm (1.9675) length of 9mm (0.350) brass tubing. The tube end is drilled to 7.8mm (0.3075) inside diameter, 10mm depth (0.400). Cut 4 slots - 15mm (0.600) long to allow for expansion when pushed on the tube.

    Solder an M3 nut inside the tube in the opposite end. This will be used for mounting. See "Cathode Resonator Assy & installation", below.

Input Cavity
Bottom Plate

    OD   =   94mm - (3.700) - (Not critical, round or square OK)
    Thickness   =   3.2mm - (0.125)
    Center hole   =   20mm - (0.750)
    Resonator ring holes circle   35mm - (1.375)
    Resonator mounting holes   6.2mm - (1/4)
    Plate mounting holes   2.2mm - (0.085)
    Cooling holes circle   62mm - (2.450)
    Cooling holes diameter   6.2mm - (1/4)
    Plate tuner hole   9mm - (0.350)

    The center hole is for access to the heater terminal. The holes on the inner circle are for mounting the cathode resonator. The next circle of holes is ventilation. The outer circle holes are for mounting to the input cavity.
    The larger hole on the left side is for the plate tuner. The 4 corner holes are for mounting the finished amp.

Cathode Resonator
assy & installation



  • Mount the completed cathode resonator to the bottom plate with Kapton insulator on the inside and Teflon shoulder insulators on the outside. Cut the Kapton insulator > 2mm oversize to prevent arcing. Voltage across these surfaces will be 350-400V.
  • Second picture shows the value of the input resonator decoupling capacitor in pF.


  • Install the heater connector inside the cathode resonator with insulators on all sides. The differential voltage between the cathode resonator and heater tube is only 6VAC so arcing is not an issue.

  • Put a spring and nut on the output tuner screw to pre-load the threads and get smooth tuning.
    Mount a solder eyelet to the heater screw and one of the cathode resonator screws.

Top View

Side View

Two PAs, one with tube installed, made following the details shown above. Proof of the pudding, so to speak!!

Water Cooled GS-23b

Description & pictures of a water cooling system.

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Last Updated: 21 December 2002
Feedback: Paul S. Goble, III, ND2X
Copyright © 2002 by ND2X and KD5FZX, all rights reserved