GS-23b 23cm Amplifier
by
KD5FZX
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GS23B / 4CX1600U 1KW PA for 23cm
Pictures and Construction Details

Pictures of the parts of the amplifier are shown below; click on each picture in turn for zoom except for the tuner body and input coupler which are shown full size.




All metal pieces before assembly.




Completed amp with water cooled tube.


All sheet insulators are made from 0.12mm - 0.005” Kapton. Teflon or Ultem are also OK.

Teflon insulators go between cathode and grid sleeves (blue insulators in mechanical drawing).


The heater connector is made from brass rod with a center-sleeve from an SO239 connector in the tube end. The other end is drilled and tapped for any screw of choice. The large end is a capacitor needed to make sure that the heater is not getting any extra power from RF. The large sleeve is only connected in the end closest to the tube by using a brass or copper strip to allow for air-flow. See picture of teflon insulators, above. Dimensions for the heater connector:

    Rod diameter = 6.35mm (0.25”)
    Total length = 145mm (5.70”)
    Large end length = 25mm (1.00”)
    Large end diameter = 18.5mm (0.728”)
    Distance from top of connector to top of sleeve 18mm (0.70”)

The cathode tube is machined in one end to match available finger-stock (in my case 22mm). A hole is drilled and tapped for a short brass screw used to prevent the sleeve from moving (see mechanical drawing left side). 4 holes are drilled below the finger-stock to allow for air-flow. Arrows on mechanical drawing indicates air-flow. The 0.12mm - 0.005” Kapton sheet is wrapped around the heater connector sleeve before assembly. Dimensions for the cathode tube:

    Tube OD = 22mm (0.875”)
    Tube ID = 19mm (0.75”)
    Total length = 155mm (6.10”)
    Diameter of cooling holes = 6.35mm (0.25”)

Top view of cathode tube assembled. Here the Kapton insulator can be seen between the sleeves. There is only 6 volt difference between the sleeves so a small gap in the Kapton wrap is ok.


Bottom view of cathode tube assembled. Solder a brass or copper strip across the bottom of the tube, narrow enough to allow for air-flow. Drill a hole in the center for the heater connector. Install fiber or Teflon insulator between the 2 pieces. See mechanical drawing. Drill and tap a small hole in the bottom for a solder lug.

Per picture, top line above, fabricate 2 Teflon rings to fit between the cathode and grid sleeve. Drill 2 holes from the side across from each other. One hole is used to secure the ring to the cathode sleeve with a brass screw (left side of mechanical drawing) and the other hole is used to secure the ring to the grid sleeve with a flat head brass screw (right side of mechanical drawing).



Grid sleeve. The top of the sleeve is machined to clear the screen connection ring on the tube. Finger-stock is soldered to the inside flush with the top. 8 holes are drilled below the finger-stock for air-flow. A groove is cut along the full length to allow the Kapton insulator sheet to overlap. Voltage difference between the grid and screen sleeves is 540V.


Here the groove for insulator overlap can be seen on the grid tube prior to silver plating. The hole seen inside is used to secure the grid sleeve to the screen sleeve insulator. See mechanical drawing above the output probe. Drill and tap a small hole in the bottom for a solder tab. Again, see mechanical drawing. Dimensions for the grid tube:

    Tube OD = 50.4mm (1.98”)
    Tube ID = 38.6mm (1.52”)
    Tube length = 100mm (4.00”)
    Groove width = 6mm (0.25”)
    Groove depth = 3mm (0.125”)
    Diameter in top = 41mm (1.61”)
    Length of reduced diameter = 15mm (0.60”)
    Diameter of cooling holes = 6.35mm (0.25”)




Put the heater connector on the GS23B and push in until bottomed out. Slide the grid tube over the cathode sleeve onto the GS23’s grid-ring. Measure the distance to the center of the bottom Teflon insulator. Mark and drill a hole in the grid sleeve at this point. Counter sink the hole so a flathead brass screw is below the surface. Tap the Teflon ring and install a short brass screw. Make sure that you have safe clearance for 500 volt between the end of the screw and the cathode sleeve.


Machine the top of the screen sleeve to match available finger-stock. Drill holes for input probe and tuning probe. Determine hole location by pushing all sleeves onto the GS23B and measure the distance to the end of the cathode sleeve from the bottom of the screen sleeve and add 8mm. The probe sleeves are made from ½” copper pipe repair sleeves. Solder the sleeves in place and cut 2 slots in each sleeve to allow for compression. Drill 3 holes for feed-through caps rated for minimum 500 volt. Drill a 12mm hole 50mm from the top of the screen sleeve. This hole is used to secure the grid-sleeve to the screen-sleeve with a flathead brass-screw and a Teflon insulator. Drill 8 cooling holes just below the finger-stock. Fabricate a flange to match a small but powerful fan and solder to the bottom.

Fabricate the anode cavity bottom plate from brass or copper plate and drill 8 holes equally spaced to match the anode cavity sleeve hole pattern. Machine the center hole to match the screen sleeve. Drill 8 cooling holes in the cavity bottom plate. The extra sleeve ring seen in the picture below the cavity bottom is not needed; it was used for initial adjustment of the prototype assembly. Position the TOP of the cavity bottom plate 100mm from the top of the screen sleeve and solder in place. Push all sleeves onto the GS23B and put a mark on the grid sleeve in the center of the 12mm hole previously drilled in the screen sleeve. Remove the grid sleeve and drill and tap for an 8-32 screw where marked. Remove any burrs in the holes.

Wrap a piece of 0.12mm - 0.005” Kapton around the grid sleeve and trim to extend 5mm extra in each end. Allow to overlap in the groove. Mark and cut a hole in the Kapton the same size as the screw-hole just drilled. Insert the grid sleeve and Kapton wrap to match the center of the 12mm hole in the screen sleeve. Fabricate a Teflon ring with 12mm OD and 5mm ID. Push the ring into the 12mm hole and insert an 8-32 flat-head short brass screw. Tighten the screw until it bottoms out to compress the Teflon. This screw and Teflon insulator as well as the Kapton insulator can be seen on the picture to the right.
Remove all metal shavings and cutting residue. Dimensions for the screen tube:

    Tube OD = 57.4mm ( 2.25”)
    Tube ID = 50.7mm (2.00”)
    Tube length = 185mm (7.25”)
    Diameter of cooling holes = 6.35mm (0.25”)

    Cavity bottom plate thickness = 3mm (0.125”)
    Cavity bottom plate OD = 89mm (3.50”)
    Cavity bottom plate Center hole = 57.4mm (2.25”)
    Diameter of cooling holes = 6.35mm (0.25”)

    Fan flange thickness = 3mm (0.125”)
    Fan flange OD = To match a small powerful fan. I used 60mm
    Fan flange ID = 50.7mm (2.00”)

Input and tuning sleeves and made from ‘ ½” copper pipe repair sleeves’ found at the hardware store. These are also called ‘ ½” slip connectors with no stop’. These sleeves have an OD of 17.8mm and ID 15.8mm. The same type is used for the cavity in picture 13.





Install feedthrough caps and chokes. A straight piece of wire or air wound choke is just fine for the grid sleeve connection.


Anode cavity installed: Distance from bottom of cavity to the centerline of tuner and coupling-probes are 47mm. Tuner and coupling sleeves are made from ½” copper pipe repair sleeves with an OD of 17.8mm and ID 15.8mm. These are soldered to the cavity and slotted to allow for compression with a hose clamp. Top and bottom has 8 screw holes at equal spacing. Drill and tap for screws of your choice (4mm or 8-32).

    Cavity dimensions:
      OD = 89mm (3.50”)
      ID = 76mm (3.00”)
      Length = 114mm (4.50”)
      Bottom to Tuner - Coupler centerline = 47mm (1.85”)

Drill 8 holes to match the cavity’s screw pattern. Countersink the holes so that flat-head screws are BELOW the top of the plate. Drill and tap 4 holes equally spaced in circle with 100mm diameter. Use screws of your choice (4mm or 8-32).

    Plate dimensions:
      Diameter = 150mm (6.00”)
      Center hole = 71.5mm (2.81”)
      Thickness = 3mm (0.125”)
      De-coupling plate hole circle = 100mm (4.00”)

Drill 4 holes with a diameter of 10mm to match the pattern in the top plate. Fabricate 4 Teflon insulators. See mechanical drawing. The center-hole diameter should be adjusted to match available finger-stock to get a snug fit for the tube. Solder the finger-stock in place flush with the bottom of the ring. The extra support ring for the finger-stock is optional. Drill and tap a small hole for a solder tab.

    Ring dimensions:
      Diameter = 120mm (4.75”)
      Center hole = 70mm (2.75”) To match finger-stock.
      Thickness = 3mm (0.125”)
      Kapton insulator thickness = 0.12mm (0.005”)
      Insulator diameter = 130mm (5.125”)
      Center hole = 63mm (2.50”)


The tuner body is made from a solid brass or copper rod. The tuner screw is made from a Ό-20 brass screw with the head ground down to a flat surface. Drill a hole to match a Ό-20 tap all the way through the body.Enlarge the hole 2/3 through the body to allow for a spring. Tap the hole. The recess seen on the cavity side is optional. Insert the screw from one side and the spring from the other side. Compress the spring and mount the knob. A washer with a small amount of grease between the spring and knob helps to get a smooth action.

Sand the end of the screw and sand a scrap piece of Kapton. Glue the Kapton to the flat end of the tuner with epoxy. Trim the Kapton to match the hole in the cavity. This is only needed for the input tuner since there is 500 volt difference between ground and the cathode tube. Note the OD below. This is because ½” pipe is actually 5/8” OD. Picture to right is tuner body prototype assembly prior to silver plating body and brass screw.

Tuner dimensions:

    Body OD = 15.8mm (0.625”)
    Body length = 35mm (1.40”) not critical
    Tuner screw diameter = 6.35mm (0.25”)
    Tuner screw length = 50mm (2.00”)


The input coupler center conductor is a solid piece of brass rod. The end is a separate piece soldered onto the rod. Drill a hole in one end to match the connector center pin. The tube is ½” copper pipe. Machine an N-Type chassis connector to slip inside the tube. Fabricate a short Teflon support (not on the picture) to hold the center conductor in place. Fabricate the end disk from brass or copper and drill a Ό” hole through it. Solder the center conductor to the connector pin. Slip the tube onto the machined part of the connector and solder it. Wash out any flux. Push the Teflon support in place. Solder the end disk in place.

Sand the end of the disk and sand a scrap piece of Kapton. Glue the Kapton to the disk. Trim the Kapton to match the hole in the cavity. This is only needed for the input coupler since there is 500 volt difference between ground and the cathode tube.

    Input coupler dimensions.
      Center conductor diameter = 6.35mm (0.25”)
      Center conductor length = 50mm (2.00”)
      Tube ID = 14.6mm (0.574”) standard ½” copper pipe!
      Tube OD = 15.8mm (0.625”)
      Tube length = 55mm (2.20”)
      Disk diameter = 15mm (0.60”)
      Disk thickness = 3mm (0.125”)


The center conductor is a solid piece of brass rod. It is tapered to act as a 20 - 50 ohm transition. The center conductor has a total length of 60mm (1/4 wavelength) and tapers from 10 to 6.35mm. Drill a hole in the small end to match the connector center pin. The tube is ½” copper pipe. Machine an N-Type chassis connector to slip inside the tube. Fabricate a short Teflon support with a shoulder to hold the center conductor in place. Solder the center conductor to the connector pin. Slip the tube onto the machined part of the connector and solder it. Wash out any flux. Push the Teflon support in place.

    Output coupler dimensions.
      Center conductor small end = 6.35mm (0.25”)
      Center conductor large end = 10mm (0.40”)
      Center conductor length = 60mm (2.35”)
      Tube ID = 14.6mm (0.574”) standard ½” copper pipe!
      Tube OD = 15.8mm (0.625”)
      Tube length = 60mm (2.35”)



Mount a powerful 60mm computer fan to the bottom flange with some aluminum bug screen in between.


Notes.

  • Brake cleaner from the Auto parts store works excellent as Rosin flux remover.
  • I highly recommend a slightly modified version of G3SEK’s tetrode board to provide tube protection.
  • The modification is for higher screen voltage and higher control grid current before protection trip.
  • The amp will produce 750W with zero control grid current.
  • The amp will produce 1000W with 30mA control grid current.
  • Most NOS GS23B’s that you find surplus are manufactured 10-15 years ago so read and follow the recommended startup procedures on ND2X website to prevent any destructive plate-screen flash-over.
  • Overheated tubes can release gas that causes flash-over so use GOOD COOLING.
  • Read the document at http://www.svetlana.com/docs/TechBulletins/technoteNo54.html before deciding to build this amp.
  • Your local machine shop can tell you where to get the metal sleeves and plates needed for this project. The total material cost was $58 at the local metal supplier McMurray Metals in Dallas, TX. They will sell any length and you pay by weight.


    Water Cooled GS-23b

    Description & pictures of a water cooling system.


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    Last Updated: 20 April 2002
    Feedback: Paul S. Goble, III, ND2X
    Copyright © 2001 & 2002 by ND2X and KD5FZX, all rights reserved