Swaping of anode cooler on the Thompson TH-308 type tubes

As some of the TH-308, TH-318, TH-328 and TH-338 tubes are very similar you may want to swap the anode cooler on a tube from a 750 W cooler to a 1200 W cooler.

You may want to remove the cooler in order to go for water cooling by putting on a specially made water cooler on your tube. Mats, KD5FZX, has done this on the GS23b tube, so it should be similar for the TH-type tubes.

This is perfectly OK to do. Here is how to do it:

You start with this:

TH338 swap cooler starting position

Start with removing the cover plate on top of the tube using a small knife and a pair of pliers. The captive screw can not be removed by itself, as it is designed only for tightening and not for removing (non reciprocal grip). Lift the plate gently by using the blade of the knife and insert the nose of the pliers and grip the plate gentle but firm. Slowly rotate the plate counter-clockwise and the screw will loosen. Remove the plate.

Remove cooler cover plate

You will now see the heads of three hex-wrench screws. Loosen them by
using a 2.5 mm hex wrench.

Loosen hex-screws

In order to remove the anode cooler (750 W) you can use a Gear and Wheel Puller. I used a three arm one with locking of the arms. Very handy.

Start by unscrewing the three hex screws for about 3 mm clearance to the cooler. Adjust the arms of the puller for a good grip on the anode cooler and lock them. Put a stiff plate on the heads of the screws (all screw heads must be on the same level) and tighten the puller center screw for contact with the stiff plate. The three hex-screws are now only attached to the tube body and will push the tube body away from the anode cooler. Now slowly continue to tighten the center screw of the puller until the anode cooler releases from the tube body.

Tools for removing the anode cooler Removing the anode cooler

Unscrew the three hex-screws completely. Anode cooler is now free for the desired swop.

Anode cooler free

Mount the desired cooler (1200 W) to the tube in question. Tighten the three hex-screws firmly, but do over tighten them so the threads get destroyed. Tighten them like you tighten the bolts on the wheels of your car, soft at first and do round after round with all three screws to get a proper tightening force. Put back the cover plate, but use a new screw that is reciprocal for easy future removal.

Put back the cover plate

Congratulation, you have now a higher power capability of your TH308, TH318 or TH328.


KD5FZX water cooling of GS23b: http://www.nd2x.net/kd5fzx-gs23H2O.html


[1] Care and feeding of POWER GRID TUBES, Laboratory staff , Varian, Eimac Division, 1967, Library of Congress No. 67-30070


Updated October 3rd, 2009.              http://www.2ingandlin.se/SM6FHZ.htm