Plisch TH308 cavity modifications:

I use a solid state driver for the TH327 cavity, delivering 300 watt to the final amplifier. Initially i was intending to use the TH308 to create this power level. We modified the Plisch TH308 / TH328 / TH338 / TH339 ... cavity for use on 23 cm. Other people where asking tips for this modifications, so i decided to re-list this page on the website.

Peter, OE9PMJ / OE9XXI published a paper concerning the Plisch TH308 cavity modifiations :

The cavity extentions needs some extra mechanical scills. Mark, ON5OT modified both our cavities ( ON4BCB / ON7UN ) using a solid brass piece, since we could not find the 100 mm od pipe. The cavity got extended by 30 mm.

On the left you see the original cavity with the output filter removed and the raw material ready to be machiened, on the right, is the modified TH308 cavity amplifier.

The cavity got cold tested, and was tuned to 1296 MHz. After a while when i was going to use the amplifier, i noticed the tuning of the amplifier was very "jumpy" , so the inside of the brass part needs to have some kind of silver plating. I believe it is not nessesary to have a real thick plating, just with "rub on silver" should be OK. The fingerstock of the plunger needs a good RF ground to the side of the plunger.

The original output filter got removed, since we do not need this part. The cavity needs a new output coupling. The place where the output coupling is made, is not very important. In Peter's design, the output coupling is made at 50 mm from the bottom of the old cavity, mine has been made at 50 mm from the top. Both setups seems to work fine.

The differences between the different Thomson tubes, 308 / 328 / 338 / 339 has to do with the DC and AC voltages. TH339 has a larger cooler though. Special attentions needs to be made to the heater voltages. I burned a transformer when i used a different tube in the amplifer.

I believe a good and stable 300 watt amplifier can be made using TH308 or TH328 tubes. Expect around 10 dB of gain when you find a good tube. Some people squeeze up to 700 watt out of this tube, but i believe you need a good stock of tubes, since commercially this tube has been designed for 50 - 100 watt of class A continious duty cycle amp.