Before coming to the USA, through G4HUP I obtained a pair of TH338 tubes. Going to microwave meetings in the North East I often saw KB2AH who had a TH338 cavity, but didnt want to sell it, even though he didnt have a tube! A few years later at a Packrats conference in PA he finally decided to sell it; Meg bought it for me for an aniversary present. It was put aside for future EME usage

Mar 2005

Found a blank chassis at the Weatherford hamfest to mount the amplifier..

Dec 2005

Built G3SEK triode control board. Modified it for the high bias voltage needed by this tube.

May 2006

Mounted cavity on chassis. Put G3SEK board in box. Found heater and relay transformer. Found the box with the 110V AC Vortex blower with Vacuum cleaner hose and adaptors, bought in London in 1986; its last action was blowing an OZ9CR 2 tube amplifier on 13cm . The biggest problem was how to force air from the blower through the TH338 anode fins. Visited K5GW who found some 3" aluminium bar to make cowling.

Nov 2006

Using a hacksaw and after tapping a lot of holes and a weekend of work had a cowling. Needed a PTFE chimney, but none of the sheet I had was thick enough. Also after running the heaters on the tube to set the heater voltage, it became obvious that air needed to be blown on the Cathode of the tube at all times. However no suitable snail blower could be found.

Mar 2007

Mentioning my missing items to K1RQG he shipped me some thicker PTFE sheet from which I made a chimney. He also found me a motorola snail blower which with a plumbing fitting i managed to force air into the cathode tube.All I needed was an EHT Transformer. 

Apr 2007

Thought I had find the ideal eht transformer from Ameritron.  Their catalogue listed a "1500V 0.5A secondary" transformer, so i ordered one. BEWARE! Subsequently I found the the manual for the amplifier it was intended for, from which I found out that the 1500V 0.5A is the full wave bridge rectified DC  output that can be produced not the secondary AC voltage. So with the KH7X expedition approaaching I searched the garage and found my old 2x4CX250B PSU (used in expeditions to 12 grids). In there were two 400-0-400V at 400mA transformers. So I recycled the chassis and removed all but the transformers and built a power supply with soft start and remote activation. This gives me 1700V or 2400V off load. 

Commisioned amplifier.Had 2 tubes. Set heater volts on 1st to 5.7V. When EHT applied the supply tripped after taking 0.9A, obviously travelling between 2 continents for 15 years had caused it's demise. So I substituted my 2nd (and last) tube. Applying EHT took no current. Good. With 31V bias I set 0.1A standing current as suggested by OK1DFC. 20W input gave 400W output taking 0.8A at 2100V on load.. I could get 500W but  the lighter loading needed caused instability issues and the Anode current trip to activate.. 

Happy, I left the PSU running as a soak test. Two hours later I came back to find the EHT had tripped. Reapplying the EHT it took a lot of anode current and tripped again. Found out the PTFE anode insulator had punched through leaving soot. Made another insulator. Applied EHT it also punched through immediately. Looked at HB9BBD web site  he comments "The plate capacitor should be built with Kapton of 0.12mm thickness.Avoid teflon because it is not mechanically stable". I think I know why now. Found W4TJ had some kapton which he mailed me. Replaced the anode capacitor, stopped arcing at decoupling capacitor, but now the tube internally arcs if i put more than 1700v off load on it

Jan 2008

It was suggested that I should run the tube with EHT for an extended period and not bias it up. I ran it this way for 48 Hours. I then increased the EHT to 2400V and it didnt arc. After some tweaking it now gives 450W output for 20W input. 

Due to the low efficiency it was found necessary to have a switch to put the big blower on all the time, not just on transmit. The only problem with this is the amount of noise in the shack, but better to protect the tube. 

The following shows the amplifier install.. The PSU on the left is the one for the TH338, the one on the right is for the 2 x 2C39 ammplifier

This page last updated Mar 22 2008