Using a Siemens RW90 TWT With an ITT TWT Power Supply Greg McIntire, AA5C North Texas Microwave Society This article describes how to modify a Siemens RW89 or RW90 TWT for use with an ITT power supply. Surplus traveling wave tube amplifiers with 10-20 wan output are the power amplifier mainstay of many amateur microwave stations (fixed and portable). Siemens, ITT, Varian, and Thomson all built TWTAs to a common interface specification for Rockwell-Collins. I found myself in a situation where I had an ITT supply and a Siemens RW90 tube without the corresponding ITT tube or Siemens supply. A study of the tube pin-out and voltage ratings and the supply indicated a possible solution. The Rockwell-Collins specification fixed parameters such as the physical size, mounting, and power/alarm connector pin out. While the TWTAs all use the same Winchester connectors for connecting the tubes to the supplies, interchangability between different manufacturers TWT's and supplies was not required. Generally they all used the same pin numbers and tube element assignments are nearly the same. The main difference between the ITT and Siemens tubes and supplies is in Anode and Collector 2 pin assignments. These lines are swapped between ITT and Siem@ns and some interesting fireworks would likely result from a direct mating without modification. To make the modification, open the Winchester connector backshell on the TWT,(before connecting it to the power supply). Cut the orange and blue wires on pins 40 find 47 at mid point inside the connector backshell to allow for insulation. After stripping a small section of insulation on the four wire pieces, swap the lines and resolder each joint with at least double heatshrink over the joints since high voltages are involved. Replace the backshell on the connector and connect the tube to the supply. Now the Siemens tube has Grid 2 (Anode) connected to pin 40 and Collector 2 connected to pin 47 which-matches the ITT supply. Adjustment of the cathode voltage is also required since the RW90 tube operates with 2,550 volts on the cathode while the II7 tube requires -2,950 volts. (When making the following adjustment, take the necessary precautions and came to protect yourself and your equipment since you are dealing with lethal voltages. This author assumes no responsibility for your well being.) Take the top and bottom-covers off the supply. RV2 on the bottom side adjusts the cathode (helix) voltage. Stand the supply on end so that you can simulta- neously access top and bottom sides. The post on the top side of the bottom board where the wires connecting the lower supply -board to the heater regulator makes a convenient test point to measure the cathode voltag Apply -24 VDC to the supply and let the supply complete the warm up cycle. Set the STBY/TRANS switch to TRANS. Then, while monitoring the cathode voltage with a,,.high voltage probe, adjust RV2 to bring the cathode supply to -2,550 volts. Disconnect power and replace the top covers. You should now be raedy to operate the set as aTWTA. Other combinations of TWTs and supplies are possible. I plan to study a Thomson supply. Also, although not part of the TWTA family used by Rockwell, I've had good luck with Varian tubes in Hughes 1177/1 277 supplies. Additional results will be the subject of future articles.