Mother Lode DX / Contest Club

Category: Tube of the Month

Norm N6JV’s Tube of the Month

  • Tube of the Month – May 2025

    by Norm Wilson N6JV – Visit the museum at N6JV.com

    In their build up after WWI,  the Royal Air Force developed a new triode transmitting tube that was capable of higher power than earlier types. The DET25 was a developmental tube similar to the “50-watters” being made in the US.  In 1925, the RAF was still flying open cockpit biplanes so the DET25 had the L4 base, with its long voltage breakdown paths, for use in wet applications. 

    DET25

    I have found two early RAF transmitters that used the DET25. The TR 4 is a single DET25 oscillator that has plug-ins to operate on the range 143 to 500 KHz or 3 to 15 MHz.  It uses either CW or interrupted CW (ICW) where the keying lines are fed through a motor driven wheel like a spark transmitter.  The TR 1091 was a MOPA design with the DET25 as the amplifier.  It operated in 1.2 to 1.5 MHz and 2 to 3.4 MHz using plug-in capacitors and coils.  It used the same CW/ICW keying system, but also had grid voice modulation.  This was a larger unit and required a larger aircraft.

    The DET25 has a published filament voltage of 7.5 volts but other sources give it as 6 or 8 volts.  Filament current is somewhere in the 1-to-2-amp range.  Plate voltage has a maximum of 1200 volts and a current given as 36 ma dissipation maximum.  The user needed to know the average of the mode they were using.  Fifteen WPM CW has an average of 45% depending on the fist.  Hand written notes indicate that in practice about 80 ma was used for a 60-to-70-watt output.

    The designation DET25 was changed to VT25 and then CV1025 (Common Valve) when the Inter-Service Technical Valve Committee in 1941 started their version of JAN numbers.  The British Army also used the designation AT35 for the VT25.

  • Tube of the Month – April 2025

    by Norm Wilson N6JV – Visit the museum at N6JV.com

    R3 – RECTOBULB

    In the July, 1927 issue of QST magazine, an advertisement was published featuring the “6EX Rectifier” by a company named the National Radio Tube Company of San Francisco, CA. The call 6EX was held by a Garrett Lewis. Rectifiers like the 280 and the 281 had become available by 1927 for receivers and low power transmitters, but no rectifiers that would handle the voltage and current required to power the larger transmitting tubes hams were wishing to use.  Arc rectifiers could be used but, they weren’t practical for most hams.  It was unusual to see a tube named alter a ham.  Hams would be assigned prefixes sometime in 1928.

    The 6EX was later designated the R-3 and started life as a high vacuum type, but the final product was mercury vapor with an indirectly heated cathode.  The maximum plate voltage was 7500 peak inverse volts at 250 ma.  The filament ran on 10 volts at 1.7 amps.  The tube had a standard UX base and a threaded stud out the top for the plate connection.

    I don’t know how well the tubes were selling, but Lewis had a ham friend in Southern California who wanted to replace the ARC rectifier he had sold him and build a new power supply. The friend was Don Wallace, 6AM, and he wanted to go big.  Lewis had also sold Wallace a re-built F328A which was a water cooled, 5 KW triode. 

    Don didn’t like QRP.  Don bought six 3 KVA pole pigs that were 220 volts in and 6000 volts out from the power company and mounted them in a rack with six of the new R-3 rectifiers.  The new power supply would be wired for six phase operation.  The schematic and rack photo are copied from QST magazine of February, 1928, where Don and Robert Kruse of the QST staff, wrote a 9-page description of the final power supply.  Six phase rectification results in an output ripple of six times the input ripple frequency.  In 1928, 360 Hz was considered a good CW note and adding a filter condenser would be very expensive.  The R-3 may have never been a commonly used tube as in a few years, the RCA 866 went into production and the R-3 became only a collectable.  Before WWII, Lewis moved to Silicon Valley where Lewis Electronics was formed and participated in tube production for the war effort.  In 1949 Lewis and Kaufman was organized and operated into 1956.  In about 1962, I bought two 3 KVA pole pigs from PG&E for $3 per KVA.

  • Tube of the Month – March 2025

    by Norm Wilson N6JV – Visit the museum at N6JV.com

    WD-24

    Westinghouse was one of the first American companies to commercially produce vacuum tubes. RCA held the triode patent so Westinghouse and General Electric made tubes for distribution by RCA.  These companies also developed experimental tubes for their own use and to develop new products and perfect construction techniques.  The featured transmitting triode was found at a swap meet.  I don’t think I paid much as nobody knew what it was other than an early, primitive triode that was probably made about 1920.  Eventually I identified it as a Westinghouse WD-24, but I have no reference source to prove it.  All tube collectors have a library of reference material and I think someone knew what it was and supplied the operating voltages and currents.  The plate could be run as high as 2000 volts at 250 ma.  The filament was 10 volts at 15 amps.  The tube stands 12 inches tall.  Westinghouse tubes had several prefix codes and often WD was used to indicate a developmental tube.

    This tube was made by spot welding rods to support the plate and grid.  The ends were connected to internal metal clamps.  In operation, the tube was mounted with straps.  This system is similar to what de Forest tubes of this period used.  The feature that doesn’t make any sense is the square box shaped plate.  Making the spacing between the grid and plate uniform, minimizes the creation of hot spots  The variable grid to plate spacing may result in an odd-looking set of performance curves.  This design didn’t catch on but it is an interesting example of early experimentation.