...The radios we'll always remember


 Posted By: Robert Nickels (W9RAN)
Posted: 12/352/2022

On the workbench 0 Comments 12/352/2022 

Signal need a boost? Build a 6DQ5 amp!


That's what vintage SSB enthusiast K5LYN did when he wanted more power from his Central Electronics station.  Since he has it set up for transceiving using the output of the SX-115 receiver to control the frequency of the 20A,  the station is dedicated to 75 meter operation anyhow, so a full bandswitched amplifier wasn't needed, and because the 20A can't provide the drive needed for a grounded grid linear, a grid-driven one would be required, and they're not very common.   So the easy solution was to build one!

Lynn had a rusty old Central Electronics cabinet and chassis avaialble which would be just the right size, and match the 20A.  The power supply would be external and the load and tune control locations were just right to be re-purposed.   Most of the unneeded panel holes were filled with red indicators that give a nice geometrical pattern no one realized was there.      He chose the 6DQ5 for good reason - they are in less demand thus less expensive than some of the other sweep tubes.    Most importantly, this "magic tube" was known to be a great choice as a linear amplifier in the early Swan transceivers.   As Herb Johnson knew, analysis conducted by W9RWZ and W9YRV that was presented in their QST article in October 1959 (below) showed that in class AB1 linear operation, a single 6DQ5 could produce "about twice as much power output as a 6146 at similar linearity and with similar place voltage".   As QST noted, the then-new 6DQ5 "has a lot to offer as an s.s.b. linear amplifier" and as K5LYN demonstrates, it still does today.     In addition to the Swan monobanders and the 240,  Hammarlund chose the 6DQ5 for the HX-50/HX-50A,  and Hallicrafters put one in the HT-40 novice rig and two in the HT-44 transmitter.

Lynn comments:  "The 4 6DQ5 amp is part of the 20A transceive station.  It is grid driven, built into the case of what was a rusty old CE10B, power supply is outboard (behind it), swamped grid input with about 500ohms, but also tuned input with the Yaesu antenna tuner."

This amplifier should be of interest to low power AM operators as well since it could be driven with a controlled-carrier rig like a DX-60 or HT-40 to produce similar PEP output, with a low resting carrier that would be within the 96 watts of combined plate dissipation (four tubes at 24 watts each).

Click on the image title or on the image itself to open the full-sized image in a separate window.

   

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