Dual 833C Triode
Linear Amplifier

Experimental, unique swamped-grid parallel-tube legal limit amplifier designed and built by W0BTU

  • Two 833C triodes in parallel. The 833C is the graphite plate version of the 833A; the only difference is the plate dissipation rating. 833A tube data sheet
  • Grid driven, class AB2. 1:4 (50 to 200 ohms) broadband ferrite-core step-up transformer develops drive voltage across 200 ohm non-inductive (of course) grid-to-ground swamping resistor. *
  • 100 watts drive and 20 mA total grid current, 1500 watts output on 160m. Total plate current is between 650 and 800 mA.
  • Plate voltage is adjustable up to to ~4500 V no load; presently using it at about 3500 V under load.
  • Zener-regulated grid bias supply, adjustable in steps up to about -100v.
  • Experimental, was built at very low cost with parts I had laying around. Currently only using it on 160 meters, but it has been operated on 80 and 40 at reduced output.
  • Unconditionally stable*. No parasitics or self-oscillation whatsoever, even without neutralization, parasitic chokes, or proper shielding between the input and output circuits. I believe this is at least partially due to the layout using the shortest possible leads made of wide copper straps, not to mention the swamped grid.
    updated content* Changed the input transformer ratio to 1:6.25 and the grid swamping resistor string from 200 to 250 ohms to get more output on 75. (With 300 ohms --which I used on 160m-- the amp oscillated on 75 and 80m.)
    Plans are to try neutralizing it using a third winding on a new input transformer.
833 833A 833C tube
833 tube outline

 Visit www.w0btu.com/files/misc/833C_linear_amplifier for more photos

Dual 833C linear amplifier

It took awhile to build this project, once I finally decided to use the 833C tubes instead of the NOS Russian GS-35Bs I have. Since I built it during a period when I had little extra money to spend, I spent more time figuring out how to build the amp with parts that I already had laying around here than actual construction time.

But I think that made the project more satisfying. I know I wouldn't have felt the deep satisfaction I do when I use this amp had I been able to afford a commercially built linear.

I don't know of anyone who has ever built an 833 linear amplifier similar to this one. I was inspired to use 833s from a construction article in the 1980 ARRL Handbook, but I had to make a lot of changes from that article to make it work right. The 50 ohm swamping resistor just didn't work, for one thing, plus the plate Z was much different because I used two 833Cs in parallel.

There were a few things that I wish I had done differently, but in the end, I was really surprised at just how well this amplifier works. No trace of instability or any blower noise (or smoke).

If you build an amp, for heaven's sake don't use the same cheap meters that I did. They are cheap Shurite meters with no damping, and they jump all over the place, making it almost impossible read the exact grid or plate current. They would be fine for monitoring a constant current, but NOT a varying one. They are almost tolerable amplifying a RTTY or AM signal, but in either SSB or CW those meters jump from minimum to maximum scale like a couple of windshield wipers gone totally berserk. I use the Westinghouse plate current meter on the panel below the amp (drilled by another ham before I got it.) The Shurite meters are better than nothing, that's about all I can say.

Surprisingly, the Dow-Key relays (which I already had from a previous 144 MHz amp project) switch fast enough to show power output even when sending a single dit at 70 WPM. But fast vacuum relays would certainly be better, especially if they were acoustically isolated from the metal chassis. Maybe someday. The external contacts on those relays are connected in series, and jumper the cathode-to-ground resistor to energize the tubes.

I never drew any schematics except the bias supply and a few control circuits, etc. Maybe I'll draw one and post it here someday. But you can figure out most of what I did from the photos, should you want to build one similar to this one.

See you on 160 meters. :-)

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  Last Edited May 1, 2014


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