Monday, September 6, 2010

The Gridless Gammatron

The Gridless Gammatron (GG) was developed in the late 1920's by Ralph Heintz (an engineer/ham from Berkeley, CA), as a means to tell RCA's David Sarnoff and his army of lawyers to "go stuff it." Instead of the usual coaxial grid, Heintz' GG employed a control, or "gamma" electrode placed opposite the cathode and anode, such that it could modulate the cathode space-charge. These fire-bottles used a pure tungsten "bright" emitter along with tantalum anode and control electrodes.

Heintz threw in with a pal, Jack Kaufman, to form the H&K vacuum tube company. The early GG's were used primarily in the high-powered transmitters that made up the communication network for the Dollar Steamship Co. Pacific Ocean fleet (Dollar owned a sizable portion of H&K). However, Heintz was pleased to sell his GG's to hams, so long as Dollar's electron tube needs were met first. They certainly did the job, only, the GG electrode arrangement never amounted to more than a "poor cousin" to the conventional triode. H&K stopped making theses GG's as soon as the RCA "Patent Police" had been called to heel. BTW, H&K continued to call their conventional gridded tubes "Gammatrons," which can be confusing as not every Gammatron is of the gridless variety.

Searching for info on the unusual Russian sub-miniature vacuum tubes that I'd been using, I came upon this link. About halfway down the page I suddenly began reading about Gammatrons.

Well, you know how it is...now I'm off and running on Gridless Gammatrons. Right away I came upon mention of a series of articles that recently appeared in, The Tube Collector.

  • Volume 6, #2, April 2004, "The 6AX5 Gammatron Amplifier/Oscillator
  • Volume 8, #1, February 2006, Working Transmitter Using the H&K Gammatron Principle using 6X5WGT Rectifiers
  • Volume 10, #6, December 2008, Tests on the 6X4W Tubes for Gammatron Operation

While I haven't yet read any of these articles, I gather these guys have figured out that the internal geometry of certain full-wave rectifier vacuum tubes could be used to mimic the old H&K GG structure. It's not a perfect match; for example, H&K placed their control electrode much closer to the cathode, whereas the FW rectifier anodes are equi-distant from the cathode.

I unearthed a sleeve of 6X4 rectifiers from my subterranean parts storage vault. Having climbed back up to the workbench I donned a strong optical magnifier. Rats! the 6X4 FW rectifier uses a pair of coaxial anodes. A Gammatron needs a pair of parallel anodes. Back in the vault I noticed two "Tung-Sol" brand 6X4W rectifiers in my box of tubes. One look through my magnifiers told me that I had struck "Gamma Gold;" I had my parallal anodes.

The first circuit I tried was a variation on John Sousa's (author of the Russian valve link shown above) AM broadcast-band oscillator. I used a 455kHz transistor IF transformer. My reading told me that it was important to reduce the energy applied to the heater in order to create a more diffuse electron space-charge around the cathode (a saturated space-charge is more "opaque" to the electrostatic field created by the control electrode). I checked my connections and fired everything up.

Nothing...as usual. These sorts of experiments almost never work right off the bat (you have to suffer a bit first :o)

Trying different ideas and varying the various circuit parameters, I happened to switch off the control electrode B+ pull-up bias just before switching off the bias to the anode. For a fleeting instant I saw something that looked like a 30 degree segment of a sinusoid on the scope. Working backwards, I found that the multi-MegOhm bias resistor should be returned to ground instead of B+. Wow...there it was...a gorgeous sinusoid at 400kHz. My GG was alive!

I spent the remainder of the evening trying to fashion a LF regenerative receiver using my new GG circuit. The best I could do was an inefficient heterodyne direct-conversion receiver. I copied one Canadian LF beacon (YWM, Maniwaki, Quebec) as well as a couple of strong stations sending digital data. In retrospect, I think my problem had to do with the lack non-linearity required for efficient mixing/detection.

I did notice that my oscillator signal strength was an inverse function of the frequency; most likely due to excessive electrode capacitance. I figured that's the reason all the chatter I've seen concerning those The Tube Collector articles was centered around the AM broadcast band.

Consulting with the Great Feathered Oracle (my pillow) that night, I decided the problem must be the control-electrode to anode capacitance. At least for narrow-band work, I reasoned that an inductor, or the inclusion of a complete LC resonator, might be used to cancel/absorb this offending feedback path. To my knowledge, no one (at least recently) has used a GG in a quartz crystal-controlled oscillator. It seemed the Pierce configuration ought to have the best chance of absorbing the stray feedback capacitance.

I wired everything up and Lo!...the Pierce xtal oscillator worked just fine; at least at 507kHz. Since I was using an RFC as the anode load it was simple enough to slip in crystals of ever higher frequency.

1.2MHz, yup.

3.58MHz, check (getting excited now)

7.0MHz, still okay

14.0MHz, yes, believe it or not

28MHz, no dice, but not too disappointed!

I put the 40m rock back in and replaced the RFC for a link-coupled LC tank. Working into 50 Ohms, the highest output power seen was 1.7mW.

The next day I found that it was possible to raise the RF output to just over 3mW; this, with a DC plate current well within the published maximum ratings.

That's when things began to go South. I began to notice the output signal level was slowly fading. With hindsight, I ought to have pulled the plug right away. Instead, I fussed about in hopes of discovering what was happening. Well, I continued to fuss and as I did I watched the output signal gradually bleed away to nothing. In fact, my 6X4W was now Fin, Morte, Tot...

Here's what I think happened.

The 6X4W cathode is oxide coated. It's not a pure tungsten (bright) emitter such as was used in the original Gammatrons. In order to mimic Gammatron behavior it's necessary to run with a cathode temperature well below the specified rating. In fact, I'd been driving my 6X4W heater with only 4Vdc @ ~400mA in order to produce a more diffuse than normal electron space-charge at the cathode.

I'm uncertain whether the failure mechanism involved was "so-called" cathode stripping or cathode poisoning. However, I'm confident that it was the lack of proper space-charge protection that resulted in the permanent loss of cathode emission.

I have one 6X4W remaining in my junkbox. Once conditions on the 80m band have become more favorable for QRPp operation, my plan is to fire it up in the "Gammatron Mode" while holding the output power well below the maximum possible level. I think it might be best to run the 6X4W in this way just long enough to complete a pre-scheduled QSO or two; just enough to say that a Gridless Gammatron has once again ridden the ham bands.


Some further references on Gridless Gammatrons

http://www.tubecollectors.org/archives/HK255.pdf
http://www.tubecollectors.org/archives/hkprice36.pdf
Radiomuseum; Dual Plate Triodes Review

3 comments:

  1. I've had quite a bit of luck using a 5845 temperature limited diode as an oscillator in a Gammatron configuration. Worthwhile results were also obtained using the tube in a simple receiver. The pure tungsten filament was immune to cathode poisoning, in addition to providing ample light.

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  2. That's super...thanks for the tip, OM! Interesting tubes, those. FB on your GG receiver. Two 5845's are headed this way from Vacuum Tubes, Inc.

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  3. I have just put some Lewis Electronics JAN35T's on ebay 181113382595 and I have some H & K '24' Gammatrons in boxes to go on if the Lewis Electronics ones go.

    I just thought I'd mention it.


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