Input Admittance of Receiving Tubes
Input Admittance of Receiving Tubes
Fellow Radiophiles,
One of the least documented parameters in RF receiving tube data sheets is the input Admittance (Admittance=I/V=Conductance+jSusceptance, Admittance=1/Impedance, Impedance=Resistance+jReactance) at the control grid g1, that loads the driving circuit. The driving circuit can be the antenna, or a tuned tank circuit. The portion of the the input Admittance, that cannot be tuned out with L or C elements in the input circuit, is the Conductance=1/Resistance at the control grid g1.
Input Conductance describes a loss at the control grid g1 and is primarily caused the lossy movement of the space charges located between the control grid and the cathode. This lossy movement of charges is worst for tubes with slow transit times. The classic way to reduce this effect is to bring the control grid as close as possible to the cathode to minimize the total space charge under the grid.
This loss at the input grid establishes the maximum power gain that can be realized by a particular stage, and also establishes how much of the precious antenna signal is wasted in this loading effect with a direct penalty in SNR (Signal-to-Noise Ratio) performance. This input Conductance is proportional to the square of the operating Frequency, so it is much worse at higher RF freqencies, than lower RF.
The tube department at RCA released "Input Admittance of Receiving Tubes" in November of 1946 with a thorough survey of input Admittance of the most representative RF tubes in the USA. Metal octal types: 6SJ7, 6SK7, 6SH7, 6SG7, 6AB7, 6AC7. Miniature types: 9001, 9003, 6AU6, 6BA6, 6AG5, 6AK5.
The paper presents and discusses a careful series of measurements, then compares the relative merits of various tube types. The highest performance tube at 100MHz was the 6AK5 (gm=5mS, gin=125uS), while the worst was the 6SJ7 (gm=1.5mS, gin=0.5mS). These measurements also show that the highest gm tube, the 6AC7 (gm=9mS), is severely limited by it's very high input Conductance of 2mS=1/500Ω at 100MHz. While this high input conductance presents a severe input loading effect at 100MHz, it is much less severe at 10MHz (Conductance=20uS=1/50kΩ), because of the square law proportionality.
An analogy can be drawn at 100MHz between the Beta=Transconductance/InputConductance=gm/gin of a bipolar transistor and the current gain of these pentodes. The 6AC7 then has a current gain of only 9mS/2mS=4.5, which is analogous to a bipolar transistor with a Beta of 4.5. By the same token, the 6AK5 has a current gain of 5mS/125uS=40, and the 6SJ7 has a current gain of 1.5mS/0.5mS=3.
The paper continues beyond the lossy Conductance study to cover input capacitance and the effects of reflected cathode and screen inductances.
The paper closes with practical RF design examples that take input conductance and reactance into account.
The partition noise that pentodes add to the cathode current as it splits between the screen current and plate current is outside the scope of this paper. See RCA tube noise from 1948 for a paper focused on tube noise.
Regards,
-Joe
p.s.: "Input Admittance of Receiving Tubes" includes embedded OCR English text that can be copied and pasted into a translator for other languages.
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MIT Radiation Lab evaluation, 1947
Dear Joe,
Here is a similar article from the ‘Components Handbook’ by Blackburn, MIT Radiation Lab., 1947. The survey also lists some experimental tubes and the British derivatives of the well known EF50 Philips workhorse. Very interesting are the comments of the author about the preferred applications of the various families and some consideration on how to evaluate the use of high transconductance tubes in some low noise circuits.
Many thanks for having kindly converted the scanned pages in a searchable pdf format.
Regards, Emilio
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Datas of european Tubes 'Inputresistance'
Hello Readers.
Hello Joe.
Joe Sousa has made a pdf-file with Datas send from me to him.
This Datas in german Language are from Funkschau-Ingenieurausgabe.
Bezahlte Originale bei Hans M. Knoll
Orignal Papers by Hans M. Knoll
regards Hans
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Further references
Hello Emilio and Hans, and fellow radiophiles.
Thank you very much for posting the additional tube-specific data on this rarely specified parameter.
More references have surface in private email exchanges and also at the Yahoo users group for the Tube Collectors Association TCA.
MAARC Radio Age editor Ed Lyon sent these references:
" Another possible source of background in input conductance is Ferris’ paper in the IRE Proceedings, January 1936, entitled, “ Input resistance of Vacuum Tubes at Ultra-High Frequencies.” Of course, at that time UHF meant anything above about 50 MHz. In the same IRE Proceedings issue (Jan 1936), D.O. North wrote an article on “Analysis of the Effects of Space Charge on Grid Impedance,” and he developed a complex analytic expression for effective grid conductance as a function of Gm and frequency. The issue is also discussed around page 440 and following pages in Sarbacher and Edson, “Hyper and Ultra High Frequency Engineering,” written in WW2 (1943) [John Wiley}.
[snip]. Wonder how the 6AG7 would fare? It had very high Gm, and was used in high-powered video amplifiers in WW2. The highly-rated British CV138 didn’t fare as well in the Valley and Wallman book as the 6AK5, and some credit was attributed to that tube’s gold-plated grid.
Ed "
TCA member and experienced Ham radio experimenter Carl, posted this at the TCA user's group:
"Good info Joe.
QST had an earlier article (maybe 1939-40) about the subject and went into
detail and graphs of the effect on input coil loading and reduced image
rejection of the 6AB7 and 6AC7 when used at HF when used in a conventional
circuit or as a swap into an existing receiver. I could dig through the QST
archives or the yearly index on my bound issues if you are interested.
Its a shame they left out tubes developed by others such as the 6SG7 and
717A (-;
[snip]
The QST article is May 1939 starting on Page 41. Titled "Input Resistance of RF Receiving Tubes". It references a RCA Application Note 101.
[snip]
Carl
"
Carl shared a file with the 1939 QST article. After digging around a little bit, it became apparent that the octal based 717A was a predecessor of the 6AK5. It was interesting to see image rejection as the central topic of the QST article about input conductance. This is particularly interesting because it was a problem that arose well before HF applications drove the pursuit of low input conductance.
My friend and TCA member Bill Courtright, kindly scanned the RCA application note 101. This file has embedded text that can be searched or translated. Bill has a long collection of these RCA application notes and hopes to scan them all, eventually.
The Radiation Lab series from MIT has been, at times, available on-line. Dr. Kent Lundberg, who is an active IEEE contributor to electronics history, has full list of all Rad-Lab volumes. The Blackburn book which is volume 17, is downloadable from Scribd. Other Rad-Lab volumes are also available for download.
A further thought that grid conductance has brought to mind is that most historians point to this fundamental limitation in vacuum tube technology as the essential impetus behind the use of crystal detectors in RADAR. The study and development of crystal detectors eventually led to the invention of European Transistrons and American Transistors.
By the way, I acquired the original tube admittance article from book antiquiarian Frank Bequaert at the recent antique radio meet in Westford Massachussets. Frank has a fine selection of antique technology books at beqbooks. I am one of his occasional customers.
Thank you all for enriching this topic with many good references.
Regards,
-Joe
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High frequency amplifiers up to 1942
Hello Joe and other radio friends,
I found this very comprehensive RCA paper about the state of art of vacuum tubes for very high frequency applications in 1933. Proposed structures can be considered the forerunner of the several small size tubes, as acorn types, all-glass types as the 384A, based types as the 717A, their miniature successors, but even of the countless planar structures, lighthouse, rocket, oil-can shapes, developed around WWII.
Another article on front-ends for UHF receivers and criteria for selecting the related tubes comes from Electronics, April 1942.
Enjoy them both,
Emilio
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