February 1933 Radio-Craft[Table of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft. |
Here is a vacuum tube chart everybody needs. Well, not really, but surely somebody out there in the RF Cafe audience will find it useful. By 'audience' I mean most likely a hobbyist who is restoring or repairing a vintage tube-type radio, television, piece of test equipment, etc., and has never even heard of RF Cafe, but finds exactly what he (or she) needs as the result of a Google (Bing, Yahoo, whatever) search. If it seems like you can find information on just about anything you need on the Internet these days, it is at last partly because of efforts like mine that uses a LOT of personal time and some personal funds to establish and perpetuate the reality. Others of you contribute in different ways; this is mine. A helpful way to make it worthwhile is to visit websites of companies that advertise on RF Cafe. A lot of great products and services are offered, and it doesn't cost you a penny to investigate. Thanks.
Francisco Pinto Basto*
A graphic representation of the following factors of tubes: amplification factor, mutual conductance, plate resistance, power output in milliwatts, power output in decibels, and R.F. performance factor. Forthcoming tube characteristics may be interpolated as the data becomes available. This procedure is discussed on the preceding page, and in the August, 1932 issue of Radio-Craft. |
In reference to the article of Mr. C. H. W. Nason. entitled "Tube Characteristics at a Glance." published in the August issue of your journal. insert page 128A. I would like to submit to Radio-Craft a handy. graphic chart for tube characteristics. I have never seen such graph in any paper; only in the "Telefunken Zeitung," April, 1930 is shown together with the triangular chart another graph employing log-log paper, where the penetration factors (in German-Durchgriff - 1/mu) are marked as abscissae and the resistances as ordinates, and the mutual conductances are given by a 45° lines net.
In my chart. Fig. 1. a log-log paper is also used. As abscissae (A) are marked the amplification factors and as ordinates (B) the mutual conductances in mA per volt. The A. C. resistances are given in kΩ by a family of straight lines (C) easy to construct.
For better comparison of power tubes. another family of 45° lines, perpendicular to the former, may be drawn. These lines (D) will give the maximum power output per peak volt squared input that a tube can deliver:
To avoid a complexity of lines, only the lines for 1, 10 and 100 milliwatts per square volts (peak) are set up. The power subdivisions P may be read on a resistance line, say, on the 1 kΩ line. Together with the milliwatts scale is inscribed a decibels scale (E) having the 0 level in 1 milliwatt per square volt, but any other level may be used.
Any tube is represented by a point which gives at a glance the three parameters: mu, mutual conductance, and resistance.
Now, suppose that we wish to compare two power triodes, for instance. the '10 and the '50. The graph shows (dotted) that the '10 gives 5 decibels above the level and the '50 3 decibels. It is to be noted that a triode gives 0.4 - to 0.8 - decibel less if the undistorted output is considered because the load is twice or three times the A. C. resistance. Without great error we can assume a 0.5-db. loss for all the triodes; consequently it may be taken into account merely the decibels difference of the two tubes and we say that the '10 gives 2 decibels more per square volt input than the '50. If the largest input voltage to be handled is low enough not to overload the '10. this tube will be preferable to the '50.
For identical power pentodes this simple comparison may be made, but if we compare a triode and a pentode, a deduction of 2 (to 4) db. must be made from the pentode's power, due to the fact that the pentode requires a load 5 (to 11) times less than the A. C. resistance. By this means the graph indicates for the '47 (deducting 2 db.) 13 db undistorted output more than for the '45. for the same grid swing.
Another family of 45° lines, perpendicular to the resistance lines' may be constructed, giving the R. F. Performance Factors:
F = ; only the scale (F) R. F. P. F. is plotted on the 10 kΩ line.
In radio frequency. it is well-known that the maximum voltage gain obtainable with a tube followed by a suitable transformer is given by
provided that the turns ratio of the transformer is properly adjusted where is the dynamic resistance of the coil and R_{p} the A. C. resistance of the tube, w and r being respectively the inductive reactance and the radio frequency resistance of the coil.
Hence, factor F may give a fair idea of the maximum voltage gain that we can obtain with an R. F. transformer coupled tube.
A little investigation shows readily that the decibels scale may be used to give a computation of the gain obtainable with a given radio frequency tube.
If G. is the voltage gain, the decibels gain is given by db = 20 log G = 10 log G^{2} =
As the decibels scale divisions are proportional to log ,it may be used also for this case.
The power lines may be plotted also in the triangular chart, referred to in the August issue, but it will be very laborious work.
Other charts employing log-log paper may be constructed, for instance, with mu, as abscissae and R_{p} as ordinates.
*Lisbon, Portugal.
Posted March 10, 2015