Dear Members and Visitors of the RM,

the evaluation of the first volumes of "Electronics" is yielding fruits:
In the April 1932 issue [1] an article appeared about the "Wunderlich Tube".
Besides some technical Information one could find a description of the use as a detector as well as a comparison of sensitivity and linearity compared to 227 tube as a detector.
I hope this could be a valuable supplement to the "Wunderlich Tube" contribution which has been already presented here ("Die Wunderlich Röhren" and "Wunderlich operational details") in Radiomuseum.

The Wunderlich tube

The new detector tube known as the Wunderlich tube has definite advantages.
The input so far as r. f. is concerned is push-pull; the a. f. output is normal.
The tube itself has two grids, of course, arranged in a co-cylindrical fashion about the cathode.
At the time of writing it is made solely by Arcturus.
Circuit details are not available for publication but imagine a center-tapped input coil tuned to the incoming signal, either at r.f. or i.f.
Between the center tap and the common cathode is a resistance.
The plate circuit is normal in that it can be coupled to a following tube by conventional means.
Incoming signals cause first one grid to take current and then the other with result that a rectifying action takes place.

The current flowing to the positive grid, when the other is negative, is Iess than would normally be expected, or which would be measured in a pair of 227's in a similar arrangement.
This rectification produces a pulsating d.c. across the center tap resistance and at the end of this resistor near the grids will be a negative potential of amplitude depending upon the incoming signal.
This negative potential can be applied to the automatic volume control system of the receiver and used to govern the sensitivity.
After detection the tube acts as an audio-frequency amplifier with the two grids in parallel.
The tube will behave much as a 227 so far as audio amplification is concerned except that a slightly less abrupt cut-off will be experienced.
It is reported that as much as 20 volts can be available for volume control without severe distortion on heavy modulation.
Stronger carrier voltages will produce stronger bias for the r.f. amplifier tubes, with the result that a fairly constant level is maintained at the input to the detector so that overloading is avoided.
The tube offers advantages from the standpoint of greater sensitivity and greater fidelity- the demodulation process will be accompanied with less distortion.
In addition there are certain manufacturing advantages according to the tube's sponsors which will make it possible with 6 or 7 tubes to duplicate the present performance of a 9-tube set.
A 5-tube automatic volume control automobile receiver is suggested.
One interesting feature is the fact that ordinary tube noise and hiss will provide the initial bias for the tubes and that the time constant can be so fixed that the set can be tuned from one local station to another before the set returns to fuII sensitivity to bring up the noise level between stations.

Notes on the Wunderlich tube *

THE Wunderlich tube can be thought of as a triode with a second grid wound between the meshes of the usual grid.
Its purpose is grid leak power detection, and gives full wave grid rectification in a balanced circuit in which negligible r.f. currents flow in the plate circuit.
This feature has two important advantages: it approximately doubles the output voltage by eliminating simultaneous plate and grid rectification and makes unnecessary the r. f. filter in the detector plate circuit.
When the grid leak and condenser are properly proportioned the voltage developed across them is almost exactly proportional to the r. f. signal amplitude and hence is a faithful reproduction of the modulation envelope.
The voltage across the load consists of a d.c. component proportional to the carrier amplitude and an a.c. component varying with the modulation.
The d.c. component places a negative bias upon the grids and is of proper polarity for automatic volume control
The values of grid leak resistance and condenser capacity must be properly chosen with regard to rectification efficiency, distortion, input resistance to radio frequencies and the r. f. input to the tube.
When it is desired to obtain extremely high quality, the resistance can be about one-quarter megohm but in general a resistance of one-half to one megohm can be used.
The power capacity of this new detector is determined by the maximum audio voltage the tube can amplify without distortion.
With the two grids connected together the tube is essentially a triode with a mu of the order of 9 to 12 and a plate resistance of between 10,000 and 20,000 ohms.
On the assumption of a d.c. rectification efficiency of 70 per cent the largest carrier voltage that can be handled is about 21.3 peak volts and when 100 per cent modulated the peak audio voltage across the grid leak would be about 10.65 volts, which would develop ample output to excite any pentode or push-pull amplifier.
The amount of negative d.c. voltage available for automatic volume control purposes depends upon the way in which the plate circuit is arranged and in the case of the Wunderlich tube can be of the order of 15 volts.
When compared with the plate rectifier commonly employed in broadcast receivers, the Wunderlich detector has the advantage of a somewhat greater rectification efficiency, particularly when the signal voltage is in the order of several volts.
The tube has ample power capacity to excite the power amplifier of any broadcast receiver now on the market, and also supplies a voltage which can be used directly for automatic volume control purposes.
When compared with the triode type of grid leak power detector, this push-pull detector has about the same efficiency, introduces less distortion because the balanced input circuit prevents simultaneous grid and plate rectification, and develops approximately twice as much output voltage.
The only disadvantage when compared with the corresponding triode rectifier is that the centertapped input circuit requires twice as great a signal voltage for excitation.
*By F. E. Terman, SC.D., Stanford University.
(later on a professor and author of numerous Radio Engineering Books)

[1] New Tubes - Detectors, Rectifiers, Amplifiers, Electronics, April 1932, pp. 118-120, 148

Regards
Achim

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