AM2 (AM2) Peculiarities
When in 1937 this new Tuning Indicator came on the market, the famous author L. Ratheiser made an in-depth description of its application in modern radio sets. In his book
“Rundfunkröhren, Eigenschaften u. Anwendungen, 1938”,
he made some proposals in order to achieve best results when using the AM2.
With a simple breadboard circuitry, I took some pictures to clarify the sometimes disappointing behaviour.
Here we see the AM2 used as a Tuning Indicator only. The negative AVC-voltage is connected to g1 of the triode. The resulting voltage swing at the anode (= deflection bars) will alter the angle of the illuminated patterns on the target.
But, the change is relatively little and higher neg. voltages will not make a difference!
At least, these patterns are bright and well focussed!
This application was used by some radio manufactures because the double-system served two functions, thus in the end, reducing costs too.
1. Tuning Indicator
A positive voltage is derived from the screen-grid(s) of a remote cut-off tube(s) and is fed via voltage dividers to the grid (gL) of the left hand indicator system. A high neg. AVC-voltage will result also in a high positive voltage at gL.
The cathode resistor provides the neg. bias for the right hand lf-amplifier. Due to current variations in the indicator system, the voltage drop over this resistor will change accordingly. As a result of this, we see a much higher voltage swing at the anode of the triode, which in turn, will support the tuning indication!
Unfortunately, this configuration doesn’t work very well as we can see on the pictures above! The clear picture for reception of a strong station changes dramatically to darker and blurred sectors for weaker stations!
Having said this, of course, there was plenty room for improvement.
I think, TELEFUNKEN saw it the same way and made progress by introducing the EM11 in the early 40’s. It was basically the same principle as depicted in Example A above. In addition, they put in a second pair of deflection bars tied to a further triode-anode. This anode was slightly different in size allowing a much wider range of indication by using separate sectors.
Hello Mr. Holtmann,
Thank you so much for your clarifiers examples .
In the Blaupunkt 4W77 the AM2 is used as magic eye and AF pre amp stage. I checked all the items in my radio searching for the AM2 disappointing behavior. Now I understand and probably can not be improved its behavior without modifying seriously the circuit.
The voltages present in my AM2 4W77 are:
Tuning indicator A (p7): 268
Triode A (p8): 148
GL (p5): +7,4
G1 (p6): +9,2
And the green pattern is as in your Example B.
Thanks again, Bruno.