The following information is for the radios that will operate on 12 vdc, not radios that will operate on 6vdc. The 6 volt version radio uses 6CM6 tubes for the modulator and the rf power amp.
Over the years I have been working on the Messenger I and II radios, I have discovered that some replaced 12AB5 tubes are actually 7061’s labeled as 12AB5. Tube substitution manuals say they will interchange. They will, but performance suffers.
History of 12AB5
Tungsol introduced the 12AB5 in 1955 as a 12 volt car radio audio amplifier tube. RCA introduced the 7061 in 1956.
If you substitute a 7061 in place of a 12AB5 in the modulator, the plate will develop a red/orange spot visible in dim lighting. Something is off in the biasing of the tube. I found a post from a forum that sheds some light on what is going on.
“In the operational data. The 12AB5 is drawing 10% of plate current for screen current (4.5 mA per 45 mA). The 7061 is drawing 25% of plate current for screen current (9 mA per 35.5 mA). You would think that similar designs (gm, Rp) would have similar screen current ratios? The higher screen current would limit the power that could be used.”
It has also been theorized that the 12AB5 was designed for audio service while the 7061 was for use as a class C amplifier for mobile radios.
How can you determine if you have a re-labled 7061? First of all, look at the length of the glass envelope. IN MOST CASES, the 12AB5 will be a taller tube, similar in length to the 6AW8. If the 12AB5 is the same length as the 7061, compare it to the plate inside the 7061. If it looks identical, its a re-labled 7061. The 7061 plate has no ribs and is flattened on 2 sides. A Tungsol 12AB5 has circular ribs.
6CM6 Tubes
If you have the 6 volt radio, it has (2) 6CM6 tubes. Johnson labeled 6CM6 tubes are taller than most other brands of replacement 6CM6 tubes. I will have to do some more investigation and report on this.