History Of The American Vacuum Tube AM

RadioEngineeringAA5 History Essay, Research Paper

AA5 History

History of the AA5

(All American 5ive) AM tube radio

Possibly the longest lived consumer electronic product design was the

five tube “AC/DC” AM radio. Virtually every household had at least a few

over the years. These radios were low cost, and one expensive item designed

out was the power transformer. Thus the series heater string, and using

the powerline directly rectified for B+ power. No power transformer also

made it possible for smaller and lighter sets to be made.

—– (This portion quoted from an article published in the Michigan Antique

Radio Club newsletter by John Reinicke)

In the 20’s the crystal set and then the Tuned Radio Frequency,

or TRF, set would provide adequate performance. The complexity and cost

of the Superhet receiver was simply not required. As a result, the Superhet

design appeared only in the most expensive receivers. See

a brief description of the Superhetrodyne radio. In the 30’s, the situation

rapidly changed. Radio had enjoyed explosive growth and the number of transmitters

on the air exceeded the selectivity of the TRf sets. The 30’s also saw

an extraordinary economic circumstance and the manufacturers of radios

realized the need to produce low cost, high performance, receivers. It

was now evident the only design that would provide adequate performance

was the Superhet. In order to reduce the number of tubes required to support

the Superhet, manufacturers designed multipurpose tubes. In April 1933,

RCA introduced the 2A7. The 2A7 was the first pentagrid converter which

combined the functions of Rf amplifier, mixer, and oscillator in a single

envelope. This tube could then be used with a pentode as an If amplifier,

a combination diode-triode as a detector-first audio amplifier and a audio

power amplifier to make a complete receiver. Add to this a rectifier to

power the set and you have a high performance receiver with but 5 tubes.

To further improve the receiver, a remote cut off pentode could be used

in the If amplifier so the If amplifier could be used as a part of the

automatic volume control circuit. The tube line up for this 5 tube receiver

would then be: 2A7, Rf amplifier, converter; 58, Remote cut off pentode,

If amplifier; 55, Diode-triode, Detector-first audio; 59, Audio output;

and 80 for a rectifier. This arrangement uses 2.5 volt filaments and therefore

required the use of a power transformer. This was the prototype “All American

Five.”

1934 saw the introduction of the 6A7 and a whole series of 6 volt tubes

to go with it. It was now possible to build an automobile radio or to combine

with a 25Z5 rectifier to build a set without a power transformer. (See

the March 1990 Chronicle article, Ballast). With the elimination of the

power transformer, it was now possible to have a truly low cost, high performance

receiver. There are those who argue the series filament version became

the classic all American Five.

(end quote) —–

The tube heaters were wired in series, sometimes with a “ballast” resistance

added to make the total voltage drop across the entire string add up to

that of the powerline, around 120V. All the tubes needed to have the same

heater requirement for this to work. All tubes had indirectly heated cathodes.

Early versions of the five tube radio used the same small signal tubes

(RF, IF, non-power audio) as transformer sets used. Tubes like 6A7, 6D6,

75, 6F7 and newer ones like 6SK7, 6SQ7. And an extra “tuning eye” 6E5 tube

if desired. All these had 300 ma heaters.

To make an

“AC/DC” radio, you would just need specially designed audio output and

rectifier tubes. Like the 43, 25L6, 25Z5 and 25Z6. That would be only two

new tubes to be developed (per radio chassis design) to make an AC/DC set.

These had higher voltage heaters, but the same current (300mA) as the small

signal tubes above. Power handling tubes like audio outputs and rectifiers

need bigger cathodes and more heater power to operate. If current is the

limiting design factor, increase the voltage to get more heater power.

But all the heaters in a series string in the above didn’t add up to

enough voltage to be fed directly off the powerline. So some sort of additional

voltage dropping resistance was used. Eighter a power resistor, “ballast

tube” or resistive wire in the power cord was used. I don’t know if anyone

used a power resistor housed in a “wall wart” (calculator charger style)

power plug.

One of the above mentioned rectifier tubes, the 25Z6, is a pair of diodes,

used in a voltage double circuit. This gets you a B+ power supply of around 250 – 300

volts. Might make “translating” a design from a power transformer design

to a “hot chassis” design. Not “AC/DC”, voltage doublers won’t work off

of a DC supply.