Prior to 1936, Philco’s part numbers for its resistors had no correlation to the actual values of the parts. Their earliest resistor part numbers used only four digits, as did nearly all Philco parts. On 1 January 1933, Philco began differentiating between resistors, capacitors, coils, etc., with two digit prefix numbers; resistors were assigned a prefix of 33-. (For further reading on this subject, see this article at the Philco Repair Bench website.)

Beginning in 1936, Philco adopted a new system of identifying carbon composition resistor values in its part numbering system. This new numbering system was applied to what Philco called “Identified Resistors,” carbon composition resistors with a small metal tag on one of the leads. On this tag was stamped the name “PHILCO” and the value of the resistor. The tag could be removed with a soldering iron; the tags are never seen on resistors mounted inside radios, however. With some practice, one may look at a Philco resistor part number if it was made after 1936, and figure out its value and wattage.

View of a prewar Philco “Identified” resistor.

A Word about Color Codes

Before we explain Philco’s resistor part numbering schemes, let’s discuss the resistor color code. The color code of the 1930s, while using the same colors as the modern code, can be confusing to read. The early tubular carbon composition resistors, which replaced the earlier “dog-bone” style, continued to use the dog-bone coloring system – body, tip, dot – except that the tip was a stripe at one end of the resistor, and the dot became a stripe in the middle of the resistor body. So a resistor that appears to have a black stripe, a brown stripe, and a yellow stripe may actually not! (By the early 1940s, the modern method of using stripes to indicate a resistor’s value had been adopted; the stripes are obvious, and should not be confused with the earlier system above.)

Look carefully at the picture above. The color over the main part of the resistor (body) is the first significant digit of the resistor’s value. The stripe at one end (tip) is the second significant digit. And, finally, the stripe in the middle (dot) is the multiplier. See the table below.

The 1936 Numbering Scheme

It consisted of the 33- prefix followed by six digits instead of the four arbitrary digits previously used.

33-xxxxxx

Each of the six digits has a special meaning:

First digit indicates how many zeros follow the first two digits of the actual resistor value.

0 – no zeros
1 – one zero
2 – two zeros
3 – three zeros
4 – four zeros
5 – five zeros
6 – six zeros
9 – divide by 10

Second and third digits indicate the first two digits of the actual resistor value.
The remaining digits indicate wattage:

339 – 1/2 watt
439 – 1 watt
539 – 2 watt
639 – 3 watt

Examples:

33-449339 – 490,000 ohms (449 – 49 followed by four zeros), 1/2 watt (339)
33-310439 – 10,000 ohms (310 – 10 followed by three zeros), 1 watt (439)
33-250539 – 5,000 ohms (250 – 50 followed by two zeros), 2 watt (539)
33-313639 – 13,000 ohms (313 – 13 followed by three zeros), 3 watt (639)

At first, wirewound resistors continued to use the 33- prefix followed by four digits, which did not correspond to the part value or wattage. By 1941, an “Identified” system had been added for certain wirewound resistors, as follows:

The first three of six digits indicated value, as above.

The last three digits indicated wattage and type of resistor:

326 or 336 – 1/2 watt, wirewound
416 or 436 – 1 watt, wirewound
418 or 438 – 1 watt, appearance like a mica condenser

More examples:

33-015336 – 15 ohms (015 – 15 with NO zeros), 1/2 watt, wirewound (336)
33-140436 – 400 ohms (140 – 40 followed by one zero), 1 watt, wirewound (436)
33-115438 – 150 ohms (115 – 15 followed by one zero), 1 watt, flat wirewound (438 – looks like a mica condenser)

A Postwar Numbering Scheme

After World War II ended and Philco resumed production of civilian items including radios, the company adopted another series of part numbers for carbon composition and wirewound resistors. These break down as follows:

The new prefix is 66, followed by a dash (-) and then the numerical part number which contains seven digits.

66-xxxxxxx

These may be read as follows:

First digit indicates how many zeros follow the first two digits of the actual value.

0 – no zeros
1 – one zero
2 – two zeros
3 – three zeros
4 – four zeros
5 – five zeros
6 – six zeros

Second and third digits indicate the first two digits of the actual resistor value.
The remaining digits indicate wattage as in the following examples:

3340 – 1/2 watt
4340 – 1 watt
4360 – 1 watt, wirewound
5340 – 2 watt

Examples:

66-2683340 = 6800 ohms (268 – 68 followed by two zeros), 1/2 watt (3340)
66-4474340 = 470,000 ohms (447 – 47 followed by four zeros), 1 watt (4340)
66-2104360 – 1000 ohms (210 – 10 followed by two zeros), 1 watt, wirewound (4360)
66-3685340 – 68,000 ohms (368 – 68 followed by three zeros), 2 watt (5340)

Larger wirewound resistors in 1946 continued to use a 33- prefix followed by four digits which had no relation to the actual resistor value.