GE M935AWL restoration

[init4fun]

From a thread at the Antique Radios forum;

"Painted tops" (usually exhaust tips on miniature tubes) did indeed represent "selected tubes."

Tomer relates the entire story in his book, Getting the Most Out of Vacuum Tubes (Gernsback, 1960). The practice was very common among older B&W TV sets. You'll often see tubes with their exhaust tips daubed in what looks like red or white paint. Many circuits "skated the rim" as far as workability went, and without the cream of the tubes plugged into them, they'd give trouble. When the cream of the tubes began to age out, they gave trouble anyhow.

With audio output tubes, I'd agree that they were tested for precise matching if used in the output of Baldwin electronic organ amplifiers. If used for other purposes (tone generator oscillators maybe?) in other parts of the circuit, they may have been tested for other qualities.