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Brands like GE, Sylvania, Tung-Sol, Motorola, Raytheon, and Zenith are often overlooked, but usually perform just as well as the more famous vintage brands, which makes them a terrific value, especially as the stocks of New Old Stock (NOS) tubes vanishes.
Indeed, some of these were made by RCA for these other labels, and others like the Tung-Sol and Sylvania have their own followers who prefer these brands over the higher priced premium labels.
Grass Instruments also had a yellow tip version which was even lower noise, but they are rare and difficult to find these days.
All of the red, yellow, and blue tips are incredibly quiet and very three dimensional in sound.
Amperex and Mullard tubes have a pair of alpha-numeric date codes printed in dark grey near the bottom of the tube.
The rarest of these are the Bugle Boy cartoon tube label for Amperex (especially if it also has the treble clef music symbol next to the Bugleboy image), and the older Mullard logo that looks like a shield, especially with the letters "BVA" below it.
Then you would be well advised to lay in a stock of the brand of your choice "for a rainy day".
Watch for the Mullard "10M" series of ECC83 tubes in the distinctive royal purple and gold boxes.
These sweet tubes were factory screened for a 10,000 hour heater life, matching internal triodes, and low noise, rather like the Telefunken ECC803S tubes.
Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, and I have seen WB and WC versions.
The W is the military type code, A, B, and C are progressively later productions. DO NOT confuse these with current production Russian or Chinese crap with the suffix WX, WB, or WC!