Unlike many other European manufacturers that used acid-etched “pin-prick” codes, Tesla typically used a two-letter stamped code printed in ink. Because this ink is topical, it is much more prone to fading or rubbing off than an etching.
The RCA Command series represents one of the very best vacuum tubes from the mid-20th century. While standard tubes were designed for consumer radios and televisions, the Command series was a “no-compromise” line built for environments where failure was not an option.
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While rebuilding a 6550 push-pull amplifier, I gathered some compelling data regarding the critical impact of dynamic tube symmetry on overall circuit performance.
In my first test scenario, I utilized a pair with a perfect Ip (Static Plate Current) match but a 10% deviation in Gm (Transconductance). Despite the identical bias points, the amplifier reached 1% THD (Total Harmonic Distortion) at only 44 watts.
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To preserve the delicate silkscreen found on vintage vacuum tubes, a more professional approach to storage is recommended. Because the labeling on most New Old Stock (NOS) tubes is notoriously fragile, even the simple act of removing a tube from its original cardboard box can cause the graphics to rub off or degrade over time.
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