By Bryan Geyer

Do recognize that
vacuum tubes are not high precision parts. Why not? Well, to start, all
of the tube makers present their products only by listing generic “average” or “typical”
performance data. They don’t provide (or control) any of the specific operating
characteristics*, so vacuum tubes of a given type can vary widely. Further, all
tubes exhibit a constant, gradual, and persistent life cycle drift; plate
current fades, grid bias shifts. So a tube’s functional day-to-day performance
is never precisely the same. This natural cyclic drift starts when the tube
initially enters service, and ends when the tube dies from cathode depletion
failure—barring all of the many other modes of premature demise that might
intervene (e.g.: open filaments, vacuum leaks, gassing, microphonics, atypical
distortion, excessive hum/noise, and damage from external mechanical shock). As
a consequence, vacuum tube circuits are not the best means to assure stable circuit
performance; there’s simply no optimally constant operating phase. Regardless,
for some 70 years tubes were all that we had. Circuit design got stale toward
the end of that era because the chassis space available for tube sockets
limited potential complexity; also because tubes are so woefully inefficient.
But creative innovation blossomed when PNP silicon power transistors finally
debuted in the mid 1970s, thereby making complementary differential solid state
symmetry a plausible alternative.

Vacuum tube commerce
has collapsed in the ensuing 43 years. All of the domestic, British, Dutch, and
German producers are now either defunct (like Tung-Sol, my employer from ’57-’60),
or they ceased making tubes decades ago. The entire world market for audio-type
tubes is now confined solely to the needs of select audiophiles and guitar
buffs, and currently fulfilled only by obscure producers in China, Russia, and
Slovakia. All are without previous market recognition. The quality,
consistency, and reliability of product from those arcane foreign sources is
speculative, and supply will persist only as long as there’s viable demand, so
the outlook for affordable access to replacement stock looks dicey. Further,
this status prevails at a time when all audio engineers concur that the
load-invariant advantage assured by driving the loudspeaker from an ultra-low
impedance source is dependent on solid-state design. (The Zout
for a solid-state power amp runs < 1/10th of the Zout for a tube amp.) Today’s
audio-type vacuum tubes represent the detritus of a dead technology; it’s time
to move on.
*Refer any vacuum tube specification sheet. For example, here’s the published data for type 6550, a common power output tube: https://shop.ehx.com/catalog/addimages/6550-tung-sol.pdf. Note that there are no minimum or maximum limits given for any of the various operating characteristics. (This traditional practice is in direct contrast with the semiconductor industry, wherein complete min./max. acceptance criteria is provided for almost every critical parameter on every registered device.)
BG (May 2019)
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