more CRYONIC info...
Marik...here's some more info culled on the net:
The series of operations through which an electron tube must pass before it
can be called a "Cryo-Valve" is lengthy and in some ways, arduous.
First, only the best sounding NOS (new old stock) and new production tubes are considered
for cryogenic treatment. Every tube is pre-selected on a "SOFIA" computer curve
tracer and only the tubes with the most linear and evenly spaced plate curves are
selected for cryo-treatment. Also, in the case of dual-triodes such as 12AX7, 6922,
only the tubes with the closest triode section matching (typically within 5%)
and quietest noise-floor are chosen for treatment. Pentodes such as EL34, EL84, 6550
are selected on the basis of having high transconductance and high standing idle current
and are matched into pairs and quads before they are frozen.
As received, a typical electron tube exhibits several problems that directly
impact its sonic performance. Most serious are the many internal stresses in the
construction materials that accumulate during most of the stages of manufacture
and; a very hard, heavy oxide-coating on through-glass pins to which direct
connection is made. Seven and nine-pin miniature tubes are typical of those pins
which are heavily oxidized while power tubes such as
KT88 and EL34 are fitted with
bases whose pins are tinned with the result that contact quality is much improved.
The degree of sonic improvement is SUBSTANTIAL.
The initial 100 hour burn-in allows the tubes' characteristics to stabilize while
providing an opportunity to "cull" any "infant mortals"
During cryogenic tempering, the tube is slowly cooled to the -117 C / -320 F
temperature of liquid nitrogen, "soaked" for many hours then slowly returned to
ambient. By means of this unique and vital process, the stresses interior to the
materials of the tube are substantially and permanently relaxed. During a
subsequent, high temperature anneal, the tube is heated to a 175 C / 350 F
temperature then slowly cooled to ambient. Although not as extensive, the
results are similiar to those achieved by the cryogenic procedure.
The "Q" of the (self) resonant (electro) mechanical systems responsible for the
output of (self) microphonic spuriae is therby drastically reduced. By this
important reduction, both the peak amplitude and the "ring down" time of these
systems is reduced with the result that the "apparent gain" of the tube is
increased - even in feedback controlled circuits - while the "dynamic noise
floor" is lowered.
Grading for noise and microphonic performance involves both listening and
intrument evaluation. Various instrumentation provides data on the noise,
microphonic level and spectral content while the overall "sound" of the spuriae
is critically evaluated. In particular, the evaluation of microphonic output
is very much an experienced-judgement call.
Dual triodes (example: 12AX7, 6SN7GT, 5751, 5687) are rated for overall noise
performance by the noise-output level of the noisier "tube" or triode section.
Thereby, it's possible for a tube given an over "STANDARD RATING" to contain an
ultra-low noise (ULN) section. ULN rated tubes however, always consist of two
ULN sections, hence the higher cost per tube.
The last few steps in the process are the standard yet essential procedures for
the evaluation of many important electrical characteristics such as GAIN, PLATE
CURRENT and TRANSCONDUCTANCE. Additional data is generated indicative of the
GAIN and DC BALANCE of each tube's sections.
...whew...and only 5 bucks more?...
