I agree that substituting a 6V6 for a 6L6 is quite a stretch. I am an active
and founding member of the Tube Collectors Association, Inc. Over the years
the topic of tube substitution guides has been discussed within our group.
The general consensus is that there are numerous "tube substitution
stretches" and downright misinformation in the literature. Sometimes it is
just a simple alpha transposition such as 6AC7 v.s. 6CA7 (there is a big
difference between those two tubes) and other times the recommended
substitution totally incompatible. It is interesting to compare recommended
tube substitutions by various authors and see how the authors differ in
their substitution findings. In my opinion, a tube substitution guide is a
valuable resource in "homing in" on possible replacements and no more than
that. One should then review the published tube specifications of the
intended replacement tube and determine its compatibility before making the
substitution.
While on the subject, as the demand for tubes waned and production
decreased, manufacturers resorted to creative substitutions so they would
not have to tool up for a particular tube. Here are some examples I have in
my collection: a 6SQ7GT (octal based duodiode-triode) that obviously has the
internal elements of a 7-pin miniature 6AV6 (duodiode-triode) complete with
the mica wafers designed for a 7-pin miniature glass envelope. I also have a
Sylvania 6J5GT (octal based medium mu triode) that has the internal
structure of a 6SN7GT (octal based twin medium mu triodes). Only one of the
twin triode sections is used and connected to the base in the 6J5GT
configutation. The other triode section just sits there supporting the mica
wafers and totally unconnected. More unusual, is a Sylvania 6SF5 (metal
shell octal based triode). Inside the metal shell is a a 7-pin miniature
6AV6 (duodiode-triode) complete with a 7-pin miniature socket! The extra
non-needed and unused duodiode elements of the 6AV6 are strapped to the
cathode. What made this tube suspicious was the tall metal shell normally
found on a metal 6V6. Another unusual one is a metal RCA 12SK7 that has
inside it a 7-pin miniature tube which is assumed to be a unmarked 12BA6.
Also interesting is that the 12BA6 tube has the four creases on the top of
the glass envelope that is common to European tubes! So you can see the "big
boy" manufacturers were scrambling to supply various tubes towards the end
of the US tube manufacturing era.
It is also interesting to note the industry's blatant mis-marking and
deceptive production of various tubes. Jim Cross has on this web-site
www.vacuumtubesinc.com a link that he authored and calls "Vacuum Tubes,
Inc., Hall of Shame". There he identifies various tubes that are "Marked
with the Intent to Deceive", "Marked as a Suitable Substitution" and "Friday
at the Tube Factory". His site is worth looking at. For those of you that
seriously work tubes, there is a great reference book for both users and
collectors alike called "Tube Lore" by Ludwell Sibley. I know Vacuum Tubes
Inc. has that book available.
Bob, K2GLO
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