[AMRadio] Re: 6V6 vs 6L6


Bob Deuel k2glo at jkasystems.com
Thu Feb 14 18:12:44 EST 2008


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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