The F2a11 is
electrically the same as the famous "post tube" F2a, which was used in
many 1950s and 1960s telegrafic amplifiers by the German Post. The only
difference is the socket.
Siemens audio amplifiers the F2a11 substituted the EL12 (as in the
Klangfilm 401 amp and changing to the 402/403 amp) and finally was
substituted by the more common EL84 in lower power applications and by the
EL34 where more power was necessary.
F2a11s are reliable
and great sounding audio tubes in push-pull amps with up to 30 watts or in
single ended stages up to 7 watts. Other famous amps using F2a11 tubes are
the Telefunken V69 and the V69a.
All F2a11s shown
were made by Siemens in their home plant.