[Boatanchors] Yet another tube question

[Brian A Clarke]

Just a moment fellas,

Selecting tubes for RF PAs doesn't need to be too accurate. What you need to
bear in mind is that if you have a set designed to deliver 400 W from two
tubes in parallel, and one of those tubes is a bit low in performance, and
the operator tries to get the full bottle, then the life of both tubes will
be shortened because of the excessive temperature rise. Balance within 20%
is probably close enough.

In audio applications, there is a very good reason for balancing tubes'
emissions much more accurately, and that is that if there is an imbalance,
then the secondary of the output transformer will be passing some net DC
that will go towards magnetising the core and thus reducing the inductance.
This will change the shape of the hysteresis curve and have two effects -
distortion on high powered peaks that wasn't there before, and loss of
bass - the OPT needs inductance in order to have enough reactance at the
lower frequencies. For stage musicians, balancing the output tubes allows
more power output before distortion sets in - very handy. Most musically
inclined folks can hear these effects quite easily - dubbing them
'audiofools' is a sign of ignorance.

73 de Brian, VK2GCE.