Some folks are concerned that their older testers (Knight, etc.) don't
have as many filament voltage taps, and they cannot quite reach a
listed filament/heater voltage. This procedure might help; and you
won't have to buy or change any parts.

There is usually a way to obtain the desired filament voltage when
your particular model tester does not have a tap/switch position for
the exact voltage that is listed on the data chart. First,set the Line
Adj. Pot. at its lowest setting (counterclockwise, so the tube's
filament/cathode won't get damaged); then set the fil. v. sw. to the
closest setting available, with the tube under test installed, and
all the other tester controls set properly. Install your good AC
Voltmeter (DMM, etc.) probes to any D (down buss), and C (center buss)
vacant tube socket connectors to read the actual filament voltage
being applied. Then, slowly increase the Line Adj. Pot. until your
Voltmeter reads the proper filament voltage. Since you are selecting
a fil. voltage that isn't listed on the switch taps, a Line Check
probably won't place the tester's meter at the mid-scale point; that
doesn't matter as long as you've applied the correct voltage to the
filament. If you can't reach the desired fil. v. in this way, then you
might try by starting with a different fil. tap. switch setting to
achieve the correct final voltage. One doesn't need to install a
different transformer, or put up with taking readings at the wrong
fil. voltage by doing this. Remember that most tubes will operate
fine at a filament voltage as low as about 85 percent below nominal,
but fil. v. shouldn't be greater than around 5 percent above nominal
(listed), otherwise filament/cathode/or grid damage might result.
Fred