the amplifier with respect to the filament of each individual 300B power tu e in
order to ensure equal current flow through oth ends of the tu e filament. Optimal
cathode point alance is attained when the hum induced in the wanted signal via
residual ripple of the heater voltage vanishes (reaches a minimum).
Thoroughly balance the cathode point of the L R amplifier with respect to the filament of
the individual 300B power tube with the BALPOT in the way described in the SETUP
section of this manual!
Check the L R amplifier for optimal cathode point balance from time to time and finely
tune the balance if necessary!
Re-balance the cathode points of the L R amplifier after changing the 300B power tubes!
For a given setting of anode voltage and current, the grid ias voltage Ug(DCRR) of a
given 300B tu e under DCRR cathode point implementation is a out 2.5V (that is
half the heater supply voltage, 2.5V=5V/2) lower than the ias voltage Ug(ACMT) of
the same tu e under ACMT cathode point conditions,
Ug(DCRR) <---> Ug(ACMT)-2.5V,
Nota ly, Ug(DCRR) is the actual voltage etween the 300B control grid terminal and
the DCRR cathode point under alanced conditions in the sense a ove (including the
effects caused y residual grid current of the tu e). It is important to take this ias
shift into account when making reference to the characteristics of the ideal (average)
300B tu e as given y the original Western Electric data sheets.
300B OPERATION CONDITIONS
Under ACMT cathode point implementation the ideal (average) 300B tu e draws an
anode current of Ia=0.065A when it is exposed to an anode voltage of Ua=425V and
a grid- ias voltage of Ug(ACMT)=(-)93V according to the Western Electric data sheets
(January 1950). This is an excellent operation point for single-ended class-A
operation which we have chosen for the 300B tu e in our amplifier. Corresponding
to a plate dissipation Pa=28W=425VXx0.065A well elow the permissi le design
maximum of 35W,
(Ug(ACMT), Ua, Ia, Pa) = ((-)93V, 425V, 65mA, 28W).
The corresponding quiescent operation point under DCRR cathode point
implementation is
(Ug(DCRR), Ua, Ia, Pa) = ((-)90.5V, 425V, 65mA, 28W)
Ug(DCRR)=(-)90.5V=(-)(93V-2.5V).
The grid bias voltage of the 300B tube is factory preset to (-)90.5V and is not meant to get
