[Amps] 4-1000 HB amp

[k1um at bellsouth.net]

I have been lurking for a while here...  I need a 2nd amp for SO2R Contesting - my current amp is the Ameritron AL1200  - 
I have been salting away 4-1000 parts over the last 40 years and decided to HB amp#2

The power supply is a remote unit with ambilical to desk top amp deck. Has variac input 220v, P dahl xfmr and home grown chasis/box on wheels 

My questrion is since the 4-1000 uses 7.5 fil at 21 amps - I want to switch resistors to the fil to yield 1/4, 1/2, 3/4 voltage and finally full fil voltage....

my calc's=
P=IE=7.5*21=157W FIL SIDE
I=157/120=1.4 AMP PRIMARY SIDE [ROUNDING UP]
SO for
3/4V=30V/1.4I =23  OHMS
1/2V = 46 OHMS
1/4V  =69 OHMS

IS my calculations about right???

KEN K1UM