[HAM] What are the appropriate ohms values for the cathode bias resistors for the different output valve types ?

[Kon Zissis]

Hi everyone.
I would like to know what the correct  ohms values and wattage ratings
are  for the cathode bias  resistors used  in valve amplifiers that have
cathode biased output stages.
What would be the appropriate  cathode bias resistor values for the
following power output valves :
EL34 , 6L6,  5881,  EL84 ( 6BQ5 )  , 6V6 ,  KT-66 ,  KT-88.
 
The Leslie 122 / 147  cathode bias resistor for the 6550's is 150 ohms ,
and the Leslie 31 H , 32H and 21H cathode bias resistor was  75 ohms for
the 6L6 , 5881 and 6V6 valves in them . 
 
Are the 150 ohms for the 6550 valves and the 75 ohms for the 6V6 , 6L6
and 5881 valves the best optimum  values for the cathode bias resistors
?
 
I have a Marshall JTM-45 guitar amplifier which  has two EL34 output
valves in class A/B push pull  configuration and I am planning on
putting in a switch so that I can select either the grid bias operation
or the cathode bias operation.
 
 IA few years ago  I did a temporary experiment  with  the cathode bias
operation in my friend's amplifier which has EL34 valves in it and the
cathode bias operation produced a very  pleasing softer and more
compressed sounding "singing' effect to the guitar sound whilst the
fixed bias  operation produced a harder attack sound .
 
I would like to do the cathode bias switching modification but I am not
sure  about what is the correct ohms value and wattage rating  for the
cathode bias resistor.
At the moment because the Marshall JTM-45 operates with grid bias , the
cathodes of the two EL34 valves are grounded  and the negative voltage
is sent to the junction of the two 220 K ohms grid resistors of the EL34
valves. 
 
With the cathode bias switch position , the ground connection will be
switched off  from the cathodes and the cathode bias  resistor will be
activated and the junction of the two 220 K grid resistors will be
grounded.
 
Thank you in advance for a reply. 
All the best.
Kon