SS Rectifier replacement question

[N5AIT Allan Stephens]

Gang:

     Following earlier discussions on replacing glass with
sand the difference in internal resistance was a continuing
subject.  Question: is there a good rule-of-thumb, B-flat
value of resistor to put in serieswith silicon rectifiers
in such a case - specifically for more common tubes such
as 5U4, 5R4... and in my case 6AX5?  Any suggestion for
such general purpose resistors?

     TIA

    73,  Al  N5AIT