In regard to running 4-400 tubes in place of 3-500 types, Rich said:
"2. Increase the anode PS potential by c. 500v in order raise the ZSAC to
the needed 75mA/tube in order to achieve SSB linearity. To do this, add
a 200Vrms or so 1A tranformer secondary winding, in-phase, in series with
the stock HV winding. [Peter Dahl can make the boost xfmr]"
Response:
It is sometimes not practical to add additional high voltage. A more
practical approach might be to isolate the screen and add a positive voltage
sufficient to raise the ZSAC to the needed 75mA/tube. This might only
require a 100 volts or so. You can use a variable voltage until you
determine the required voltage - then build a fixed supply. A little higher
voltage will be ok, since you can then simply change the cathode bias to
reach the desired ZSAC. Remember, although the current demand is slight, it
will need to be regulated. Also, check the resulting imd to verify that
there is no deterioration.
73, Colin K7FM
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