>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]"
>It is sometimes not practical to add additional high voltage.
** good point. In a TL-922 it looks like there is room, the filter C is
good for 4kV, and the bandswitch has a breakdown potential of 5kV, so it
should be possible.
>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.
** my guess is 10 - 20 volts positive.
>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
Rich, AG6K, 805-386-3734, www.vcnet.com/measures