Valiant problem

[W9BHI]

What would cause the final plate current to drop slightly when modulating?

Don W9BHI

[KK4YY]

A slight drop in current can be caused by a slight drop in voltage.

The Valiant uses a common high voltage power supply for the final and modulator. So the modulator loads down the PS (when modulating) and drops the HV slightly thus causing a slight drop in the final current.

Sounds like no problem... as long as it's slight. Slight should be less than... let me pick a number... 5%. More than that and I'd start looking for problems.

Sooooo, how slight is slight?

Don
KK4YY

[W2PFY]

I had the same problem in a BC-610D. The PS only has 6 mf of capacitance. I had a couple caps around. One was 30 mf and the other was ten. I paralleled them up and put them across the existing capacitor so now I have a total of 46 mf. The DC voltage still dips ever so little but it also helped my PEP output as the caps store the extra energy needed for the modulation peak current demand. Try a 30 mf cap in addition to whatever capacitance you have in your transmitter. It should help!  

If this results in no change, I would look at the wiring before the transmitter. Possible things to look for, what is on the side where your transmitter is getting its power? i.e. electric water heater as an example. Maybe a dedicated line to your ham shack is in order?

[W8IXY]

Both my Valiant and DX-100B show a slight downward kick under modulation, which is due, as Don said, to the common HV supply.  Be sure the grid drive is correct and the PA is loaded properly, and the PA tubes are good as well.

73
 Ted  W8IXY

[ke7trp]

Load the valiant down a bit so the current stays the same on mod.   280 or so seems to do it.  Full 330 always results in downward kick.