Hi Phil,
Thanks much for the info.
25 watts MOSFET dissipation for two 4D32s is amazing for a class A modulator! Plus no filament power. I will have to look into this more closely. What is the carrier power out when running 25 watts dissipation?
So the heatsink is not that big. But what if we use like, 1500V instead of 600V? Obviously the 11N90s will not handle it. But how about using those IGBT HV devices? I'm told their prices have come down. I played around with them when building the 4D32 PDM rig using them as PDM cathode modulators. They did get hot though at 2000V on the rig and popped, even with a copper spreader/ heatsink. 1500-2000V will certainly ramp it up. It all depends if the rig is to be used as a low power driver or as a stand-alone rig that can use the power to get thru.
Have you tried passing a squarewave thru and how does a 50 Hz triangle look?
If 600V is the limit, then I can still use a low power 50-75 watt rig that can pass squarewaves to drive my 4X1 linear amp. It will also double as a low power daytime summertime rig. What's not to like?
Rick: I see where you're coming from with the class A modulator plans. The critical loading adjustent would be my first reason not to go with efficiency modulation. It's like putting a test tone thru every time you want to tune it up on a different freq. TNX, OM.
With the cascode circuit seen below and as previously posted, this will handle up to 1600 volts cathode voltage. It also has HV surge protection built in. This is the modulator I will be using for my 4-400 cathode modulated transmitter.
Cathode voltages only rise to about 0.25 X Vp to 0.35 X Vp depending on the tube gain. Of course tetrode cathode voltages may rise to the 0.35 X Vp figure.
For a dual 4D32 about +200 Watts carrier can be expected for 650 Volts Vp and 0.5A total plate current. Now for a Vp of 650 volts Vp (as per tube specs), only one Mosfet is needed.
As always, depending on individual tube gain(s) some tweaking is always required.
AC0OB
