Series Modulating a Class C 4-1000A

[K1JJ]

I believe KLR's coil choices were about 62? mh each.  When I first wound them I found they measured too small, like only 35mh each or so. So then wound a third coil to add to the first. Bottom line is now 70mh input and 35 mh output.  Frank and I kinda optimized the modeling based on these real whirl values.

T

[WA1GFZ]

Interesting that I ended up at 72 uh and 28 uh running at 2 ohms

[John K5PRO]

Voice of America, at it's transmitter site in Sri Lanka, had a destructive transmitter fire over 10 years ago, that started in the modulator. A capacitor fed the fire, which was thought to have started due to some sort of failure of a big solenoidal inductor. I cannot remember if it was a PDM but it didn't use a big plate modulation transformer. I remember hearing from one engineer about speculation of some sort of parasite or unload at high frequency which cause a "tesla coil"-like effect from the inductor. There was pointing blame at tube (TH537) and at transmitter (Marconi) by each other. VOA eventually eliminated their tube modulators in those rigs with big IGBT-switched audio modulator that was composed of many series power supplies that could be binary added to get the plate voltage for the RF tubes. I think maybe Continental Electronics got the job.

[K1JJ]

Frank, John, et al,

Well, made some more progress today.

I found a new spot where the PDM filter coil was arcing. One of the connecting wires between coils was sitting on the 3/16" thick Plexiglas that insulates the coils from the aluminum groundplane. It's amazing, but under modulation the wire was developing a 2" diameter blue plasma thru the Plexi to ground. It wasn't really arcing thru, since there was no damage to the Plexi, but it was loud and taking power like an arc.  The Tesla coil effect is tremendous when it happens.

I moved things around and now there is no arcing - so far.

I put a variac on the big supply so I can ramp things up slowly. Brought the voltage up to 5KV (2KV on the pdm 4X1 RF final) and it all stayed glued together. I measured the 4X1 PDM switch tube modulators at about 90.1% efficiency. The RF final is running the normal class C  80% or so. 

The positive peaks are tremendous and everything looks OK for now.  I tried it into the 2:1 swr antenna and had no arcing problems.

Glad I didn't give up and go with that wasteful series modulator scheme the other day.

Maybe I'll try ramping up the HV further in a day or so.

I'm ordering a shutdown board from QIX industries to sense overcurrent and kill the T/R line. That will add a bit of safety against future arcs, I hope. 

With the RF final, screen, grid and filament supplies all floating at full HV potential above ground, this rig is really a handful. It's almost as potentially squirrely as adding a blower and nitrous oxide to a car... :-)  But I just love the challenge, as frustrating as it can be sometimes.  The payoff will be transformer-less, clean, high level modulated audio -  and power efficiency that goes as far as possible for tube technology.... and a cool view of the tubes in operation thru the Plexiglas.

T

[WA1GFZ]

Very Cool! I sent Steve a bunch of my simulation files so he could check them out. I hope he proves me wrong on the first inductor because after a second look it even looks better at 40 uh to 45 uh input inductor so 2 phase they will need to be 80uh to 90uh.