The AM Forum

I have been thinking about using solid state circuitry to replace modulation transformers. This is just an idea, so bear with me until I make my case. The idea is to take a 100 watt AM plate modulated rig, like a DX100 or Viking 2 as the basis for the conversion.

The circuit topology would involve several changes to the rig, but the tube lineup would not change, and the visual appearance of the rig would only change a little. So here is what I am thinking:

* FW bridge the power supply for a B+ of 1200 - 1500 vdc
* Run RF final in series with the modulator, with the RF plate on top, and modulator cathode at ground
* Make the modulator switch mode PDM (class D or H?) similar to early work by Steve KA1SI using tubes
* Add a filament transformer for the 6146 RF tubes insulated for 1500v or more
* Remove old Mod Transformer and replace with painted black metal box with all the new circuitry within
* Add a carrier level pot to define max upward mod % versus carrier power
* Add some form of level limiting to the audio as is done with the audio on the class E rigs

I think the circuitry could be standardized such that we could convert 100 watt class AM rigs over such that no modulation transformer is required, and then have some flexibility and capability that the stock rigs do not have.

So in the box would be the filament transformer, PDM driver power supplies, PWM board, FET tube driver board, and toroidal core audio LPF. The solid state circuitry could be rather small, and possibly surface mount.

This is just an idea I have been pondering for some time. I wonder what folks think about this, and is anybody willing to explore this concept to the point of constructing a prototype? The whole concept deviates big time from the KISS principle, but is in my opinion cool, and a viable replacement alternative for those pesky modulation transformers. If done well, broadcast quality with high upward modulation capability could be achieved (at reduced carrier power output).

Regards,
Jim
WD5JKO

You should contact Steve QIX ex:ka1si
I know he is/has helped at least one other fellow with doing the PWM / Tube route.
And sure he has done it.

I'm sure he'll pop in and comment.

A very worthy Ideal, a standardized Mod with minimal chopping, sounds good take alot of pics and post alot.

73
Jack.

PDM is probably over kill for 100W class rigs perhaps series modulation would be a simpler approach since efficiency isn't that big a deal in a low power rig. Same idea though with replacing the mod transformer with the series modulator.

Think again. With the B+ twice normal you have too pull half the current as before to not overload the power transformer. With a series analog (class A) modulator, the RF pa modulator currents have to be identical (no voice). This means the original modulator tubes in class A parallel would be dissipated beyond their ratings, and the power transformer would overload at 2X the VA rating. I am assuming keeping the same 100 watts out as before.

With a series PDM configuration, the power supply would run half normal current at twice the voltage as stock, which is the same VA as stock. The PDM modulator running at 50% duty cycle (no voice) would provide half the B+ at twice the current to the 6146 RF PA's. This is like the 'buck' power supply topology. So you could make say 1200V B+ at 100ma to provide 600V @ 200ma to the RF tubes when the PDM modulator is switching at 50% duty cycle. Since the modulator tubes would be running in switch mode, they will be cool and well within ratings.

The series analog class A modulator really is only practical at a lower power level since the modulator needs to be a brute (High PD) for just a small RF output, say 25 watts.

Jim
WD5JKO

I think it's a good idea and I too have been considering a HV design for tubes.

I think there are two things one needs to consider:

A. A series string of mosfets to get the voltage needed,

B. The impedance of the PWM filter has to match the tubes plate impedance.

Phil - AC0OB

Steve's first PDM rigs used tubes in both the modulator and final.

The PDM transmitter is a series modulated transmitter.  The only difference is in the class of service of the modulator. The classic series modulator stage operates in class A, whereas the PDM modulator operates in a switching mode.

FETs in series, good luck with that

I have seen Steve's SS ClassE schematics but never his tube switcher's. Steve has always been on the forefront of modulation and RF technology.

One of the Collins ClassF3, 5kW BC transmitter's, used two 3CX3000F7's in series, one as the switching modulator and the other one for the final. The final's plate was grounded through the tank circuit and the switcher's cathode was connected to the -13.5kV PS. In between the mod tube and the final was the 70kHz filter.

One could have a cascode arrangement of say two 1000Volt MOSFETS. The cascode switcher is little more complex since you have to set the bias of the top and bottom transistor's gates.

Phil - AC0OB

Hi - This can definitely be done (with tubes or solid state), and has.

There are a couple of ways to attack the problem.

The first, and most straight forward is to do what you're talking about - bridge the power supply and use a CAPACITOR input filter to get more high voltage (about 2000 volts is optimum for 6146 tubes with 700 volts at carrier).

Use a 6DQ5 (or a pair) as a pulse width modulator.  This can be accomplished by either floating the RF amplifier cathode circuit (which will involve floating the heater supply as well), or you can float the modulator cathode and driver circuits, and use high voltage opto isolation, or a pulse transformer to get the PWM signal into the floating circuitry.

I did this with a Viking II back in the 1980s, and floated the RF amplifier cathode (and heater) circuit.  Worked very well!!!

The 2nd way to do it is to completely eliminate the high voltage power supply, and construct a combination high voltage power supply / pulse width modulator using MOSFETs.  I did this with my "PWM Valient" last year.  The power supply produced 2000 volts on modulation peaks.

This was an experimental thing that I clip leaded together to see if it could be done.  There are some technical issues with this approach - notably, there is some energy consumed in snubbing circuits that needs to be mitigated (put back into the power supply), and I never got around to designing that part.  But, it did work very well.

Both approaches produced outstanding audio.

I also did this using a solid-state pulse width modulator comprised of 1600 volt IBGTs in a fairly complex series arrangement (2 devices).  There are pictures of this transmitter on my WA1QIX web site.  This used a 2000  volt power supply, and 6 6DQ5s in parallel for a 500 watt transmitter.  I finally abandoned the project as being too complex to be worth it, and concentrated on getting class E working correctly at high power!!  However, the transmitter did function well for about 1 year.

Regards,

Steve

Steve,
My last trip to Ca. I spent a day working with a guy from Rocketdyne. He deisgned power supplies for spacecraft. I was taking notes as fast as I could write on the recovery of energy.  This guy was in his mid 70s and drove quite the Italian machine. I'll scan it and send you some stuff sometime.
I also built a PDM Viking II right after I met Steve in '82 when he planted the PDM idea. I used a pair of 6DQ5s but my problem was the modulator beat the crap out of the fianls that usually need one or more replaced every few years. I checked the 6DQ5s a couple years ago and they were good as new. I cathode drove them with a FET. Ran 1550 volts on the B+ with a bridge on the transformer.