The AM Forum

    Contemplating, Cogitating, Throwing stuff against the hypothetical wall:
Heising circuits are typically drawn with a direct DC connection from the modulator tubes to the high side of the reactor, with the modulator tubes drawing plate current up through the Heising choke from the same source as the RF PA. But what if we DC isolate the modulator plates from the choke and feed them from their own supply, relieving the Heising choke of the extra current burden? Less choke saturation, and more current available to the RF PA.
    Since the Heising choke works via the expanding and collapsing magnetic flux from the modulator, at audio frequencies, it would seem to me that the DC component really serves no purpose and only adds to the burden on the choke.
    My schematic is obviously simplistic, but it shows the general idea.
    Is there any reason this wouldn't work?

Just curious if your intent is to use dual tubes as a class A modulator?

If so, the image below is a workable concept. Using a Push Pull Transformer

as shown will work when the class A anode current matches the RF PA anode current.

The fields in the transformer halves are then equal and opposite with a near net zero

DC bias on the core. It works well so long as the heat from the class A tubes is tolerable.

This approach might not be practical at 100 watts or higher unless you need a room heater.

Jim
Wd5JKO

   I was just thinking that your schematic looked a lot like a modified G-50 modulator, and then I noted the caption.
   Indeed, that would be the plan: a big space-heater pair of 813's running class A. The notion of capacitor coupling to the choke is just to utilize all the current capability of the choke to feed the RF PA, rather than passing the modulator current through it as well.
   Not any sort of "shovel-ready" project; just going through my inventory of stuff and thinking of potential uses for it all. I've got a big BC-1 mod reactor, and though the classic way to employ it would be with a mod transformer, I had this weird "what if?" thought. A pure Heising configuration like this would be inferior in a number of ways to a conventional mod transformer/reactor scheme, but it might be a bit of "let's do something different" fun. A pair of 813's in class A...yeah... it would be a winter rig only.

You could always try a couple of back to back Anteks for your missing mod trans... I have a proposed lashup worked out for an 813 x 813 rig I will be modifying soon. K9YQQ has been doing this lately on 3 or 4 rigs. I plan to do this if I do not run into a more traditional mod trans. Been looking for a CVM0 for a year with no joy. I don't have a mod reactor in the current design, but would like to add one.

John

   I actually do have a couple of mod transformers, a UTC VM-4 and a Thordarson T-21M65. Foolishly missed a chance to score the matching mod transformer to my BC-1 reactor at Nearfest; kicking my posterior a bruise over that one.
   The Heising thing is just a thought-experiment; a "what if" curiosity (I'm not a cat, so it won't kill me). Heising is simpler, but with mod transformers available I can't see any advantage - other than saving weight and bulk of a mod transformer - in actually building this thing.

Looking at your schematic diagram there may be a problem with unbalanced DC in the AS-4T320 secondary, toroidal transformers can easily saturate.  To prevent this a suitable choke and capacitor combination should be used to keep the final DC off of the secondary winding.  I use an AS-8T800 with push pull parallel 805s into an RCA 1 to 1 modulation transformer, Heising (impedance) coupled to my Globe King 400 final, the frequency response is very good.  Also, you may consider using one or both 6.3 volt windings on the AS-4T500 for feedback to an earlier stage.

Bruce - WA3JVJ

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