I am working on a spice model at the moment. At this point I only achieved a model of the three EF86 / EF804s stage and the PSU. With the values of the original schematic this stage has a gain of 1 with a 10k : 600r OT.
OK nice. So 4:1 works from a gain standpoint and, with the follower beforehand, probably no need for anything like an 8:1. Most of the designed for NWDR units I've played with seem to have really quite low output impedance, in the region of 30R if that helps.
Using 200H inductors for the plate choke and anode works well. Even though a 100Hz sine already looks slightly distorted. 1kHz looks clean. I will work out this model as far as possible.
I'm not sure what the impedance of an EF86 strapped for triode is off hand, but 200H seems instinctively big enough to not cause too many problems at the low end but? I tend not to trust distortion results from a sim myself and it's very possible you may not need to worry about it in real life.
With respect, I assume the Vacuvox U23 design mainly allows for settings on the front panel that were originally on the inside and not needed at hand in broadcast use.
Ah OK, good to know, makes sense.
My plan is to use Edcore TXs and Hammond chokes. I will build a first version with turret boards and point to point wiring. As soon as I get it running I will layout a pcb.
Sounds good. I don't know where you're located but, if in Europe, there might be some alternatives from Carnhill worth a shot. I noticed recently there was an inductor in the £20 region that might be a good candidate for anode loading and folks have used their 10K:600 with good results.
Would you rather skip the input pad and use a 1:1 TX on the input? Could you explain the advantage of having the four secondaries of the interstage TX wired in series rather than having a single winding?
It might be that Rohde & Schwarz were just using the best transformers available to them then... Could be the mic to p-p grids had better balance than a line to grids? I would bet that interstage they used too just required that each secondary be terminated for good response rather than it being a purposeful design decision.
There's usually a design evolution you can see with these NWDR units and a look at the U73 schematic which came later might give you some clues. Speaking of which, I think someone on here had the 1:1 transformers from that unit analysed and copied. I remember them being quite reasonable in price for what they were, and pretty much spot on to the original coils.
Your spice looks good
