The AM Forum

   Picked up this Hammond H301533 modulation transformer - brand new - at a local hamfest. The Hammond documentation came with it, but it's pretty rudimentary. Apparently there was a review of it in ER, and I've ordered that back issue. That transformer was sold through ER's store, as indicated by the original purchaser's sales receipt, which was also included.
   The secondary has three taps: 3K, 4K, and 5K. The primary is interesting: impedance is fixed at 5K; it has a center tap for the modulator plate supply and taps for the modulator plates, of course, but it's also got two taps - marked "Sc" - which I assume are for the modulator screens. I've never seen that before. What's up with that? What would be the advantage of supplying the modulator screens through taps on the mod transformer primary?
    It's only a 25 watt transformer, but the frequency response is listed as 150HZ to 15kHZ, where Hammond's current offering - their only current mod transformer - is spec'd at 150HZ to only 7kHZ.
     Anyone have any experience with this transformer?
   

Ultra Linear connection.....?


https://www.iwistaoblog.com/2018/05/ultra-linear-taps-of-iwistao-output.html?m=1

I realize the above is for a speaker transformer but I would think the end result would be the same.

Would make a nice modulator, for sure.


--Shane
KP2/KD6VXI

The taps labeled "Sc" are likely intended to be connected to the screen grids of the output tubes, to achieve what is commonly known as "ultra-linear" operation.  This is a form of distributed loading originally patented by Alan Blumenthal, but then later developed into a product by Hafler and Keroes:
https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_02/073_Keroes_Hafler_UL_US_1955.pdf

The screen tap connection provides a form of feedback that reduces distortion and also reduces output impedance of a pentode amplifier.  It slightly reduces the efficiency, below that of a pentode, but provides much more linearity and lower distortion than the triode connection, at almost the power level of a pure pentode stage.  It was used for many years and was a common upgrade to the D T N Williamson amplifier circuit.  One caveat is that the tubes chosen must have a screen voltage rating almost as high as the plate voltage rating, if maximum output is to be realized.  Acrosound, one of Hafler's and Keroes' companies, (Along with Dynaco) produced a multitude of high quality output transformers, as well as amplifiers employing this design feature.   For 6146s, they produced the TO-350 transformer, which had a separate screen winding, thus allowing the screen grids to be powered from a separate, lower voltage, regulated source.

You will find many circuits applicable to your transformer here:
http://www.tubebooks.org/file_downloads/acrosound.pdf

The example with push-pull 807s using the TO-300 transformer was very effective when driving a quad of Taylor TZ-40 class-B modulators in push-pull parallel, rivaling the original speech amplifier employing a quad of 2A3s.

    Interesting thoughts - thanks. Looking at the diagram, I sort of figured it might be some sort of negative feedback, since any audio voltage that gets coupled back to the screens would be out of phase from whatever is on the plates.
    That little transformer was among a pile of stuff that was virtually free. Looks like I might have found a jewel among the junk. I can envision making a nice little exciter-class rig with that. Looking forward to getting the ER back-issue with the article about it. It's issue #314, as I remember. Just ordered it this morning. Meantime, I'll look through some of my other back-issues of a similar vintage for any adds that might be there.

It's probably this one:


Hammond 25W Modulation Transformer P/N H301533 Offered by Electric Radio

Dave Ishmael reviewed that transformer. 

Phil - AC0OB

That's the one. The transformer was apparently a limited edition by Hammond. Not sure if ER was the exclusive distributor. The adds can be seen in the ER issues from that era.