The AM Forum

Looking at building the GE Double Sideband Junior transmitter http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2013%20No%202.pdf   Why?  Because I have all the parts in my junk box except the driver transformer, which is where I'm getting stymied.

Circuit calls for a "driver transformer transformer with a turns ratio of 5.2 to 1, primary to half secondary; and connect the primary as secondary and vice versa.  Thordarson 20D97 or equivalent".  So not having a driver transformer in my junkbox, I went into the Hammond catalogue.

As the transformers are listed in terms of Primary Impedance to Secondary Impedance; and knowing that Transformers always reflect impedances from one winding to another by the square of the their turns ratio or, expressed as a formula: Zp/Zs = ((Np/Ns)squared); where Zp is primary impedance, Zs is secondary impedance, and Np/Ns is turns ratio; I calculate the two CT primary driver transformers turns ratio as follows:

124D  7K C.T. to 15.8K C.T = turns ratio of 1:1.47
124E 15K C.T. to 33.8K/135K = turns ratio of 1:1.5/1:3

Given that the circuit calls for a 5.2:1 transformer, which as only half of the Secondary is being used I'm really looking for something with a 10.4:1 turns ratio (or flipped around a 1:10.4) the Hammonds wouldn't work as there is not sufficient turns in the Secondary.

Have I got this right?  And if correct, where could I source this?   73  Alex

I have Stancor a A-4416 that you can have or perhaps swap for a small filament tranny.

Check out the number in the catalog and see if you think it will do the job.

I believe it's 5:1 primary to 1/2 secondary so it's similar to the Thordarson you mentioned.

The Stancor is a little heavier and beefier, I think

Dave

If the  ratio is 5.2:1, primary to 1/2 secondary, then the ratio would be 5.2:2, primary to the whole secondary, or 2.6:1 total primary to secondary turns ratio.

Thanks for the replies David and Don and for clearing the confusion!  David, I sent you a separate email on the transformer.

73  Alex

If that thing works out to somewhere around 2.6:1. that is close enough to the 3:1 step-up just about standard interstage transformer used in just about any and everything where you are driving class-A or possibly AB1 grids. (that dont require any grid current)

As you get more and more into the realm of grid current that ratio starts to change rather dramatically, and driving big-assed class-B triode grids often requires a step-down transformer in order to provide enough grid current to the outputs without bogging the driver stage down too badly.

Turns ratios are your friend here much more than rated impedances, as the impedance that the primary shows is a direct function of the load that the secondary sees. General "rule of thumb" here: step up for non grid current applications, step down for applications requiring grid current.

I hate that primary to half secondary, primary to whole secondary 1/2 primary, etc crap. It just adds to the confusion when you are doing the numbers game.
I just wring them out with a known AC voltage and see what the total full primary to full secondary ratio is and go from there.

I took a lookie to see which one I keep on the shelf.

It is a Hammond PT-124D (I still have 1 NIB left)

these are a way kool little transformer. Both windings are center tapped, making it a very useful little animal. It is not really winding specific, so you can use either side as a primary or secondary. You can also use half windings for step ups or step downs. You can use it from single ended to push pull, push pull drivers to push pull modders, single ended class A drivers to P/P AB1, AB2, or small class B modders or just about any combo you can think of depending on which way you turn it and which combo of windings and taps you use.

I have put these things into V-2s, Valiants, Rangers, Apaches, and a bunch of other stuff including my big HB rig. They will work for just about anything if you pick the right combination of taps and windings for the job at hand.

Hammond still makes all of the 124 series interstage transformers.

Pat
N4LTA

Well, I sent off that Stancor A-4416 this morning.

Sorry for the long delay. I went out of town right after we discussed it.

Not sure how long it will take to get from California to Ontario, but it's on the way.

I think that DSB high level balanced modulator concept is interesting.

I wonder if anyone has tried it and if there is any DSB suppressed carrier on the air.

It seems like it would work with most tetrodes or pentodes -- even high powered tubes.

Dave

You willing to sell that to moi? ;)

Nada, I have too many rigs that use one now.

Dont they make them anymore? ? ? ? ?

Yep, but I just thought if it was surplus Id be interested as I'll need one for the next V-1 project or something else.
No biggie