The AM Forum

Hey fellers,
A 1935 Super Pro followed me home from Dayton!
Well, time to fire it up, the power supply is re-capped, rx tubes tested AOK.
Now to the final steps...

It runs a pair of 42's in PP to a rx mounted output transformer that is 8 ohms
output.
Next.
The voltages for several tubes are supplied via a "loop" through a field coil mounted
on the speaker.
The original speaker was a Jensen C8R or C10R. No data on the inductance
or resistance of the filed coil tho.
I have successfully in the past substituted a field coil & output matching coil
by building an "outboard" circuit.
This was a 2000 ohm 50 watt resistor in series with a 10 H choke.
Next I incorporated a nice UTC output transformer to match plate load to a 8 ohm
speaker.
SINCE this SP-10X has its own output speaker, can ya think of any reason
why a 10 H choke & adj dog-bone 50 watt resistor (say about 2K ohms) would
be ok to use?
I do have the manual and pin voltages for tubes, so I can adj the resistor for appropriate voltages to the circuit...
thanks for the thoughts!
73 steve
8tow

I would think the field coil is just acting like a power supply choke. The current through the field coil is an electromagnet for the speaker. I don't see the need for a large series resistor??? I would think you dust need to duplicate the resistance of the winding and I bet it may not effect the plate voltage all that much. Been a lot of years since I cut one open but remember the wire was small. 
It must have been before they invented cheap magnets.

Should work fine Steve.  You don't need the resistor unless your filter choke is very low resistance.  I may be able to help you with an old dynamic speaker from an old radio.  If we don't get together before then, can bring one to Gary's place in July.

73,  Jack 'GT

Hey Steve!

Didn't know you were at Dayton, would've been nice to meet up.

Congrats on the SP, I think Buddly has the same or a similar model and could perhaps give you some insight. Not sure how he has his hooked up.

You're going to love the Push/Pull audio those 42s make. They were the top dog receiver in their day.

Hey great thoughts guys...
I will pull out a choke this eve and see what the DC resistance is on the thing.
The thing is, in the 1936 Lafayette Radio catalog, that Jensen speaker
was listed, but 2 different resistances available, 1500 ohms & 2500 ohms.
None of the Super Pro documentation says which they used!
gettin closer to making it play tho!
73 steve

This should be simple to solve by empirical examination.

First, what B+ voltages to the tubes want to see? (tube manual and a look at the circuit will reveal all!)
Second what is the B+ before the presumed field coil?
Is there a cap after the field coil return?
The difference in voltage between what the B+ supplied to the tubes needs to be vs. the voltage before the field coil is the required (if any) vdrop.

A quick look at the schematic (if there is one) or the power supply should tell you where this voltage is derived from - as someone just said, it may serve as a second "choke" or not depending on what is going on in the PS.

Most field coils are fairly high Z, especially if they ran on B+ level voltages, since they couldn't draw too very much current at that voltage...

If it comes down to it, I have a Jensen A-12 (iirc) field coil 12" speaker that I can measure the DCR on, but it's unlikely that you'll need that...

          _-_-bear

Steve.... I'll get this boxed up and on its way to ya.

As u can see it's a 12" but from the "A" series but shud wrk OK FINE !!!

I put the field coil on the bias supply and it sounded PDG considering the state of the cone.

My SP100 has an audio output xformer with the PP 6F6s.
Enjoy !!

Yeah, that's the one(s) I have too!

I accidentally toasted the VC on one by using the wrong socket on a Leslie amp and it ran the FC voltage into the VC. Boy was I PO'd at myself!! Grrrrr...

That's a VERY nice speaker that has some decent bass response!

The leg in the back comes off... Mounts fine in a regular box from the front of the basket.
Use a box of about 2-3cu ft and it will sound good.

Shipping it - screw the face of it, cone side, to a plywood board!! otherwise it will get mangled. Best to drill holes through and use machine screws. Then bag and box... double box to absorb the throwing about... can't put peanuts in with the speaker part, they'll mangle the cone. So, peanuts between the inner box (which should be a dead tight fit, no slop at all) and the outer - good to go, get thrown about by gorillas...  ;D

             _-_-bear

Thanks Bear..... good advise !!

That's pretty much it.  The dynamic speaker needs a stronger magnet than they were able to produce at reasonable cost back in the early to mid 30's, so they formed an electromagnet using the field coil.

About the time of WW2, an alloy was developed called ALNICO (aluminium, nickel and cobalt) that would make a powerful enough permanent magnet to serve the purpose.

Some electrodynamic speakers have a "hum bucking" coil of a few turns, wound on the same core as the field coil, wired in series with the voice coil to cancel any hum modulation of the magnetic field that might be introduced by the field coil, which usually doubled as a power supply filter choke.

Oh, the leg in the back coming off has nothing to do with the shipping ideas...
just so that whomever gets it knows that it is not mandatory... might be useful in shipping, more anti-crush plywood?

           _-_-

the audio out of that thing will amaze ya. just make sure you get the i.f. cans aligned right, adjust it on the 6kc setting, then use the k-nob. If you adjust at 3 KC you'll find you cant get maximus bandwidth with it set on maximus. You should get a peak as you twiddle the k-nob through min to 6 kc and back again. Your min selectivity should b right around 3 kc nose and max should be around 12 to 14.

good indication of maximus bandwidth is to set it on a good BC station and start checking for where you start hearing hi hat cymbals in music, should be around one notch back from maximus.

I've owned a 100, 3 200's, 3 400's, and 2 600's. I'll take the 200 any day over most other receivers of that era. The resistance to overload and images in strong RF fields is remarkable on a Superpro.

I'll take the super pro over the sx-28a as well, up to 25 Mc. Be careful, you have full b+ on the back chassis screws marked for the field coil. You'll also find extra screwholes on some of he power suck plys to mount a actual 2nd filter choke instead of a field terminal. It was an option. I think it was 1500 ohm dc rx. But it's not that critical.