[AMRadio] B*I*G modulation transformer

Donald Chester k4kyv at hotmail.com
Sat May 22 09:56:53 EDT 2004

>if that is a moduation reactor, you indeed have a treasure. They are
>exceedingly hard to find. Typically they are nothing but a huge choke,
>30-50 hy's, 1 amp or greater for current. You can put them in the secondary
>of your modulation transformer along with a oil filled capacitor and take 
>HV off of the mod tranny. I've been looking for one for a while, and I 
>I have one located now.

Mark's descriptions sounds more like a mod TRANSFORMER than a reactor.  
Wonder if the specs mention whether or not the secondary is designed for DC, 
or if it requires a separate reactor?

Mod reactors shouldn't be that hard to find, with all the retired  tube-type 
broadcast transmitters that have been replaced with solid state units.  Some 
hams have converted these rigs to operate on the ham bands, while many are 
simply parted out, and no doubt many have simply gone to the landfill or 
metal scrap yard.

Inductance may range from less than 20 hy to as much as 100 hy, and current 
rating as low as 250 mills to several amps, depending on power and 
voltage/current ratio at the final.

A good rule of thumb for inductance is a minimum of 8 henries for each 1000 
ohms of modulating impedance.  For example, a kw transmitter running 2000 
volts on the final @ 500 mills operates at 4000 ohms modulating impedance 
(Z=voltage/current).  You would need at least 32 henries @ 500 ma.  A 
30-henry unit would be close enough.

Less than optimum inductance results in loss of low frequency response.

Some hams simply series up several power supply filter chokes, but that 
tends to be unnecessarily bulky.  Power supply chokes may have too much 
capacitance, or the iron may be lossy at high audio frequencies, causing 
loss of high frequency response.  In that case, find a low-inductance choke 
(0.5 to 2 hy) at the appropriate current rating to put in series with the 
main reactor, on the final amplifier side.  The small one will take over as 
the reactor as the larger one becomes non-functional at higher audio 
frequencies (kind of like the woofer/tweeter combination in a speaker 
system).  At lower audio frequencies the smaller reactor adds negligibly to 
the total inductance.

I haven't run dc through a modulation transformer since the mid 60's.  Even 
a transformer designed to carry the dc will function better if a reactor is 


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