The AM Forum

As I am a new comer to AM could someone explain the purpose/function of a 'reactor'.
Is there another name for it?

Bob

if you mean an audio reactor, it is just a high value choke that functions to develope a voltage across it at audio frequencies (reactance at audio frequencies is high??).   values of 50 henries or so are not uncommon. The term reactor is based on it's function in a modulation circuit as opposed to chokes in a power supply or RF circuit though they are the same type of component.

In modulation setups, it is advantageous to keep the DC current off of the secondary of the modulation transformer to prevent saturation of the core, so a capacitor blocks the DC from the modulator, and and the reactor allows the Plate DC to bypass the cap, but not the audio.

smoothing Chokes can and are successfully used as reactors, even stacking chokes in series to achieve a high value of inductance works.
Swinging chokes do NOT work as reactors. I don't know the physics behind that one though

Hi Bob

Welcome to a great bunch of folks.  A reactor in AM service is not radioactive (just kidding ;D).  It is a circuit that removes the class C DC from the secondary of the Modulation transformer.  Some called a Heising reactor after Raymond Heising who modulated a class "C" stage using a choke er reactor with the modulator working in class "A."  There are other ways of doing this but most of us use a modified form of Heising.  See example in my Eico project.  Here I was using an audio service transformer backward where the speaker winding was used as a primary and the peviously designated primary is now being used as a secondary.  This (the DC blocking cap in the circuit) removes the class "C" DC current so that this particular transformer that doesn't work well with an unbalanced DC current going through it.

The results was a very nice freq response with fairly low THD.  These reactors are scarce and if you find one grab it even if you don't have an immediate use for it.

GL  Al W1VTP

The swinging choke is a saturable reactor.  It is designed to saturate with DC flowing through it when used in a choke input supply.  At low currents the inductance is very high, to maintain  critical inductance so that the filter doesn't become more like a capacitor input filter, which would cause the plate voltage to soar at light loads.  At heavy loads, the inductance drops drastically, but less inductance is needed when the supply is delivering higher current.  Sometimes a second, smoothing choke is used, since the inductance of the swinging choke may not be enough at high current loads to adequately filter out the ripple.

The advantage of the swinging choke over one with a constant high inductance, is that the swinging choke tends to be more economical, smaller in size, and to have lower DC resistance than would a higher fixed inductance choke with the same current rating.

Because the swinging choke saturates at full plate current to drastically lower the inductance, it would make a very poor modulation reactor.

The swinging choke is constructed very similarly to the smoothing choke, except that the gap in the core (there to reduce saturation from the DC) is much narrower, to maintain a high inductance at low current, but the increased saturation causes it to drop to a low value as higher current flows through it.

Modulation reactors tend to have relatively high DC resistance  compared to smoothing chokes, but the DC load is constant when running AM, so this doesn't cause the plate voltage to vary.  It may actually give a slight edge to the modulation capability when using a common power supply for modulator and final, since the voltage drop through the reactor results in a slightly lower plate voltage on the final as compared to the modulator.

In the old class-A Heising circuit, a power resistor was usually inserted in series, between the modulation reactor and rf final, to drop the DC voltage to the final, allowing 100% modulation capability.  The resistor was by-passed with a capacitor so that the resistor would drop the DC voltage but not the audio voltage.

Impedance (Z) in an AC electrical circuit is defined as R + jX where R is the real resistance and X is the imaginary part (Reactance) of the complex impedance.

For an inductor:

The X part of the term is the amount of resistance to AC current at a given frequency

R is the DC resistance measured in ohms and does not change with frequency.

X is the AC reactance and is zero for DC and rises as AC frequency increases( for an inductor) and is also measured in ohms

The name reactor is used for an inductor which has useful reactance at a given frequency usually audio.

Reactance is calculated as 2 x pi x f(frequency) x L(Inductance in henries)   so a 20 h inductor would have the following reactance at 100 Hz

2 x 3.1415 x 100 x 20 = 12,566 ohms

Inductive reactance is  positive and capacitive reactance is  negative in the term Z=R +jX  and

j = the square root of -1 (which make it an imaginary number)  j is the electrical engineering equivalent to i in mathematics


Pat
N4LTA

In simple terms the modulation reactor serves the same purpose as the plate choke in a typical pi network. It is there to keep audio from being fed back in to the PS and so the reactance must be rather high. The modulation transformer secondary is isolated from DC by large caps, typically oil filled of 4uF or more.

Carl
KM1H

Thank you all for the explanation of the reactor but I think I am still missing something.

I have just looked at the circuits in my 1963 ARRL handbook but it doesn't show a modulation reactor. Is it simply replaced by a RFC with the B+ fed through a blocking capacitor in low power applications?

Bob

It was seldom featured in an ARRL publication which might help explain why mod transformers are in short supply ::) In some cases the transformer was designed for DC in the secondary, particulary at low power.

It was often mentioned in CQ, the W6SAI Handbook, and of course used in AM BCB transmitters.

B+ wont flow thru a blocking cap so Im not sure what you mean. Do you have a reference or post a schematic here?

Carl
KM1H

I didn't explain that very well.

The modulated B+ is fed to the plate of the output tube via a RFC with a DC blocking capacitor before the tank circuit.

Bob

The problem in building a large modulation transformer that does not use  a reactor is that heavy DC current must flow in the transformer secondary. This heavy DC current saturates the transformer core. When the core is saturated, it doesn't work well as a transformer - AC flowing can only change the magnetic field in one direction - as the DC is saturating the core in the other direction and does not allow the magnetic field to rise and fall in both directions as the AC current flows in both directions.

This saturation of the core was once used to build magnetic amplifiers.

Modulation transformers are built with an air gap to keep the core from saturating but this is more expensive and requires a lot of iron for large power modultion transformers.

With a modulation reactor - the DC flows through a large choke to the RF output tube with the audio transformer in parallel with the choke but seperated with a large value high voltage capacitor that can pass all the needed audio frequencies. A standard push pull audio transformer can be used that is physically much smaller than a corresponding modulation transformer.

A push pull transformer has no sum DC flowing in the primary. They are fed with DC at the center tap and equal DC currents flow in opposite directions to the tube plates. Sice the equal currents flow in opposite directions - the sum  DC current is zero.

Most amateur transmitter circuits in the handbooks did not use a modulation reactor.  They used a modulation transformer designed to carry the full DC to the final through the secondary.  This requires a gap in the modulation transformer core similar to the gap in a filter choke or modulation reactor.  The gap prevents saturation but also limits the low frequency response of the transformer.  A mod transformer designed for use with a reactor usually does not have a gap in the core.  The laminations are stacked like the ones in a power transformer.

The RFC that carries the B+ to the final amplifier tube(s) has nothing to do with whether a modulation reactor is used or not. Its purpose is to carry the DC to the  final without blocking RF.  The mod reactor's purpose is to carry the DC to the final without blocking the audio.

Just as you can series supply the B+ to the final by "series feed", that is, running the DC through the modulation transformer secondary without a modulation reactor, you can dispense with the RFC by series feeding the B+ to the final through the RF tank circuitry.  Of course, when  doing that, the tank coil and capacitor are hot with modulated DC as well as RF.  I use series feed in the PA of my homebrew transmitters.  The tank coil and tuning capacitor float above ground and are hot with modulated B+.  That's why I have an interlock on the door that opens the RF compartment for changing the plug-in coils.

The advantage of series feeding the rf is that it is very difficult to design an rf choke that does not resonate somewhere in one of the amateur bands.  This usually manifests itself by the plate choke burning up when you are operating one particular band.  That choke has to be able to withstand the full rf voltage off the plate of the final amplifier tube throughout the transmitter's frequency range without burning up or arcing over.  I believe the T-368 uses a series fed tank circuit for that reason.

Now I understand. Thanks Hugh and Don for the extra information.

Bob