[AMRadio] what to use for a mod reactor???

[Donald Chester]

>From: "Jeff Edmonson" <w5omr at w5omr.shacknet.nu>
>
>...just in case you're not sure how to connect it, I tie one end of  the
>modulation transformer (that WAS going to the B+ supply) to ground.
>The other side is connected to a large capacitor (5uf @ 5kV) on the Final
>side of the B+ line to the final.
>
>So, from the B+ supply, attach to one side of the choke, the capacitor to
>the other side, that's connects to the mod xfmr and then B+ from the choke
>to the final.  This shunts the DC off of the secondary of the modulation
>transformer, allows the transformer to react more linearly, and prevents
>core saturation on the secondary of the modulation transformer.
>More Core in the transformer = broader range of frequencies, generally
>speaking.

Actually there are 3 ways to connect it.  The above corfiguration has the 
disadvantage that one winding of the modulation transformer is always 
grounded, putting high voltage stress on the insulation between windings.

The way I connect mine is to connect the final amplifier end of the mod xfmr 
directly to the final, and put the series capacitor (1-4 mfd) from B+ side 
of the mod xfmr secondary to ground.  The mod reactor goes from the final 
amp B+ line to the HV power supply.  This arrangement keeps high voltage DC 
on both windings of the mod xfmr, reducing the HV stress on the insulation 
between windings.  However, when the transmitter is first switched on to 
transmit, the capacitor must charge up, so there IS HV stress the first few 
moments of each transmission, but the potential difference is not sustained.

The third method is to wire the mod reactor in parallel with the secondary 
winding of the mod transformer, and break the connection on the transformer 
secondary and insert the capacitor.  This can be done at either the B+ or 
modulated final end with little difference in performance.  This 
configuration keeps the HV on both mod transformer windings at all times, 
with no transient periods, but has a disadvantage in that the audio is 
returned to the HV power supply instead of ground.  That means that the 
coupling capacitor and output capacitor of the power supply are in series, 
and that hum from the power supply is more likely to show up on the signal.  
The varying load on the power supply from the modulator tubes can induce 
harmonic distortion into the audio. The first two configurations return the 
audio directly to ground, bypassing the HV power supply altogether from the 
path of the audio.  Of course the disadvantages I mentioned of the latter 
method are present when a modualtion reactor is not used.

Use at least 8 Henries of inductance for every 1000 ohms modulating 
impedance.  For example, running 2000 volts to the final @ 500 ma gives 4000 
ohms modulating impedance, so you would need at least 32 henries @ 500 ma.  
A BC-610 runs @ 8000 ohms modulating impedance, so you would need a minimum 
of 64 Henries @ 250 ma!  These figures are for broadcast quality, so a ham 
rig could use somewhat less, but I wouldn't use anything less than half to 
75% of those figures.

You can use a string of filter chokes in series, but that is bulky, and 
sometimes power supply chokes have so much stray capacitance that you lose 
high frequency response.  Ask me or John, WA5BXO sometime to explain how we 
made a nice compact mod reactor @ 30 Henries using two identical 12-Henry 
filter chokes.

With so many old tube type BC transmitters being junked as they are replaced 
by solid state units, modulation ractors and transformers should not be that 
hard to find.

Don K4KYV


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