From: "Jeff Edmonson" <[EMAIL PROTECTED]>
...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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