[Collins] 32S-3 Neutralization Circuitry

[Laird Tom N]

I have been looking for the changes that were made in
the 32S-3 neutralization circuitry to allow the later
6146W / 6146B tubes to be used in the transmitter.
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>From the WC9M archives:

In January of this year I got back KWM-2 Serial # 434 from Dennis Brothers.
Dennis reworked this unit that I had gotten on a trade in the year 2001. It
required so much work that I decided not to take it on myself. One of the
many things Dennis did was rework the PA neutralization circuitry... He
added the following components:

...."HEAVY DUTY PA NEUTRALIZATION MOD"......

C-183   1000pfd, 6KV

C-184   1.5 to 12 pfd, 1500V

R-127   470K, 2W

This unit is one of the hottest transceivers I have ever listened to.

Dennis did work for Collins for 25 years. He goes so far as to
test select components using a spectrum analyzer. John W4NU
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Except that Dennis used a 12 pf variable (which can either accommodate
the wildest 6146B or W or make for touchy neutralization) where Collins
used an 8.1 pf variable, (then 2 watt resistor instead of 1 watt and 6
KV fixed capacitor instead of 2 KV) this is the difference between early
32S and KWM-2 and the '78 KWM-2 manual. It should be a little more
stable with heating and with the large capacitor should have enough
range to accept the variations in later less well controlled tubes.

PA neutralization won't have any effect on how it hears though.

The changes chronicled in part 7 of the on-line KWM-2 manual do make
interesting reading. Jerry K0CQ
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> I've done a little digging. I downloaded the latest manual for the KWM-2
> from the CCA page (nothing there for 32S3) and the parts list. Section 7
> has an extensive section on updates and variations over the years. It
> mentions changes to capacitor values in the RF feedback divider, but not
> the changes in the PA neutralization circuit. Its worth reading though a
> 12 MB download on a 28.8 dial up takes lots of patience. Be sure to save
> it first.
>
> The 6146B, having greater power dissipation, has different
> interelectrode capacitances just because there's more and larger
> hardware inside. Better shielding could decrease feedback capacitance.

> There has been a change in the PA neutralization circuit from the
> earliest (as shown in the yellow book on SSB) to the latest. The early
> neutralization circuit used a 10 pf 5 KV Centralab 858 series capacitor
> in series with an 8-50 PF ceramic variable capacitor. That gives an
> effective range of 4.44 to 8.33 pf. There is a 100K resistor to ground
> from the junction between the two parts. As a resistor it has no effect,
> though it will add a bit of C to ground and that T network will have a
> lower effective capacitance range. I measured a 100K 1/2 watt carbon
> composition resistor as having 0.4 pf capacitance. Neglecting the
> resistance, only using its capacitance I compute a capacitance range of
> 4.35 to 8.28 pf. Certainly the two ceramic capacitors will be somewhat
> temperature sensitive, and the greater the setting the more the RF
> current for heating.
>
> The new circuit uses a 1000 pf 2KV ceramic, a 470K 1 watt resistor to
> ground (has full RF plate swing applied), and a 1.4 to 8.1 pf 1250 volt
> variable capacitor. 1250 volt tiny variable capacitors are no more stock
> these days than the 8-50 ceramic. The 0.4 pf of the resistor 470K has no
> effect at all (just shunts the PA tuning) so the effective C range is
> 1.4 to 8.1 pf. Better on the low side than the old circuit, the same as
> the old circuit on the high C side neglecting that shunt capacitance of
> the resistor and its mounting.
>
> It would be possible to pad either the 10 pf (say with 2 or 3 pf with at
> least a 2K voltage rating) or the ceramic variable (with maybe 18 pf) to
> increase the maximum C range. The RF current in the neutralization
> circuit would increase leading to more drift of the capacitor not padded.
>
> In the later circuit it would be practical to pad the air trimmer with 2
> or 3 pt to add that much to the maximum capacity range, though any
> padding capacitor may add to the temperature drift.
>
> It should be possible to start with a 25 or 30 pf 500 volt variable
> capacitor and pull plates to make an 8 pf 1250 volt. You have to
> essentially double all the air spaces (plus the thickness of the plates
> removed). Starting with a little larger capacitor should allow a little
> larger neutralization capacitance range.
>
> The effective range of the air space neutralization capacitor will also
> depend on precisely where its mounted whether the shaft or the plates
> are in the plate compartment or in the grid compartment. And if the
> shaft is in the plate compartment, which side of the circuit the shaft
> is connected to, if it has a shaft.
>
> It might ba as effective as major circuit modifications to connect a
> piece of #16 buss wire to the junction of the two capacitors in the old
> circuit or the junction of the air capacitor and the grid circuit bypass
> and run it through a convenient hole into the plate compartment and
> adjust how long it is and how close to a PA tube to add a few PF of low
> loss capacitance.
>
> If I needed a little more neutralization C in either circuit, I'd be
> inclined to use the buss wire (insulated at critical places with Teflon
> sleeving). Many a tetrode and pentode PA has been neutralized that way
> at frequencies from HF through UHF. Jerry K0CQ
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Hope this helps,

Tom Laird WC9M
Moline, IL.