6146 variants and neutralization

6146 variants and neutralization ... 6146 variants and neutralization Neutralization. The ...
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The majority wisdom seems to indicate that the neutralizing circuit in early vintage units isn't designed to work with 6146Bs or 6146Ws. I reviewed the schematics I have in the military manual (TM11-5820-529-15). There are two sets of schematics in this book with two distinctly different neutralizing circuits. It appears that the earlier KWM-2s have a 1.8-8.7pf variable cap while the later version has an 8-50pf cap. Aside from measuring the value of the cap in my unit I wonder how one can tell the difference? I asked a fellow ham very familiar with Collins gear who told me that the smaller cap in early units was a ceramic trimmer like those used in the RF input circuits while the larger cap is an air variable. My unit upon inspection has an air variable.

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:

Dennis did work for Collins for 25 years. He goes so far as to test select components using a spectrum analyzer. John W4NU

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.

The changes chronicled in part 7 of the on-line KWM-2 manual do make interesting reading. Jerry K0CQ

I've done a little digging. I downloaded the latest manual for the KWM-2 from the CCA page (nothing there for 32S-3) 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 be 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.