[Boatanchors] Valiant 6146 Question

[Glen Zook]

The redesign of the neutralization circuit in the 32S- series and KWM-2- series actually involved going to a smaller value variable capacitor that was then an air variable rather than a ceramic trimmer.  Also, there were other components in the circuit that were changed as well.  Where the ceramic trimmer is involved as the neutralization capacitor, there is well over a 95% chance that this capacitor will "burn up" if the 6146B/8298A or 6146W tubes are used.

Heath had a "special run" of tubes made for use with the HW-101 transceivers that were marked as 6146B/8298A but were actually 6146A/8298 tubes.  There was a special notification that came with those tubes stating NOT to use any other 6146B/8298A tubes in the equipment.

It is possible to use the 6146B/8298A and 6146W tubes in equipment that was designed for use with the original 6146 or 6146A/8298 tubes.  However, doing so is often not "plug and play" and, at times, requires modification of the neutralization circuitry.  That is why I caution people when using the later versions to be VERY careful to make sure that the tubes can be neutralized and, if neutralized, that the neutralization "holds".  Not every amateur radio operator is technically capable of recognizing neutralization problems and the same is true of possessing the ability to modify the neutralization circuitry.  If the operator is not comfortable with making the changes necessary to get proper neutralization of the 6146B/8298A or 6146W tubes, then I definitely suggest returning to either the 6146 or 6146A/8298 (the 6293 can also be used without the neutralization problems).

I have had experience with numerous amateur radio transmitters that were designed for the earlier tubes and were then used with the later tubes.  In one situation, there were 3-different transmitters (not Collins) from the same manufacturer, the same model, and that had serial numbers within 10 of each other in which 1 of the transmitters was happy with the 6146B/8298A and the other 2 transmitters required modifications to the neutralization circuitry.  What I surmise is that the component tolerances "added" such that the one transmitter had sufficient "range" in the neutralization circuitry that there were no problems and in the other two that there was insufficient "range" to get the tubes properly neutralized.

In the case of commercial equipment that is "certificated" ("certificated" is the new "buzz word" for "type accepted"), one cannot make changes to the neutralization circuitry else the certification is violated.  When that happens, use of the equipment on the commercial two-way radio frequencies becomes illegal.  When that equipment is used on amateur radio bands, then it is perfectly legal to modify the circuit.  However, use of that equipment on the two-way radio bands is then illegal.  Not only Motorola, but General Electric and other manufacturers ran into the same problems when trying to use the 6146B/8298A, 6146W, and the 12-volt heater equivalents (6883B/8032A/8552) in place of the 6883 or 6883A/8032 in equipment.  I use the example of Motorola in the article because trying to eliminate the earlier versions of the tubes resulted in a definite financial burden which then resulted in the earlier versions of the tubes to again be stocked.

As for "mixing" examples of the tubes in r.f. circuitry, especially where the final amplifier tubes are used in parallel, pairing a 6146B/8298A or 6146W with a 6146, 6146A/8298, or 6293 will almost always cause instability.  You can "mix" a 6146 with a 6146A/8298 with no problems.  But, when trying to pair the later versions with earlier versions, instability will almost always be a problem.  Are there examples where mixing the tubes caused no problems?  Yes.  However, the chances of problems caused by mixing the tubes are very great and, as such, the problem can be avoided by not mixing the tube types.  As modulators, there can definitely be some distortion caused by mixing the later tubes with the earlier tubes.  Fortunately, the amount of distortion usually goes unnoticed within the frequency range that the vast majority of amateur radio equipment utilizes.

Where amateur radio operators are concerned, there is a very wide range of technical ability.  For those operators who are technically inclined and have the ability to trouble-shoot as well as to make modifications, etc., then using the 6146B/8298A or 6146W presents no difficulties.  But, for those operators who are less technically inclined, I definitely suggest using extreme caution.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Jim Wilhite <w5jo at brightok.net>
To: 
Cc: boatanchors at mailman.qth.net 
Sent: Wednesday, December 19, 2012 10:59 AM
Subject: Re: [Boatanchors] Valiant 6146 Question
 
One thing most of us forget is that when you change final tubes you should neutralize them in any or all transmitters.  Each tube, no matter the manufacturer will have very small differences.  Neutralization is not all that critical on 160-40 but when you are at 14 Mc or above it should be done especially for 10 meters.  By doing that you minimize the potential for troubles with the B or W tubes.

The problem that Glen cites was present is some Collins HF gear.  Collins produced a retrofit kit for the transmitter(s) in question so the later B version of the 6146 would not pose problems.  If I recall it was a larger neutralization cap but it has been several years.