[Collins] output tubes

[Glen Zook]

If your KWM-2A has the later neutralization circuit which uses an air variable then your unit will work with the 6146, 6146A/8298, 6146B/8298A, any 6146W, and the 6293.  If it has the older neutralization circuit which uses a ceramic trimmer then you definitely should avoid the 6146B/8298A and any 6146W with a code date after mid 1965.

RCA originally said that the 6146B/8298A was completely compatible with the older versions of the 6146.  Unfortunately, this did not "pan out".  However, the military changed the specifications of the 6146W  from the 6146A/8298 to the 6146B/8298A starting in July of 1965.  Various contracts with the several manufacturers that were making the 6146W tubes were modified at different dates from mid 1965 to January 1966.  However, nothing in the nomenclature was changed to indicate whether or not the tube was a 6146A/8298 equivalent or 6146B/8298A equivalent.

When the 6146W was changed to the 6146B/8298A those units in use by the military started "burning up" the neutralization circuit.  Because there was no easy way to tell which version of the 6146W was being used Collins had to modify all of the KWM-2, KWM-2A, 32S-3, and 32S-3A units to have the new neutralization circuit.  They also had a field conversion kit to change the neutralization circuit in units already in the field.  Once the neutralization circuit is changed to the air variable circuit then any 6146W can be used as well as the other members of the 6146 family.

On the surface the only characteristic of the 6146B/8298A that is different from the 6146A/8298 is the control grid (grid 1) capacitance to the plate.  It is specified as a maximum of 0.22 pf in the 6146B/8298A whereas the 6146A/8298 has a maximum of 0.24 pf.  Of course the 6146B/8298A is rated 33% higher in input and output power.  

Unfortunately, the 6146B/8298A is VERY prone to "taking off" and the original neutralization circuit used by Collins Radio could not usually "handle" this.  It is the same in other transmitters designed for the 6146 or 6146A/8298 in that the vast majority of those transmitters are "very unhappy" with the 6146B/8298A.  Now individual transmitters work fine with the later tubes.  But, that is usually not the case.  I have worked on like 3 identical model transmitters with serial numbers within 10 of each other.  Of those 1 was "happy" with the 6146B/8298A and the other 2 just would not work.

The 6293 is an excellent replacement for the 6146 and 6146A/8298.  It is basically a 6146A/8298 that has a heavier constructed plate.  The 6293 is rated at 1,000 watts peak input power when used as a pulse modulator.  Using a 6293 will usually result in between 5 and 10 times the tube life of the 6146 or 6146A/8298 and the cost is usually the same or even slightly lower than the 6146A/8298 when bought new.

If you have not read the article on the 6146 family of tubes that was in Electric Radio "a while back" (and has been republished in several other publications) go to

http://k9sth.com/Page_2.html

Then scroll down the the heading "Articles".  The 6146 family article is the very first link.

Glen, K9STH

Website:  http://k9sth.com


--- On Wed, 7/9/08, yash at aol.com <yash at aol.com> wrote:

Moveing along on my KWM-2A project,I started looking ay the output tubes . The rig has 6146B in it now. I went to several site and found the 6146W was a good replacement. Then reading another site.the 6146 W was a BAD choice. Got the same answer from several of the listed vendors, but no real facts.

Since I have to replace the existing outputs ,what verson of the 6146 SHOULD I be using. Iam confused with all the differant answers so I thought I would as here ,for the right one.et