[Collins] refurbished s line

[Glen Zook]

You can definitely use the 6146, 6146A/8298, or the 6293 tubes.  The only difference between the 6146 and the 6146A/8298 is the fact that the 6146A/8298 uses what RCA called the "dark heater".  This was developed so that the tubes could be more reliable in mobile applications.  With the dark heater the filament voltage can be varied from a low of 5.0 volts to a maximum of 7.5 volts and the power output will remain within 10% of the value obtained at the nominal 6.3 voltage rating.  This is per the RCA published 6146A specifications 6146A 5-63 which is effective May 1963.

You can even mix a 6146 with a 6146A/8298 in the transmitter without any problems.  However, do NOT try to mix any of the earlier versions with any of the 6146B/8298A versions.  You WILL definitely have problems.

The 6293 is the same tube as the 6146A/8298 except that it has a heavier plate construction which allows it to be used as a pulse modulator with inputs up to 1000 watts with a pulse time of a maximum of 3 microseconds with a maximum plate voltage of 5000 volts and a peak plate current of 1.5 amps.  This is per RCA published specifications 6293 4-63 dated April 1963.  When operated as a "normal" 6146A/8298 the 6293 generally lasts between 5 and 10 times as long as the 6146A/8298.

You can also use a 6146W with a code date of before 1964 because those tubes are identical with the 6146A/8298.  However, the military changed the specifications on the 6146W from 6146A/8298 to the 6146B/8298A starting in mid 1964.  However, those specifications were changed as contracts were renewed with various tube manufacturers.  Therefore, the exact date that the tube construction was changed varies with each manufacturer and there is no specific date on which the tubes were changed.  The military did not change the nomenclature on the 6146W when the actual design was changed because RCA originally claimed that the 6146B/8298A was directly interchangeable with the 6146A/8298.  Unfortunately, this was not true.

What happened in the 32S-1, 32S-2, 32S-3, 32S-3A, KWM-2, and KWM-2A was that when then 6146B/8298A and 6146W after mid 1964 were installed the ceramic neutralization capacitor in them "burned up".  This was caused by the inherent instability of the 6146B/8298A that requires a different neutralization circuit.  Because of this Collins Radio had to redesign the neutralization circuit of the 32S-3, 32S-3A, KWM-2, and KWM-2A all of which were in production at the time.  To allow the use of the newer 6146W tubes by the military in equipment already in the field Collins came up with a modification kit.  Although there were several component changes the easiest way to tell if the transmitter can safely use the 6146B/8298A series of tubes is to examine the neutralization capacitor.  If it is a ceramic trimmer then the 6146B/8298A series MUST NOT be used because there is well over a 95% chance that the neutralization capacitor WILL burn up.  If  the
 neutralization capacitor is an air variable then the 6146B/8298A series tubes will work fine.  However, due to power supply considerations the tubes still must be operated under 6146 or 6146A/8298 specifications.  That is the transmitter is not capable of 33% more power even though the 6146B/8298A can handle the power.

For the article that appeared in Electric Radio a while back, and which has been reprinted in several publications since then, go to

http://k9sth.com/Page_2.html

and scroll down to the heading "Articles".  The article is the first link below the heading.

As for the 4-diode balanced modulator:  The earlier versions of the 32S-1, 32S-2, KWM-2, and KWM-2A came from the factory with the 2-diode version.  However, in "mid stream" Collins did change the balanced modulator in all of those units to the 4-diode version.  If your 32S-1 has the 2-diode version it is definitely advantageous to go to the 4-diode version.

Glen, K9STH

Website:  http://k9sth.com


--- On Mon, 12/15/08, Dr. Gerald N. Johnson <geraldj at storm.weather.net> wrote:


And they will need a complete alignment after you replace all the tubes, including the three driver, pa, and feedback neutralization adjustments. Don't try to run any pa tubes but 6146, no A, no B, no W.
 
Now, some of the mods, like the 4 diode balanced modulator, are a serious improvement of the transmitter and covered in a service bulletin from the era of the transmitter.