[Boatanchors] Valiant 6146 Question

[Jim Wilhite]

You made me think a bit more Glen.  I dug out my old ARRL handbook that had tube specs in it and looked up the 6146.  The Interelectrode Grid 1 to Plate in the old tube is .22 pf and in my later book (c. 1970s) it is .24 pf.  The Grid to cathode in the old is 13.5 and the newer version is 13.0  The plate to cathode is 8.5 in the old and 8.5 in the new.  

So there were redesigns of the tube later along accounting for a small change in neutralizing and, as you describe in the Collins equipment particularly, the cap was already marginal.  

Jim
W5JO
  ----- Original Message ----- 


  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.
  Glen, K9STH