I have a 75A1 Collins that has/had a mod involving replacement of V-11 (6SJ7) with a 6SA7; the original tube-socket is rewired with new components, and with the index key broken off of the NEW tube, it's inserted off-index. I'm told it was a product-detector mod. Does ANYBODY have any info on this, or can tell me where to find any documentation, so I can fix it, or put it back the way it was. Tom