[Drake] Drake 2-C question

[bonddaleena at aol.com]

Hi. I received an e-mail from a chap that had recently acquired a 2-C 
receiver. It 'kinda' works on one band. In suggesting some of the usual 
suspects, a strange fact has emerged.
His radio uses a 6BZ6 tube as the RF amp. My radio has a 12BZ6. He sent 
me a pix of his chassis and it is indeed stamped "6BZ6".
My info and schematics indicate a 12BZ6 was used there. Now the 
filament transformer does have a center tap, so it's possible that 
Drake made a running change.
My ser # is: 085X and his is: 163X.
He is going to measure the fil voltage and get back to me.
If Drake did indeed switch to the 6BZ6 later in production, what would 
be the reason?
Cost, performance, heat, availability?

This one's got me scratchin' my gray head...

thanks!

ron
N4UE