IF tube oddity: 6CB6 and 6AG5

[JohnCT]

My Andrea TVL16 was originally designed with the three IFs to use 6CB6 tubes. The inside label and the appropriate SAMS call them out as 6CB6 for all three IFs.

In my example, I found the first IF to be a 6CB6, but the second and third IF tube sockets have 6AG5s installed instead - all tubes original to the TV (RCA 274 ID and date codes match all the rest of the tubes).

Closer inspection of the tube compliment chart pasted inside the cabinet shows that the original label was rubber stamped over and the second and third IFs show that the 6AG5 was indeed spec'd for those sockets in a later revision.

I have new tubes of both varieties but the TV does NOT want them swapped. Any swapping leaves a very bad ringing and other picture issues. Apparently, the gain and capacitance differences between the tubes means that a re-alignment would need to be done if I wanted to put all 6CB6s in or all 6AG5s.

It's interesting that only the second and third IF were changed to the 6AG5, not the first IF - so I assume this revision wasn't done for tube availability reasons.

Near as I can tell, the 6AG5 is a little lower in gain. Would it be correct to assume that the designers decided a little less gain was preferable in the second and third IF slot in this TV?

Would there be another reason this was done?

Any insight appreciated.

John

EDIT: looks like they're wired differently internally. I had seen a sub book where they would interchange, but apparently that data is wrong. So I guess it's not just alignment, Andrea must have wired the chassis differently for the revision. Still wonder why they would have changed tubes midstream.