[Guest]

hello. i have a question.

why do some philco tubes have lettering like XXL, XXFM, FM1000, etc, etc and are also called 7X7, 7A4 , etc ? what was the reason for this ? was it a kind of marketing idea ? are the "letter named" tubes the exact same tube as the "numbered tubes" like shown below and in accordance with my tube substitution manual ?

XXL = 7A4
XXFM = 7X7

for example, 12AU6 and 12BA6 are interchangeable and will work in each other's circuit even though they have different characteristics.

is the XXL versus 7A4 ; XXFM versus 7X7, the exact same tube technically or do they have different characteristics like the 12BA6 and the 12AU6 but will work in each other's circuits ?

since i am going to recap and restore a 42-395 radio, should i insist on the XXL and XXFM labeled tubes ?

i am not one who likes to use substitutes. i like to keep everything original as possible.

since the FM in those early 40's sets can be very difficult, finiky, and problematic, i would like to keep the tubes as original as possible to rule out any varations.

thanks for any help.

steve

email: vinyltocdr@msn.com