[Guest]

WOW, very interesting. so that explains why i do not get my local FM stations where it says 92-99 mhz states on the dial. thanks.

tubes are bad though, on my good calibrated tester under load, they are about half way into the red zone, not even close to the yellow.

does anybody have any answers to my original question about:

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 ?

before i recap, retube, realign, restore, can anybody help me out on using the XXL versus 7A4 issue as described above ?