[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

[exray]

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.

I hope you are aware that the FM band on the pre-war sets no longer exists!

[Guest]

absolutely. the FM band on the 42-395 goes from 20-something MHz to 98 or 99 MHz. with that said, i should be able to get my two locals on 92.1, 95.5, and 96.5.

[Ron Ramirez]

Actually, it is (or I should say WAS) 42 to 50 mc. What you see on your Philco's dial scale is something which was called "channel numbers." This was obtained by dropping the leading 4 and the decimal point, leaving the last two numbers. Hence, 42.1 mc became 21 on the dial, 49.9 mc became 99, etc.

[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 ?

[exray]

I think they are exactly the same but I stand to be corrected. I often see XXLs labelled as "XXL/7A4" which wouldn't be the case otherwise. I don't know why the XXL moniker was chosen unless it was for marketing purposes or maybe it had not been "accepted" by the tube-numbering convention at the time it was released into the field.

There's a number of 'merged' numbers in the tube world. As example, in old radios the 39 and 44 were combined since any differences between them were negligible. In that case it was a matter of different manufacturers submitting their new tube somewhat simultaneously and they turned out to be the same for all practical purposes.

12AU6 and 12BA6 are only closely related, not at all the same tube. One is a sharp cutoff and the other is a remote cutoff. Something like that wouldn't normally be noticed except in difficult AVC situations. Then they behave quite differently.

-Bill

[BDM]

When I received my grandfathers 42-350, I thought the same thing. I kept wondering why I couldn't receive the lower FM broadcast stations, only a steady hiss. Then one day I caught the MI state police (state police transmitter was within 5 miles of my home then). Had me confused as I knew the police weren't on the FM band. This was 30 years ago..........Then I learned the truth later in life....I was shocked LOL

[Texasrocker]

It is always a good idea to have a trusty ol GE Tube Manual to reference all the electrical characteristics when in question of "subbing" tubes. Even the best quality tube-testers have been known to "lie" to me before. The best test for any tube is in the circuit,( according to proper tube placement via schematic or tube layout chart),... unless the filament is open, and/or the wrong tube has been inserted by previous hacker. Also, on any loctal-type alum base tubes, it is imperative to make sure ALL the tube-pins are all clean and shiny before placing them in a tester or circuit chassis socket to be tested. Those type tube designs werent the best from the get-go. Corrosion o'plenty developed!! Sometimes, all it takes for a "test bad" tube to be qualified as a "test- good tube", is simply by careful cleaning of the tube pins, and resoldering/ reheating the tubes pins-tips! I have found many loose/or corroded pin connections in the past, and saved me from ordering un-necessary tube replacements just by taking time to do a good mechanical-inspection of the tubes that tested bad. The GE Tube Manual tells the truth on "subs".

[Fred R]

In early Philco books, I see the characteristics are slightly different between the XXL and the 7A4. Later books list them with identical characteristics. Like Randel, I have some XXL/7A4 branded tubes in my 42-350. You will not notice any difference. XXFM is identical to 7X7 so no problem there. FM1000 is unique and nothing replaces it. Another Philco equivelant is 14AF7 and XXD.

I built an FM converter off of Chuck's website. The pre-war Philco FM sets are just so-so performers. There is no limiter stage so they are sensitive to noise but it is neat to hear them play.

Fred