[Boatanchors] AC/DC Series Filament Voltage Values

[jeremy-ca]

I like those tube choices, especially the 6SB7Y mixer which IMO is the best 
of the octal style. Ive used it to update many vintage radios.

The filaments can safely vary +/- 10% from nominal according to RCA tube 
manuals.

That 6SG7 may have its own tolerance issue, try another one in that socket.

If that radio will be used in the 12-30 MHz range Id suggest a 717A for the 
RF stage; another resistor calculation is required.

I own a National NC-46 and the PP 25L6's in that also have similar 
differences so it must be the nature of the beast.

Carl
KM1H



----- Original Message ----- 
From: "Bill Stewart" <bstewart at ipass.net>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, March 04, 2008 4:39 PM
Subject: [Boatanchors] AC/DC Series Filament Voltage Values


I am rebuilding a 1950 vintage 10 tube AC/DC superhet and have the series 
fil. sting hooked up and tested. I found some fil. voltage differences which 
brought up a couple of questions. There is a  46 ohm dropping resistor in 
series with the fil. string. (The schem. calls for a 42 ohm...only had a 46 
ohm in stock). I have the tubes in the sockets with just the fil. wiring 
hooked up...no other wiring has been done.

Here is the lineup (also the sequence they are wired in the string) and fil. 
voltages w/115vac input:
*25L6GT     27.8vac
*25L6GT     23.6
6SG7         6.3
6SG7         6.5
6SB7Y       6.47
6SG7         7.04
6SF7          6.13
6SL7          6.43
6SL7          6.17
6H6            6.34
*I swapped the two 25L6GT and the fil. voltages swapped also.

 As can be seen a couple voltages are off nominal (25 or 6.3 vac)...the 
25L6GTs, one high and one low.
 One 6SG7 is 7.04 vac while other tubes may be right on or vary only a few 
tenths.
1. Is there a general tolerence on fil. voltages on these type of tubes? Is 
any tube damage likely to those with the  higher fil. voltages...guess that 
would depend on the tol. Seems I read in 1920's QST's that low fil. voltage 
could also damage a tube...not sure about my memory or if that would apply 
to more modern tubes.
2. Is there a way to equilize the voltages on the higher voltage tubes 
without disturbing fil. voltages down stream.
3. Am I looking for a perfect world, which doesn't exist in this situation?

That does it for now...I'm sure I'll be asking more as the project 
continues. Many thanks for any help.
73 de Bill K4JYS



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