[Boatanchors] AC/DC Series Filament Voltage Values

[Revcom]

Bill,
Sounds about right. Tubes are NOT precise devices, esp that old.
Some purists may argue you need to add resistance here and there,
but I'd say you are OK.  This would be that group that pays $1900.00
or more for a power cord I suppose. Adding a few ohms to the series resistor
is a good idea as line voltages are quite a bit higher new days then
when that unit was made.  Also changing tubes will change the meas-
ments again.

My AC mains are running 122.57V RMS today,  have seen it up towards 124-125
and as low as 118.5.  Depends if power company puts on any
capacitor banks or not to correct for induction on loads in area.

Rod
K0EQH


----- Original Message -----
From: "Bill Stewart" <bstewart at ipass.net>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, March 04, 2008 3:39 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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