Tube filiment voltage ???

[Eric]

>>      I remember (well I read that... I'm not
THAT old...) that ENIAC
>> used a reduced filiment voltage to prolong the
life of the tubes so that
>> the MTBF would be acceptable.


Yes, they ran the 6.3v tube heaters on 5.8 volts
routinely.
Remember, ENIAC was made entirely of cheap 1944
radio
tubes--6L6s, 6V6s, 6L7s, etc. They were NOT meant
for
such use! Starting in 1947, special computer-rated
tubes
were introduced which were 1000 times more
reliable than
ENIAC tubes.

How do I know this?
1) I am a volunteer at the Computer Museum History
Center.
  www.computerhistory.org
2) I am currently retubing their small piece of
ENIAC with
  the correct tubes............453 of them in a
single rack.
  (You can see a picture of it in the November
issue of WIRED.)
3) I have prepared a computer tube exhibit for the
museum, showing
all kinds of these special tubes. You would be
amazed to see how many
types there were.......hundreds, some very exotic.
4) Have also been writing a series of articles for
Vacuum Tube Valley
magazine about tube computers.

Does that qualify me?

>> Any audio reasons to give this a try ??? Neat
new sounds ??? Waste of
>> Time ???


You can get more life out of small preamp tubes by
running the heaters colder.
Depends on type. 15% low can extend the tube life
by 2-3 times, IF your
circuit works OK at the lower temperature. Most of
my tube synth circuits will
work fine this way.

The rec.audio.tubes resident Steve Bench claims
that he has
run some tubes on about 3.5-4 volts---lowering the
heater temperature SOMETIMES lowers the 2nd
harmonic distortion dramatically. Esp. in
direct-heated tubes

There's more info at his website , look under
"starved filaments".
http://hometown.aol.com/sbench101

Lower heater voltage is not always recommended
with power tubes--
you'll get less power out, and some need the high
temperature to
activate the getters and keep the vacuum clean. It
seems to work better with
DH tubes than those with cathodes.

>BTW. Is really AC needed or would DC suffice?
I've kind of woundered about that
>for some time.

AC is usually better, esp. with directly-heated
filaments--the AC
distributes the emission along the length of the
filament better.
Hum reduction is a matter of design and of using
hum-balancing
pots or other methods.
DC is OK with indirect-heated tubes, such as
12AX7, 6L6 etc.

--Eric