Edmund H. Ramm wrote:
Can anyone please enlighten me as to why there is an 8BN8 heated
by only 6.3 instead of 8.4v, i.e. underheated by 25%, used in R2B?

What forced the designers to accept cathode poisoning instead of
using a 6BN8 valve?

73, Eddi ._._.

Beats me. You sometimes see detector tubes like the old 6H6s run at
slightly lower filament voltage because apparently the noise drops off
significantly and the extra filament voltage serves no actual benefit in
a detector.
This doesn't seem to directly apply in the 2B. Two of the sections of
the BN8 are simple diodes and could easily be replaced with their
silicon sisters. The triode section, however, must be operating at
significantly lower gain as a result.
If I have the story correct, the early model 2As used a 6AV6 in place of
the BN8. Later 2A models used the 6BN8.
Apparently the guys at Drake saw a need to keep working on whatever
concern they were having and going to the 8BN8 made sense from a
production standpoint. It "evolved" rather than being "planned" so to
speak.
Or it could have been an issue with availability of the tubes at the moment.
At any rate, I don't think 'cathode poisoning' would be an issue in this
case.

Pure speculation from WX4A.

-Bill

I have 2, 2Bs and SN 12xx has a 6BN8 but chassis is stamped 8BN8 and SN 86XX
that has a Sylvania 8BN8.
Neither radio has been on in years but may be in the next year or so....
Just a wild guess, just maybe Drake got a real good deal on 8BN8s?
A serious overrun of series string TV tubes could have been a deal too good
to pass up?
Heathkit used to use surplus in their stuff and maybe Drake got a Heathkit
designer!
Just in case I don't get around to it, has anyone measured a difference
while using a 6BN8 instead?
tnx
hank wd5jfr

"Edmund H. Ramm" wrote in message
...

Can anyone please enlighten me as to why there is an 8BN8 heated
by only 6.3 instead of 8.4v, i.e. underheated by 25%, used in R2B?

What forced the designers to accept cathode poisoning instead of
using a 6BN8 valve?

73, Eddi ._._.
--
e-mail: dk3uz AT darc DOT de | AMPRNET:
If replying to a Usenet article, please use above e-mail address.
Linux/m68k, the best U**x ever to hit an Atari!

"Edmund H. Ramm" wrote in message
...

Can anyone please enlighten me as to why there is an 8BN8 heated
by only 6.3 instead of 8.4v, i.e. underheated by 25%, used in R2B?

What forced the designers to accept cathode poisoning instead of
using a 6BN8 valve?

73, Eddi ._._.
--
e-mail: dk3uz AT darc DOT de | AMPRNET:
If replying to a Usenet article, please use above e-mail address.
Linux/m68k, the best U**x ever to hit an Atari!

Somewhere buried in an old engineering text I've got a discussion of
grid-leak biasing a tube with higher-than-normal resistance (2x to 5x), then
reducing the filament voltage to reduce the contact potential and (I
believe) the tendency of the grid to go into secondary emission and block.
This was used both for noise reduction and to increase the circuit's input
impedance. I'd give you details but I can't find the @$#% book right now!

Is the tube grid-leak biased? Is the bias resistor significantly larger
than 1M-ohm?