I've never seen an example of this. Can you provide a specific model?
Both tubes are dual diodes, and they both have standard 6.3 v 300 ma
filaments. A one ohm resistor would drop 0.3 volts. A ten-ohm
resistor would drop 3.0 volts, which would put the filament way below
nominal operating voltage.
It's remotely possible that they wanted to reduce the filament voltage
and hence the cathode temperature to reduce noise output in detector
circuits, but that's some really wild speculation on my part.
In 48 years of repairing radios, I've never encountered an open
filament on either type of tube (that I recall), so I wouldn't
consider that to be a common problem.
73 de Leigh W3NLB
When the normal current is very small, these tubes can build cathode
interface and begin to act strangely. Lower cathode temperature staves off
this effect and prolongs normal operation.
On the other hand, when used as a rectifier or relay protector or other high
current function, the resistor should be omitted.
Hallicrafters did it on the S-53A and SX-62, if I recall. Probably others.
> Both tubes are dual diodes, and they both have standard 6.3 v 300 ma
> filaments. A one ohm resistor would drop 0.3 volts. A ten-ohm
> resistor would drop 3.0 volts, which would put the filament way below
> nominal operating voltage.
> It's remotely possible that they wanted to reduce the filament voltage
> and hence the cathode temperature to reduce noise output in detector
> circuits, but that's some really wild speculation on my part.
I guessed the same thing, but I don't know for sure, either.
> In 48 years of repairing radios, I've never encountered an open
> filament on either type of tube (that I recall), so I wouldn't
> consider that to be a common problem.
> 73 de Leigh W3NLB
National NC-98: 6AL5, 5 ohms
Hallicrafters SX-43: 6H6, 6AL5, 6.8 ohms each
Hallicrafters SX-71: 6H6, 6AL5, 6.8 ohms each
Hallicrafters SX-99: 6H6, 68 ohms (must be 6.8 ohms)
Canyon Lake, TX
I saw a circuit in one of the Popular Science books for using a 25Z5 as a
detector diode in place of a crystal detector. It used a bell transformer and a
slide adjustable resistor to set the filament voltage around 6V to alter the
characteristics enough for it to function as a detector.
The same book has a circuit using a 6H6 & a filament transformer to make a
6-8 Ma 90 VDC supply "for a 1 or 2 tube receiver", and a project that specified
using a "bad" 6J7/6K7 as a power rectifier in an RF preamp.
"Brian" <k6...@n2.net> wrote in message
Of course, on AM with all the OTHER awful noises, who's to hear it
over 3 miles from the 5KW transmitter??
On Wed, 14 May 2003 08:27:26 -0700, "Bob" <bal...@cnmnetwork.com>
Extremely intelligent life must exist in the universe.
You can tell because they never tried to contact us.
A Google search yielded this interesting paper, a survey of vacuum
tube Spice models, some of which involve diode contact potential:
Now, take an old speaker magnet and move it around the tube while
listening to the noise on the amplifier. The magnet will be able to
REALLY jerk free electrons away from that cathode and it'll get a LOT
louder in certain positions.....(c;
Ah, those old classroom tricks to keep 'em from falling asleep in
class......Most never HEARD an electron crash into a plate, before.
Hey, izzat the way they made the 'jet plane' phasing noise in 'The Big
>Ah, those old classroom tricks to keep 'em from falling asleep in
>class......Most never HEARD an electron crash into a plate, before.
That reminds me...
When does the W4CSC troubleshooting class continue?
I once had a GR noise generator that used a miniature tube mounted
inside a magnet as the noise source. Must have worked on the same
Magnets don't attract electrons, but they can change their path when
they move, as in a CRT. Does the magnet just focus the electrons onto
I would assume they bend them into a spiraling sheet. That's what
they do inside a magnetron. The spiraling sheet of electrons spins
round inside the maggie giving up energy to each of the maggie's
microwave cavities as it passes and being modulated at microwaves by
it. One of the cavities has the output coupling loop in it where the
BIG power streams out on an unsuspecting world.