Filament Resistor with 6H6 or 6AL5

[Brian]

In several radio schematics, mostly boatanchors, I've noticed a
resistor of anywhere from a few ohms to several dozen ohms in series
with the filaments of a 6H6 or 6AL5 dual diode, even when the tube is
powered from a 6.3-volt transformer winding. The RCA data sheets don't
mention the need for this resistor. Are these tubes particularly
susceptible to popping the filament at power-up, or what?

Brian

[Leigh W3NLB]

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

[Bob]

The purpose of the resistor is to reduce noise and to reduce the 'contact
potential' the tubes produce. This is due to some conduction of the tube
with no voltage applied.

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.

Bob

[Frank Dresser]

"Leigh W3NLB" <w3nlbRE...@amsat.org> wrote in message
news:58h4cvs1qj9vp154n...@4ax.com...

>
> I've never seen an example of this. Can you provide a specific model?

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
>

Frank Dresser

[Walter Sutton]

National HRO-7 and other HROs. Both 6H6s (dectector and NL)/

Walter

Leigh W3NLB <w3nlbRE...@amsat.org> wrote in message news:<58h4cvs1qj9vp154n...@4ax.com>...

[Brian]

> I've never seen an example of this. Can you provide a specific model?

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)

Brian

[Ed Engelken]

Leigh W3NLB <w3nlbRE...@amsat.org> wrote in message >

> I've never seen an example of this. Can you provide a specific model?
>

==================================================================
Well, how about the Hallicrafters S-53, S-40B, SX-42, SX-62A, National HRO-60,
NC-125, and probably others. Those are the ones I currently have on hand.

Best Regards,

Ed
Canyon Lake, TX

[Neutrodyne]

>Subject: Re: Filament Resistor with 6H6 or 6AL5

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.
Neutrodyne

[Uncle Peter]

You beat me to this post. I was wondering why my S-53 had a
filament dropping resistor on the 6H6 myself!

Pete

"Brian" <k6...@n2.net> wrote in message
news:1ef466b6.03051...@posting.google.com...

[Larry W4CSC]

I agree. These noisy tube detector diodes are at the lowest signal
point in the big audio amplifier chain. Any shot noise a really hot
cathode creates to that plate is gonna be a real hiss by the time it
gets to a speaker between words in a quiet play.

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>
wrote:

Larry

Extremely intelligent life must exist in the universe.
You can tell because they never tried to contact us.

[Brian]

> The purpose of the resistor is to reduce noise and to reduce the 'contact
> potential' the tubes produce. This is due to some conduction of the tube
> with no voltage applied.
>
> 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.

Fascinating.

A Google search yielded this interesting paper, a survey of vacuum
tube Spice models, some of which involve diode contact potential:

http://digilander.libero.it/paeng/spice_models_for_vacuum_tubes.htm

Brian

[Larry W4CSC]

Have you ever seen it for yourself, Brian? Take a 6AL5 and apply
6.3VAC to it. Put your DVM or VTVM from cathode pin to plate and read
the voltage caused by the hot electrons banging into the plate that
were boiled off the cathode. As there is no electrical connection
between the filament pins and the cathode or plate, hook an audio
cable up from ground(cathode) to center lead(plate) and plug it into
your stereo amp. If it's not loud enough in the AUX jack, plug it
into the mag phono jack for that extra stage. It's a great white
noise generator to fool around with your equalizer slidepots...(c;

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.

Larry

[Bill Sheppard]

>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.....

Hey, izzat the way they made the 'jet plane' phasing noise in 'The Big
Hurt'?<g>

Bill (oc)

[Joe Bento]

On Thu, 15 May 2003 02:43:59 GMT, nos...@home.com (Larry W4CSC) wrote:

>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?

Joe

[Brian]

> 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;

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
principle.

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
the plate?

Brian

[Larry W4CSC]

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.