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Old 5th May 2012, 11:08 PM   #1
nazaroo is offline nazaroo  Canada
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Default Heaters & Filaments: AC vs. DC

In an interesting post on another forum, I came across this disturbing phenomenon. I haven't confirmed the claim, but it seems to deserve examining.

IN my view this may only be relevant to cathodes in 'modern' beam power tubes, and not in classic thoriated tungsten or carbon heater/filaments.

Quote:
Originally Posted by Yankee-Rob
Just thought I'd pass this along...

We thought we'd box clever and cheat a little on a new series of amps - in order to reduce the build time, PT weight and size we thought we'd use 12VDC to heat the tubes... no need to laboriously twist heater wires... just hook the damned things up in series and forget about it... WRONG
Click the image to open in full size.:

Pre-amp and PI don't seem to be affected but power tubes don't like it... why? Something called 'plating' occurs... where the cathode becomes charged and thermionic emission becomes inhibited... takes a while to happen but eventually the amp starts sounding a bit thin and weedy... I'd been warned by the guy who helped us develop the DC heater supply that this might happen and sure enough it did...


So now we merrily twist our heater wires (we're use a very thin silver plated PTFE jacketed wire) and have just made up some jigs to knock up heater wire assemblies before offering them into the amps and hey presto.... very quiet operation and no plating... we're still acheiving the goal of smaller and lighter PT's by virtue of using 12.6VAC (in series of course) and there is no audible difference...
So according to this experimenter,
there is a definite cathode-smothering effect on some common beam tetrode/pentodes, traceable to D.C. heater-current.

The explanation appears to be 'magnetization/saturation' and/or a static field causing attenuation of cathode emission and plate current.

If this is true, it should be traceable to:

(1) materials used to manufacture the heater filament.

(2) physical size, length, and/or orientation/polarity of the cathode-heaters and/or cathode arrangement.


Any thoughts?

This seems important, as it implies that for yet another reason, we can't generalize regarding "best practices",
without specifying the actual tubes to apply the concepts to.

Last edited by nazaroo; 5th May 2012 at 11:12 PM.
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Old 6th May 2012, 12:04 AM   #2
nazaroo is offline nazaroo  Canada
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This is especially disconcerting, as experienced builders like Tubelab.com have noted the following:

Quote:
Originally Posted by Tubelab.com
Observations so far:

The 10 amp filament puts out a serious AC magnetic field that gets into everything. Hum is evident in the speakers and on the analyzer BEFORE you turn on the power supply. As soon as you plug in the filament transformer the hum appears. Moving the transformers or the tube around has no effect. The hum balance pot has no effect on this hum. DC filaments WILL be required.
This suggests strongly that D.C. heater supplies are actually preferred,
from the perspective of noise-free amplification.
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Old 6th May 2012, 01:16 AM   #3
popilin is offline popilin  Argentina
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I have not thought about the problem, well I think slowly.
But 10A AC surely is not the problem, from Maxwell's equations, we can see that the source of magnetic field due to AC is voltage, not current.
The noise is from the AC electric field.

Returning to the initial problem, how to charge a cathode if it is at a potential close to zero if fixed bias is used?
What is the potential difference of cathode to filament?
We should probably look that way, the DC electric field due to cathode-filament voltage can retain/push some electrons produced by thermionic effect.
With an AC electric field the effect is less perceptible.

Moral: Make the voltage cathode-filament negative, floating filament at a potencial higher or closer than cathode potential, and enjoy DC.


Best regards
Johann

Last edited by popilin; 6th May 2012 at 01:45 AM.
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Old 6th May 2012, 02:01 AM   #4
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Quote:
Originally Posted by popilin View Post
Moral: Make the voltage cathode-filament negative, floating filament at a potencial higher or closer than cathode potential, and enjoy DC.
That's what I was thinking. Elevating the heater circuit above ground(EDIT: above cathode voltage(s).) should remedy this,no?
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Old 6th May 2012, 02:05 AM   #5
popilin is offline popilin  Argentina
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Quote:
Originally Posted by DigitalJunkie View Post
That's what I was thinking. Elevating the heater circuit above ground(EDIT: above cathode voltage(s).) should remedy this,no?
Classical Electrodynamics says yes.
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Old 6th May 2012, 05:26 AM   #6
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Sometimes it is a little too easy to shoot from the hip when offering explanations about what really is just a snippet of information.

I don't think that one marketing blog lends much information to allow a reasonable appreciation of a technical issue. Not that that should stiffle discussion, but better for the discussion to start with eeking out what the blogger was actually discussing for starters, and have others described such an issue in the last 60 years.

I also suggest digging out those text books again about EM fields and induction, and also looking around for a few tutorials on the subject - as most will indicate it is actually the heater current that 'does the damage' so to speak.

And who knows what extreme configuration tubelab was referring to - a heater for one tube? or many tubes? I have a heater supply requiring more than 25A from a PT that also services HT, and there is no hum issue at all - but as I haven't indicated the configuration, it could be hard for you to guess the type of amplifier I'm referring to.

Ciao, Tim
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Old 6th May 2012, 07:57 AM   #7
popilin is offline popilin  Argentina
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Quote:
Originally Posted by trobbins View Post
I also suggest digging out those text books again about EM fields and induction, and also looking around for a few tutorials on the subject - as most will indicate it is actually the heater current that 'does the damage' so to speak.
You do not need a textbook for something as simple, furthermore Maxwell's equations are in the forum, here

Designing Transformers with J.C.Maxwell

If you can follow the math, you'll see that equation (21) clearly indicates that the AC magnetic field source is the AC voltage.
While the equation (28) tells us that the DC magnetic field source is the DC current.
As shown in equation (14), the AC voltage generates an AC electric field, which affects the grid, introducing noise.

But this reasoning is flawed, was thought to derive the equations of transformers.
Equation (4) shows a time-varying current produces a time-varying electromagnetic field, and then electromagnetic radiation, which induces noise through the grid.

I apologize, Tubelab.com reasoning is correct, only that it is poorly expressed.
It was not my intention to offend anyone, but it happens I'm going crazy doing several transformers, I'm kind of obsessed with transformers, the tree will not let me see the forest, also as I said, I think slowly.

Thanks Tim for making me see my mistake.
I think the following reasoning was correct.

Best regards
Johann
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Old 6th May 2012, 08:18 AM   #8
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by DigitalJunkie
Originally Posted by DigitalJunkie Click the image to open in full size.
That's what I was thinking. Elevating the heater circuit above ground(EDIT: above cathode voltage(s).) should remedy this,no?
Quote:
Classical Electrodynamics says yes.
Popilin and Junkie:

I think you've got the right idea, but one thing backwards:


If you want to drive away electrons from the heater,
and leave them available for tube current to plate,
then you want your heater more NEGATIVE, i.e.,
repelling the electrons and leaving them floating
in the vacuum ready for attraction to screen and plate.

Thus if the effect were purely electrical (electric field),
I would have to say you "float" your heaters (and entire D.C. supply) BELOW Ground, say about 50 volts!.

If they are separated from the Cathode electrically,
and you have a -ve Bias supply already, why not use it to
submerge your heaters -40 volts below ground?

But if the effect is in part magnetic (and lets see how this could come about,
other solutions must be added.

We all know that D.C. current magnetizes many metals and alloys,
i.e., iron, steel, nickel, etc. and that both heaters and cathodes
are made of various materials.
It may be that in some tubes, either the heaters, or the cathodes (or both) can be magnetized.

If so, the next question is:
How or why would this magnetization affect the electron release and/or
the electron cloud?

Finally, could the original poster be mistaken or unclear in
his description of the phenomena?
Could he be referring to the well-known (or supposedly better understood)
polarization of the heater in a Directly heated cathode,
whereby bias (and current) is affected across the filament surface?
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Old 6th May 2012, 01:03 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by popilin
from Maxwell's equations, we can see that the source of magnetic field due to AC is voltage, not current.
Any current generates a magnetic field, both AC and DC. So an AC current generates an AC field. In addition, an AC voltage generates an AC magnetic field.
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Old 6th May 2012, 01:21 PM   #10
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Quote:
the source of magnetic field due to AC is voltage, not current
With a shielded power cable with grounded copper shield, I can energize the inner conductor with voltages over 69 kV and would not be able to tell the conductor is energized using either electric or magnetic field sensitive equipment. So much for that magnetic field. Remove the shield and I can detect the presence of voltage. But pass 1A through that shielded cable, and I can clamp it and measure that current by means of the magnetic field which blows right past the copper shield.

I think the fact that Maxwell's equations recognize an AC magnetic field propagates an AC electric field and vice versa adds to the confusion on this subject, but IME you can simplify in practical application by knowing that current produces the magnetic field in significance. AC voltage produces an electric field in significance.

That is why you can shield with copper from energized wires, but twist to cancel magnetic fields in heater leads. If the heater leads are not carrying the copious current, they don't induce into the magnetics of the amp.
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