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PP DHT output stage, is DC heater necessary?
PP DHT output stage, is DC heater necessary?
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Old 28th February 2021, 08:09 AM   #61
v4lve lover is offline v4lve lover  Netherlands
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PP DHT output stage, is DC heater necessary? PP DHT output stage, is DC heater necessary?
I have a weird transmitter tube on my desk.


1.1V 3.8A EEK..


YL1190 @ The Valve Museum
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Old 28th February 2021, 08:21 AM   #62
sser2 is offline sser2  United States
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6A3, the idea of filament movement by electrostatic or electromagnetic force is a brilliant one! That could be the real culprit, not heat modulation.
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Old 28th February 2021, 08:59 AM   #63
6A3sUMMER is offline 6A3sUMMER  United States
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How much magnetic field is there with a 1.5 Inch long filament wire that is carrying 1.2 Amps (like one of the filament wires of a 300B)?

The magnetic field rises and falls at the alternation rate, not at the frequency rate (there are 2 alternations per cycle).

And, there are several of those wires in a W or 2W filament.

Just sayin'
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Old 28th February 2021, 09:05 AM   #64
v4lve lover is offline v4lve lover  Netherlands
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PP DHT output stage, is DC heater necessary? PP DHT output stage, is DC heater necessary?
Is that a homework assignment? If i remeber correctly its the Lorentz force.


Lorentz force - Wikipedia


Edit i just realized its a pointless endeavour because you need to know what the spring tentsion holding up the fillament on top is exactly, and the exact elasticity of the tungsten wire used. Furthermore you can only model this quite easily for a single wire in a cylindrical anode because the distance of the greatest mass (The plate) is uniform. Wheras in Oval plate structures you would need to get fancy with multiple integrals whos output is squared....






You can ofcourse model this, and get a number, but then you have a number. And what does this number tell you? very little unless your the ultimate expert of course. But by that time you have spend so much time working it out. It would have been cheaper to use DC.

Last edited by v4lve lover; 28th February 2021 at 09:19 AM.
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Old 28th February 2021, 10:27 AM   #65
Rod Coleman is offline Rod Coleman  United Kingdom
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PP DHT output stage, is DC heater necessary?
Quote:
Originally Posted by sser2 View Post
That could be the real culprit, not heat modulation.
Thermal effects were tested experimentally by Dmitry Nizhegorodov in 2003.
He showed that even by increasing the frequency to 600Hz, the ac-frequency components remain at the same amplitude, and as such thermal modulation can be dismissed as cause.

On Correlation Between Residual DHT Filament Hum and AC Frequency


The true original of hum and IMD from ac-heating can be traced by careful inspection of the triode curves.
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Old 28th February 2021, 10:36 AM   #66
baudouin0 is offline baudouin0
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I would have thought the most likely explanation is the that the heater generates its' own magnetic field and simply wobbles around changing its' distance to the grid in places and by a small fraction.


There's also an effect that one end of the heater is high and the other end low - the lower end will produce disproportionally more current due to the non-linear transfer function than when both ends are at the same potential. This will produce 100Hz.



I agree that that 50Hz IMD can be noticeable if it exceeds a certain level. Below a certain level its masked by the tone itself. This depends on the frequency of the tone and the frequency of the hum.

Last edited by baudouin0; 28th February 2021 at 10:49 AM.
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Old 28th February 2021, 10:43 AM   #67
v4lve lover is offline v4lve lover  Netherlands
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PP DHT output stage, is DC heater necessary? PP DHT output stage, is DC heater necessary?
That could very well be, I read an article about AM interference caused by a light bulb: Exerp bellow. Full text here: Can incandescent light bulbs cause interference? | EMC and Regulatory Compliance



Recently the European market surveillance authorities (AdCos) on EMC have been investigating the potential of incandescent light bulbs to cause radio interference. The investigation began with reports that some incandescent lights may be the cause of reported interference to FM broadcast reception. The findings show that a specific type of incandescent bulb can interfere with radio reception. The type is referred to as a 'squirrel cage' Edison bulb in the report.


As an Electromagnetic Compatibility engineer, this came as a bit of a surprise to me. It's been accepted by EMC experts that an incandescent light bulb on its own wouldn't emit radio signals, in fact, the European standard covering interference from lighting states;
"incandescent lights "are not expected to produce electromagnetic disturbances"
yet this new data proves otherwise. So what's changed? After a bit of research I found the answer, nothing, we've just stumbled on an old problem inherent to the once obsolete but now trendy retro vintage style vacuum Edison bulb.


In the United States, the problem was known in the 50s to cause interference with T.V. reception. Although not in production in the U.S. at the time, there were still a number of the straight tungsten-filament Mazda bulbs in use. Tungsten-filament bulbs of the Mazda type were initially more costly than carbon filament bulbs but used less electricity. A direct comparison to the LED vs Incandescent light bulb today

This Popular Science article from 1953 titled "U.S. declares war on static", a campaign of hams and hobbyist sponsored by the FCC shows an example of how to identify T.V. interference from an obsolete light bulb. Today's filament light bulbs use an inert gas and relatively small coiled filament rather than a hard vacuum and a long zigzag filament structure.

An investigation of the science and physics behind the phenomenon indicate this is caused by the filament oscillations coupled with thermionic emissions from the heated filament in a hard vacuum, similar to properties found in vacuum tubes. This post "Rustika lightbulb FM measurements " provides data and analysis and links to other works.


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Old 28th February 2021, 10:58 AM   #68
Rod Coleman is offline Rod Coleman  United Kingdom
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PP DHT output stage, is DC heater necessary?
Quote:
Originally Posted by baudouin0 View Post
There's also an effect that one end of the heater is high and the other end low - the lower end will produce disproportionally more current due to the non-linear transfer function than when both ends are at the same potential. This will produce 100Hz.
<snip>
Yes, that's what I mean by inspecting the curves.

If the centre-point of the filament is the the effective return-point of the anode-current, the two ends swing (in opposition) in time to the line waveform. If everything were prefectly symmetrical, there would be no hum, and no line-derived IMD. But symmetry is not perfect...

It is not simply Vgk that varies with the line waveform; Vak does as well - and since the amplitude is non-negligible, even for 2.5V filaments, the effect is more complex than at first sight.
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Old 28th February 2021, 11:02 AM   #69
baudouin0 is offline baudouin0
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Screenshot from 2021-02-28 12-04-07.png


This distributed simulation using a 12AU7 with 'fake' heater as cathode shows up the problem nicely. Even through its perfectly balanced its doubles the frequency of the hum due to the non-linear grid voltage to plate current curve.

Last edited by baudouin0; 28th February 2021 at 11:09 AM.
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Old 28th February 2021, 11:16 AM   #70
Rod Coleman is offline Rod Coleman  United Kingdom
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PP DHT output stage, is DC heater necessary?
That's a good view of it!

You could probably extend the model to push-pull, and use varying degrees of side-to-side mismatch (maybe even by adding small values of Rk, different for each side), to find the effects of real-world valves.
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