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Old 31st May 2011, 06:54 PM   #41
M Gregg is offline M Gregg  United Kingdom
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As you say there is a difference with AC and DC heaters, its interesting that the DC components in the supply of the heaters can also make a difference!

This is 50Hz and again the heating and cooling is not the issue.

I think you would have to show a modulated idle current to prove this, I know it shows it in the link. However many here will argue the AC/DC heater issue also. I will look more closley at the link!
Best wishes.

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M. Gregg
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Old 1st June 2011, 08:08 AM   #42
tomchr is offline tomchr  United States
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Originally Posted by Rod Coleman View Post
You are right - and Dmitry Nizhegorodov's work proves it.
His "study" doesn't "prove" anything. Yes. He posted some oscilloscope shots on a website and attached some graphics. But not anything I would consider documentation. It's not even clear what the oscilloscope shots show, what the vertical or horizontal scales are. Gimme a break.

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His study shows that even when you null out the fundamental signal (with the usual pot) you get 2nd harmonics of the filament current at the output!
Of course you do. You create an artificial ground reference for the signal that couples capacitively into the tube. It's intuitively obvious that this will make the interference signal twice the frequency at half the amplitude. No surprises here.

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Check out his measurements and see for yourself how clearly he proves that this is a modulation effect and not a thermal effect.
Again, he doesn't prove anything. He merely shows poorly documented results of some measurements.

Rod, rather than postulating, hypothesizing, and hand-waving, would you please share YOUR results? Specifically, I would like to see THD+N vs frequency and output power with an "amateurishly implemented DC supply" and your constant current source. If this "current mixing effect" really causes the distortion to skyrocket like you claim, it would be easy for you to measure and show the difference with/without your regulator. Any computer sound card along with free FFT software would be able to pick this up. Without hardcore data, your posts are really just fear mongering and pseudo-science.

I don't understand why you are pushing this issue so hard. Last I posted anything related to DC filament supplies, I recall you did the same thing. Shouldn't it be up to the individual designer which circuit topology he chooses to use? What's your angle there? Just curious...

~Tom
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Old 1st June 2011, 10:18 AM   #43
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Originally Posted by tomchr View Post
His "study" doesn't "prove" anything. Yes. He posted some oscilloscope shots on a website and attached some graphics. But not anything I would consider documentation. It's not even clear what the oscilloscope shots show, what the vertical or horizontal scales are. Gimme a break.
Read what he says carefully, and it makes perfect sense. The scope shots show a residual 2nd harmonic of the filament voltage, and their phase relationship to the incoming filament supply waveform. The output levels are separately graphed, and presented in dBV, and plotted for various frequencies... I have attached the output graphs so you don't have to search for them.

What Dmitry presents us with is a study of output magnitude and phase of DHT amplifiers, when the filament is fed from a selection of frequencies from 20 to 6000Hz.

This is useful - to spell it out - to show that :

- there is a current-modulation effect on the output of DHTs;
- 2nd Harmonics are generated
- the effect is frequency independent.
- the phase relationship is stable and unchanged from the incoming signal.

The last two items are of great interest, since they prove that this is NOT a simple capacitive coupling effect.

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Originally Posted by tomchr View Post

Of course you do. You create an artificial ground reference for the signal that couples capacitively into the tube. It's intuitively obvious that this will make the interference signal twice the frequency at half the amplitude. No surprises here.
Intuition proved wrong by the measurements shown!

If it were a cap-coupled signal, the effect would increase with frequency, since the load at the grid is a resistor (50 to 100K).

What's more, the 8pF or so of input capacitance of the 2A3 is over 800Mohm of reactance at 20Hz. The grid has a typically 100K pulldown. And yet -40 to -60dBV of 2nd harmonic was measured at the amplifier output.

I would be interested to see your calculation that shows how such a feeble coupling could give such a large output as the -40dB V measured. For normally configured 2A3 amplifiers you'd get less than -140dB!


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Originally Posted by tomchr View Post
Rod, rather than postulating, hypothesizing, and hand-waving, would you please share YOUR results? Specifically, I would like to see THD+N vs frequency and output power with an "amateurishly implemented DC supply" and your constant current source. If this "current mixing effect" really causes the distortion to skyrocket like you claim, it would be easy for you to measure and show the difference with/without your regulator. Any computer sound card along with free FFT software would be able to pick this up. Without hardcore data, your posts are really just fear mongering and pseudo-science.

I don't understand why you are pushing this issue so hard. Last I posted anything related to DC filament supplies, I recall you did the same thing. Shouldn't it be up to the individual designer which circuit topology he chooses to use? What's your angle there? Just curious...

~Tom
NATURALLY, every designer should make his/her own choices about design work.... where did I try to step in the way of choice? But in an open forum, if you present a solution, and label it optimum, you have to be able to accept criticism of it. Please see the difference between criticism and arm-twisting - everyone is free to ignore either of us, if they don't like what we say.

Tom, my Angle is simple. The forum is here to present ideas, debate the origin of problems, and discuss solutions. A big part of that is constructive criticism, and critical analysis. Filament heating is one of my pet topics, and there is no shortage of DIYers here who can vouch for the effectiveness of my solutions. I have offered some criticism of a solution you presented, and you seem to have interpreted it as an attack, and say that you are "not interested" in discussing the cardinal points of my analysis.

As for presenting measurements, what reward would I have for all that considerable labour? I presented Dmitry's measurements, and you tried to misrepresent and belittle them, effectively labelling him a charlatan, despite it being a sincere and useful analysis.
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Old 1st June 2011, 02:10 PM   #44
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by M Gregg View Post
As you say there is a difference with AC and DC heaters, its interesting that the DC components in the supply of the heaters can also make a difference!

This is 50Hz and again the heating and cooling is not the issue.

I think you would have to show a modulated idle current to prove this, I know it shows it in the link. However many here will argue the AC/DC heater issue also. I will look more closley at the link!
Best wishes.

Regards
M. Gregg
How do you distinguish 2nd order harmonic noise from electrical induced emission from second order harmonic noise from thermal induced emission?

H. J. Van Der Bijl in The "Thermionic Vacuum Tube-Physics and Electronics" clearly explains the relationship between temperature and emissivity of at filament/heater. If the ac power through the the heater/filament does cause thermal changes it will be at a second harmonic of the line frequency and therefore indistinguishable from emissivity changes from second order harmonic emission from voltage e-field changes.

Furthermore, he explains the difference in emissivity from one end to the other of a heated filament/heater, and how a potential difference along the heater effects emissivity along the heater (his analysis was done on an AC signal iirc, but equally applies to DC).

Finally, given that there is a correlation between switcher noise on the filament and noise at the anode, what is the relationship between the 8mV 500KHz-800KHz switcher noise and audible sub-harmonics?

Are there any?

Last edited by TheGimp; 1st June 2011 at 02:23 PM. Reason: Corrected mv reading
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Old 1st June 2011, 02:48 PM   #45
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Quote:
Originally Posted by TheGimp View Post
How do you distinguish 2nd order harmonic noise from electrical induced emission from second order harmonic noise from thermal induced emission?
This question is addressed by Dmitry's tests: the thermal effects will be subject to a thermal time-constant, so increasing the frequency should decrease the effect. The results of his test were that the output was constant, regardless of frequency - which suggests zero thermal effect, 100% electrical modulation.

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Originally Posted by TheGimp View Post
H. J. Van Der Bijl in The "Thermionic Vacuum Tube-Physics and Electronics" clearly explains the relationship between temperature and emissivity of at filament/heater. If the ac power through the the heater/filament does cause thermal changes it will be at a second harmonic of the line frequency and therefore indistinguishable from emissivity changes from second order harmonic emission from voltage e-field changes.

Furthermore, he explains the difference in emissivity from one end to the other of a heated filament/heater, and how a potential difference along the heater effects emissivity along the heater (his analysis was done on an AC signal iirc, but equally applies to DC).
This is a relevant fact, and contributes to unwanted artefacts in the DHT output.
These add to this conduction-generated noise effects.

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Originally Posted by TheGimp View Post
Finally, given that there is a correlation between switcher noise on the filament and noise at the anode, what is the relationship between the 4mV 500KHz-800KHz switcher noise and audible sub-harmonics?
Usually, a switching chip responds to changes in the input voltage by altering its PWM duty ratio. If there is noise and ripple in the raw dc supply, the PWM ratio bounces along to this waveform. An amplitude-to-pulsewidth conversion in other words
This means it is delivering energy to the output filter in pulses with low-frequency content, not just the 800kHz carrier frequency. The amount in the residual just depends on the quality of the supply input, and the (analogue) loop response of the switcher.

This is added to other noise leakage paths, eg:
during the ON time portion of the PWM, the input is connected to the output by a low impedance path, allowing other conducted emissions through to the filament in a piece-wise fashion.
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Old 1st June 2011, 03:55 PM   #46
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Originally Posted by tomchr View Post
I don't understand why you are pushing this issue so hard. Last I posted anything related to DC filament supplies, I recall you did the same thing. Shouldn't it be up to the individual designer which circuit topology he chooses to use? What's your angle there? Just curious...

~Tom
Because he sells pc boards of his design.
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Old 1st June 2011, 03:59 PM   #47
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Because he sells pc boards of his design.
Thanks for the plug.
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Old 1st June 2011, 04:05 PM   #48
tomchr is offline tomchr  United States
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Originally Posted by astouffer View Post
Because he sells pc boards of his design.
Bingo! I would have preferred if he'd be willing to admit this himself. If his customers can hear a difference between constant voltage and constant current, more power to them (and him). But don't claim there is a *HUGE* quantifiable difference without backing it up with data. That's all I'm asking.

Some people also swear that they can hear a difference when they suspend their speaker cables on myrtlewood stands. Any other kind of wood or other inferior dielectrics just don't sound the same.

~Tom
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Old 1st June 2011, 04:39 PM   #49
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by tomchr View Post
Some people also swear that they can hear a difference when they suspend their speaker cables on myrtlewood stands. Any other kind of wood or other inferior dielectrics just don't sound the same.

~Tom
Can you prove it...LMAO

In the red corner we have....LOL

Regards
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Last edited by M Gregg; 1st June 2011 at 04:41 PM.
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Old 1st June 2011, 04:46 PM   #50
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Rod Coleman View Post
The scope shots show a residual 2nd harmonic of the filament voltage, and their phase relationship to the incoming filament supply waveform.
All I see on that page is the blanket statement that some residual of the heater waveform will be present somewhere in the amplifier. I'm not contesting that. In fact I'm supporting that.

You make the assumption that what he is measuring is due to this "current mixing effect" that you have thought up. Based on my understanding of how vacuum tubes work I just don't see that argument as being rooted in anything that goes on physically in the tube.
Yes, the cathode current provides electrons for the filament coating to release. The filament coating releases electrons because it's heated by the filament current (DC or AC). But the process by which these electrons are released to the space-charge cloud surrounding the cathode is completely random. It's a stochastic process. To argue that you can distinguish one electron from another and follow this one electron as it travels happily from the heater wire to the anode is absurd and not supported by science.

Quote:
Originally Posted by Rod Coleman View Post
NATURALLY, every designer should make his/her own choices about design work.... where did I try to step in the way of choice? But in an open forum, if you present a solution, and label it optimum, you have to be able to accept criticism of it. Please see the difference between criticism and arm-twisting - everyone is free to ignore either of us, if they don't like what we say.
Rod. At no point did I say my solution was optimum. If I did, please point to it. I am willing to accept criticism if it is rooted in science. I am not willing to accept fear mongering, snake oil, and pseudo-science that is deliberately targeted to drive customers towards one solution over another.

You claimed earlier that a DC heater supply would cause all sorts of nastiness and result in severe degradation of the THD of the amplifier. You mentioned THD in excess of 4 %. Please back this up with data. If you are unwilling to take this data, would you allow me to borrow one of your regulators? I am perfectly willing to take the measurements for you and provide a true A/B comparison.

I'm measuring 0.05~0.06 % THD+N with DC heating at 1 kHz, 1 W into 8 ohm. This is measured on this 300B amplifier speaker output using an HP8903A audio distortion analyzer. Once I get further with this filament heater design, I will be posting THD+N vs frequency and output power just like I have for any of the other amplifier designs I've posted here. I will also be posting complete schematics of the amplifier and filament regulators as I have for my other designs.

Enjoy.

~Tom
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