triode_ai,
That is a very well established fact:
One end of the filament's DC voltage versus the grid voltage; is very different than the other end of the filament's DC voltage versus the grid voltage.
So . . .
More of the total plate current comes from the negative end of the filament, versus the plate current that comes from the positive end of the filament.
Just a plain old Physics fact. There is no "Magic" here.
That effect is greater with a 10V filament DHT, versus a 2.5V filament DHT.
Think 2.5VDC, and type 45 tubes and Type 2A3 Tubes. Perhaps that is why they are so popular.
Happy continuing discoveries to all.
That is a very well established fact:
One end of the filament's DC voltage versus the grid voltage; is very different than the other end of the filament's DC voltage versus the grid voltage.
So . . .
More of the total plate current comes from the negative end of the filament, versus the plate current that comes from the positive end of the filament.
Just a plain old Physics fact. There is no "Magic" here.
That effect is greater with a 10V filament DHT, versus a 2.5V filament DHT.
Think 2.5VDC, and type 45 tubes and Type 2A3 Tubes. Perhaps that is why they are so popular.
Happy continuing discoveries to all.
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I had, when I started the DC heating, a different resistor from one side as the other to earth.
- As to discoveries; the total voltages did not match! I was puzzled a lot. [And found out that a big capacitor across the filament also was no way to go. Dumb sound.]
AC with a traditional filament centre potmeter-hum balancer, was nicer (warm) while the new age DC (gyrator/CCS) made the sound more flashy. But that is now well known.
- As to discoveries; the total voltages did not match! I was puzzled a lot. [And found out that a big capacitor across the filament also was no way to go. Dumb sound.]
AC with a traditional filament centre potmeter-hum balancer, was nicer (warm) while the new age DC (gyrator/CCS) made the sound more flashy. But that is now well known.
triode_al,
Also well known, but often ignored, is that AC filaments on DHTs cause Intermodulation at 2X the power mains frequency.
50/60Hz power mains put 100/120Hz intermodulation upper and lower sidebands on each and every instrument's musical note, and instrument's natural musical harmonics.
How far down those intermodulation upper and lower sidebands are, depends on the tubes filament volts (2.5V, 10V, etc.); the tube's bias voltage; the signal voltage amplitude that drives the grid; and how close the grid is to the contact potential voltage.
Lots of variables.
Also well known, but often ignored, is that AC filaments on DHTs cause Intermodulation at 2X the power mains frequency.
50/60Hz power mains put 100/120Hz intermodulation upper and lower sidebands on each and every instrument's musical note, and instrument's natural musical harmonics.
How far down those intermodulation upper and lower sidebands are, depends on the tubes filament volts (2.5V, 10V, etc.); the tube's bias voltage; the signal voltage amplitude that drives the grid; and how close the grid is to the contact potential voltage.
Lots of variables.
jhstewart,
The next time I am at my friends lab who has a HP3585, and if an amplifier such as a 2A3 or a 300B single ended amplifier with AC filaments is available, I will take a picture and post it.
Too many of the people I know now use DC powered 2A3 and 300B filaments.
The next time I am at my friends lab who has a HP3585, and if an amplifier such as a 2A3 or a 300B single ended amplifier with AC filaments is available, I will take a picture and post it.
Too many of the people I know now use DC powered 2A3 and 300B filaments.
Intermodulation distortion from AC heating of DHTs?
Here's a good example of AC heated 300B PP at 1W output is shown in the Post by Euro21 (picture at the right-side).
https://www.diyaudio.com/community/threads/sound-of-300b-set-by-satoru-kobayashi.258870/post-4030013
If you measure this intermodulation yourself, you can experiment with the music signal level, and see that it increases more rapidly as the signal amplitude reaches higher.
The post also shows a DC heated SE 300B at the left.
Not quite true. It can be easily shown in an experiment that the current at the negative end of a DC heated filamentary cathode is by the cathode current (the sum of screen, plate etc. currents) bigger than at it's positive end, regardless where the cathode is grounded. This is the cause why all data in the sheets relate to the negative filament pin. This means that if the positive end is grounded, the cathode current flows through the heater supply, hence requires some sophistication in the supply design - which is absolutely not the case if the negative end is grounded. Here a constant heater voltage of correct value suffices. This also means that the filament gets somewhat hotter at it's negative end. Anyway, I can't judge if this - small - temperature difference shows an impact on wear, lifespan etc. Furthermore this means that »balancing« a DC heated filamentary cathode by a potentiometer, a voltage divider etc. is an absolutely unneccessary gimmick with no outcome
Best regards!
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No - The anode current runs into the filament coating, and mixes with the heating current. This anode current does not enter by the filament substrate (wire) - but it does leave by it , and it does not have to divide between the two ends.
Kirchoff is fully respected, but you have to analyse it with all inputs and outputs considered.
Kirchoff is fully respected, but you have to analyse it with all inputs and outputs considered.
Uhhh, I protest ! Changing more than one variable at a time renders the outcome inconclusive. Two different amps, and not even the same basic topology. Looking more closely one might even argue that the apparently noisier of the two graphs actually has the noise content farther down.
The default gap between heater wires and anode wire of an average tube is quite big.
Even more so because we operate in vacuum.
So what you're saying would only be true I think if we're getting close to conduction between two pins.
Which is not a good place to be in to begin with at all.
Although I think that the creepage distance (over the bottom from the glass) would be sooner a problem.
Even in this case, we are in a super clean vacuum, so the so called pollution degree will be very close to be none at all.
Looking at the permissible maximum voltage there is quite a bit of safety margin on this (for good reason).
I can see problems happening in this regards with AC signals, since this creates an induced magnetic field.
Some proper literature or investigation would be more helpful.
Here I found some paper;
https://www.one-electron.com/Archives/Tube/TubeJournals/AE of AGOET 1956 Heater-Cathode Leakage.pdf
(see attachment below)
Although they clearly talk about heater-cathode leakage.
However, in the conclusion (page 4)
- On page 4 left side, they also clearly state that DC current would prevent such leakage problems!
Which is in line in what I was already expecting.
For AC signals, yes I can totally see small amount of leakage appearing.
For DC, there is just no path at all to go anywhere.
Besides this, I haven't being able to find any well documented papers, except for some subjective and individual tests with more like a hypothesis than actual proof.
So I am more than happy to believe this, if people can provide actual data and proper literature.
Btw, this still doesn't mean there is a difference at the end or beginning of the heater wire.
This induced magnetic field happens over the entire wire.
The only nuance that can be made is when there is bends and such.
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Heater-cathode leakage current, and your introduction to the article, and the article itself - are applicable only to indirectly heated valves.
The discussion in this thread is about Directly heated triodes - DHTs - where the cathode and the heating electrodes are one and the same. The oxide coating is applied to the heating filament directly, and the anode current mixes in the same metallic substrate as the heating current.
This direct connexion presents a number of problems, but more than offset by the opportunity for lower distortion.
A fragment from a power stage (GM-70 SE) shows the Directly heated triode schematically.
Heating current flows from pin 2 to 3.
Anode current arrives separately on the oxide coating, which is a separate "input" in Kirchoff terms, but the anode and heating currents are mixed in the filament substrate.
The discussion in this thread is about Directly heated triodes - DHTs - where the cathode and the heating electrodes are one and the same. The oxide coating is applied to the heating filament directly, and the anode current mixes in the same metallic substrate as the heating current.
This direct connexion presents a number of problems, but more than offset by the opportunity for lower distortion.
A fragment from a power stage (GM-70 SE) shows the Directly heated triode schematically.
Heating current flows from pin 2 to 3.
Anode current arrives separately on the oxide coating, which is a separate "input" in Kirchoff terms, but the anode and heating currents are mixed in the filament substrate.
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Not ideal of course, Ian!
-but it addresses the question asked (what the spectrum of AC-heat looks like). @6A3sUMMER has seen the IMD effect for himself, so I suspect he can vouch for its general appearance.
Using my own data for the excercise might have been unconvincing for others, given my activities in DHT heating supplies...
Highlighting a PP endstage is useful, I'd say, as there are some who believe that PP can cancel the IMD.
We're still talking about DC heated tubes with a filamentary cathode, aren't we? As I said and have proven experimentally, the cathode current occurs exclusively, always, toujours, sempre, immer and in every tube at the negative filament end, regardless where the filament is grounded, regardless of some »gap«, whatever you meant with this, regardless of the cathode to anode distance, and regardless of how many electrodes (grids) are in between.
Best regards!
That is not obvious from the start post itself at all!
With a direct heated tube, that is pretty obvious yes!
If we connect one side of the (shared) cathode/heater to the ground (or whatever), the current that goes through that node is the heater current + anode current.
That additional current is also significantly more than just a couple of micro-amps.
So depending on how those are connected, more current will run through it, if they are the same material/size, since P=I²*R , yes this will create more heat.
Again, laws of Kirchoff, world is luckily still not exploding..... 😅😀
(I was seriously getting worried lol)
But it's also obvious I think that this wasn't clear for a lot of people to begin with (reading what discussions are going on)
Maybe change that, I don't know all tube models on top of my head (an also not all abbreviations sometimes 😉 )
Because this is a total non-issue for indirect heated tubes.
So much for a miscommunication 😀 😀
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Read everything carefully.
Take all that you read with a grain of salt.
Many of us have written about the effects of AC on the filaments of a DHT.
Single ended is the most sensitive; but push pull is not always immune either.
1. Intermodulation onto each music note or test tone, is at 2X power mains frequency, that is one effect.
2. Another effect is Hum, caused by unequal distance of the filament to one side of the plate, versus the distance of the filament to the other side of the plate. The plate is magnetic steel (try the same tube type that has a carbon anode).
Like this:
The AC current in the filament causes a magnetic field. The filament wire is attracted more to the closer side of the plate, versus the larger spacing to the other side of the plate. That causes movement of the filament, changes the spacing to the control grid, and that modulates the plate current.
Proof: Did you ever wonder why the hum of one DHT tube nulls out with the adjustment of the Hum Pot; and another tube's hum can not be nulled out even if it is installed in the good tube's socket?
It is all in the spacings, equal or un-equal.
All tube elements spacings are equal, but some tube spacings are more equal than others.
Just my opinions and my experience
Take all that you read with a grain of salt.
Many of us have written about the effects of AC on the filaments of a DHT.
Single ended is the most sensitive; but push pull is not always immune either.
1. Intermodulation onto each music note or test tone, is at 2X power mains frequency, that is one effect.
2. Another effect is Hum, caused by unequal distance of the filament to one side of the plate, versus the distance of the filament to the other side of the plate. The plate is magnetic steel (try the same tube type that has a carbon anode).
Like this:
The AC current in the filament causes a magnetic field. The filament wire is attracted more to the closer side of the plate, versus the larger spacing to the other side of the plate. That causes movement of the filament, changes the spacing to the control grid, and that modulates the plate current.
Proof: Did you ever wonder why the hum of one DHT tube nulls out with the adjustment of the Hum Pot; and another tube's hum can not be nulled out even if it is installed in the good tube's socket?
It is all in the spacings, equal or un-equal.
All tube elements spacings are equal, but some tube spacings are more equal than others.
Just my opinions and my experience
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Back to DC heating of DHT filaments:
(To stay on the original thread title).
Example:
A 300B, and DC on the filament, with a self bias resistor and bypass capacitor that are connected between ground, and the other end of the self bias resistor and bypass capacitor are connected to the junction of two 25 Ohm resistors, with the other ends of the 25 Ohm resistors which are connected to the ends of the 5 Volt filaments.
Simple, you have seen that schematic multiple times, Right?
Well, at the ends of the filament . . . you will not measure bias voltage + 2.5V; and then bias voltage - 2.5V at the ends of the filaments, . . .
it shows an unbalanced current.
That is well established, Right?
Moving away from the thread subject of DC on DHT, to AC on DHT:
What is far less commonly known and understood, is when the same circuit has 5VAC applied to the 300B filament.
5VAC is 7.07V peak
You would expect the voltage at the ends of the filament to be the bias voltage + 7.07V peak, and bias voltage -7.07V Peak at the other alternation
Should be that way at either filament end, Right? . . . Surprise . . . Wrong.
Measure it, and the Peak voltages at any given filament end will not be +7.07V above the bias voltage, and -7.07V below the bias voltage.
It will be uneven at the + and - peaks of the AC voltage (uneven at one filament end; and also uneven at the other filament end).
Forget about the complex theory, just measure it, and believe.
If you do this measurement with AC powered filaments, it might help you to believe and understand about DC powered DHT (the original thread title).
Just saying
Have Fun!
(To stay on the original thread title).
Example:
A 300B, and DC on the filament, with a self bias resistor and bypass capacitor that are connected between ground, and the other end of the self bias resistor and bypass capacitor are connected to the junction of two 25 Ohm resistors, with the other ends of the 25 Ohm resistors which are connected to the ends of the 5 Volt filaments.
Simple, you have seen that schematic multiple times, Right?
Well, at the ends of the filament . . . you will not measure bias voltage + 2.5V; and then bias voltage - 2.5V at the ends of the filaments, . . .
it shows an unbalanced current.
That is well established, Right?
Moving away from the thread subject of DC on DHT, to AC on DHT:
What is far less commonly known and understood, is when the same circuit has 5VAC applied to the 300B filament.
5VAC is 7.07V peak
You would expect the voltage at the ends of the filament to be the bias voltage + 7.07V peak, and bias voltage -7.07V Peak at the other alternation
Should be that way at either filament end, Right? . . . Surprise . . . Wrong.
Measure it, and the Peak voltages at any given filament end will not be +7.07V above the bias voltage, and -7.07V below the bias voltage.
It will be uneven at the + and - peaks of the AC voltage (uneven at one filament end; and also uneven at the other filament end).
Forget about the complex theory, just measure it, and believe.
If you do this measurement with AC powered filaments, it might help you to believe and understand about DC powered DHT (the original thread title).
Just saying
Have Fun!
I would have thought the opposite . . . . . that you had done your research for development of the obviously successful supply circuits and you were basing your statements on first hand knowledge.
I find this hard to visualize. Can you show a test circuit scheme? Thanks.
This is a Physics "electric field" problem.
Anything less than that, prevents an understanding of the cause.
I guess someone needs to draw out something better than just a simple tube base diagram.
Want to try thinking about it in another way . . .
Draw out a DC powered filament, and a Plate, all in a glass envelope.
It does not get any simpler than a Diode!
No grid is necessary to prove both the theory, and the actual measured results.
"You should make things as simple as possible, but no simpler." - Albert Einstein
We are already at Post # 80
Good Luck!
Anything less than that, prevents an understanding of the cause.
I guess someone needs to draw out something better than just a simple tube base diagram.
Want to try thinking about it in another way . . .
Draw out a DC powered filament, and a Plate, all in a glass envelope.
It does not get any simpler than a Diode!
No grid is necessary to prove both the theory, and the actual measured results.
"You should make things as simple as possible, but no simpler." - Albert Einstein
We are already at Post # 80
Good Luck!