Dear All,
I noticed that in more than one amp design the heaters of the drivers (12AX7 or 5842) are fed with a lower voltage compare to specs. Something like 5.5V instead of 6.3V. I understand that this is done intentionally. What's the theory behind this ? I could not find any reference about.
Thanks,
Davide
I noticed that in more than one amp design the heaters of the drivers (12AX7 or 5842) are fed with a lower voltage compare to specs. Something like 5.5V instead of 6.3V. I understand that this is done intentionally. What's the theory behind this ? I could not find any reference about.
Thanks,
Davide
Reducing heater voltage to reduce noise is another Hi-Fi myth with a kernel of truth. The original reference is "Signal, Noise and Resolution in Nuclear Counter Amplifiers" by A B Gillespie and refers to reducing grid current noise in electrometer amplifiers by reducing heater voltage (and therefore grid emission). The fact that reducing heater voltage crucifies gm and therefore raises shot noise within the valve was not a problem because noise in electrometer amplifiers was dominated by grid current. The only audio application that would benefit from this strategy is the head amplifier in a condenser microphone.
That's very interesting, learn something new every day! Long ago I decided to heat using the nominal rated filament voltage because I was unable to establish that there was any measurable reduction in noise levels in a phono stage I was working on at the time. Subjectively I was convinced there had to be (audio dogma?), and I thought that the test equipment I had was just too noisy to reveal the difference. (HP 400GL, 330D, etc..) Now it appears that probably wasn't the case.
Pardon my ignorance, but how does lowering the heater voltage lower the grid emission? Is the grid close enough to the heater to be heated to an extent where it emits electrons?
Also, I'm not clear on how lowering the gm would impact the shot noise. Shot noise is caused by the random movement of electrons, hence, is proportional to the anode current. Assuming the anode current is the same before and after lowering the heater voltage, the shot noise from the anode current should be the same. However, if lowering the heater voltage reduces the grid current, it would lower any shot noise caused by this current.
Am I understanding this correctly?
Thanks,
~Tom
Yes exactly- lower voltage means lower temperature, and grid current noise is one of the (usually minor) sources on noise in valves.
However, EC8010 may be being too sceptical. Lowering the heater voltage to reduce noise does not just appear in computing texts, but also Cherry & Hooper's Amplifying Devices and Low-Pass Amplifier Design, 1968.
Shot and flicker noise is proportional to cathode temp', bandwidth and anode current^2 [yes!], and inversely proportional to gm, so it is a balancing act of minimising the cathode temp and and anode current, without reducing gm by the same factor (trial and error).
In fact, it might be fairer to say that the belief that running valves at high anode current automatically leads to low noise is the real audio myth! (well ok, not a myth, but an over-simplification).
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Well, remember that in consumer audio circuits, the source impedances tend to be low (e.g., phono cartridges) so that grid current noise tends to be negligible. Thus it really is worthwhile to run high current in input stages.
Yes. That's why some low noise tubes use gold plating on the grid wires. Moreover, the grid can become contaminated by cathode emission, increasing its own propensity to emit.
Yes. That's why some low noise tubes use gold plating on the grid wires. Moreover, the grid can become contaminated by cathode emission, increasing its own propensity to emit.
I think the main advantage of running lower heater temps is extended valve life. There is a chart somewhere that shows the relationship. Running at 10% over will considerably lower valve life, running 10% lower will have the opposite effect. Tungsten bulbs experience the same behaviour and within Britain light bulbs sourced from Europe (all of them now) have considerably shorter duty life than in main land Europe.
I have also heard it said that triodes run under voltage are more linear - but I cannot remember where I heard that and would not take it as anything other than hearsay.
Of course when small signal valves have lives of 5-10yrs this may not be an issue for you - but it would be ill advised to run them over-voltage.
Shoog
I have also heard it said that triodes run under voltage are more linear - but I cannot remember where I heard that and would not take it as anything other than hearsay.
Of course when small signal valves have lives of 5-10yrs this may not be an issue for you - but it would be ill advised to run them over-voltage.
Shoog
An arm-waving explanation
I'm afraid I can't agree with the anode current squared statement. If you think about it, the anode current is composed of the sum of all those quanta of charge (electrons) and because they have a Poisson distribution, the variance is equal to the total number of electrons. Their standard deviation is equal to the square root of the variance, so the relative standard deviation (which is effectively the "noise") is the reciprocal of the square root of the total number of electrons. Thus, shot noise is proportional to the inverse square root of anode current. Add some fudge factors in to make the real world fit our system of units, note that equivalent noise resistance is a more useful parameter, and include the effect of voltage gain within the valve, and you find that the inverse square root relationship magically transforms into the well-known req.=2.5/gm.
Just to add a date; Gillespie was 1953 and dealing with seriously high source impedances, so grid current noise was the problem, not shot noise.
I'm afraid I can't agree with the anode current squared statement. If you think about it, the anode current is composed of the sum of all those quanta of charge (electrons) and because they have a Poisson distribution, the variance is equal to the total number of electrons. Their standard deviation is equal to the square root of the variance, so the relative standard deviation (which is effectively the "noise") is the reciprocal of the square root of the total number of electrons. Thus, shot noise is proportional to the inverse square root of anode current. Add some fudge factors in to make the real world fit our system of units, note that equivalent noise resistance is a more useful parameter, and include the effect of voltage gain within the valve, and you find that the inverse square root relationship magically transforms into the well-known req.=2.5/gm.
Just to add a date; Gillespie was 1953 and dealing with seriously high source impedances, so grid current noise was the problem, not shot noise.
I am only quoting Cherry, although it sounds like you are applying a simplified model of a triode.
Cherry provides the following for the shot/flicker mean-square equivalent input noise generator:
d(vin) = [3.5 x 10^-20 / gm + P(1 / f) ] df
The first part is shot noise and is related to the cathode temp (normally 1000K multiplied by the Boltzman constant and an empirical constant etc)
P is a flicker factor which depends on tube type, and is equal to KI^2 / gm^2, where K is a constant (something around 10^-8) and I is the anode current.
f is a given frequency.
df is the bandwidth.
Yes, a simplified model that does not allow for the smoothing effect of the space charge around the cathode. Nevertheless, the principles still apply when considering the shot noise due to the granularity of anode current.
As for flicker or 1/f noise, it varies wildly from valve to valve, and I'd be quite happy with any assertion that it was related to anode current - or the FT index, for that matter.
As for flicker or 1/f noise, it varies wildly from valve to valve, and I'd be quite happy with any assertion that it was related to anode current - or the FT index, for that matter.
6N4, Heaters Voltage 4.7V !!
An externally hosted image should be here but it was not working when we last tested it.
Will tube hiss increase if the heaters are run over-voltage?
I have a pre with 6 12ax7's in it, and have swapped the caps in the CRC filter heater supply to mundorf, low ESR. Originally, 5000 uF sprague atoms (unknown ESR and 35 year old), now 6800 uF Mundorf with 55mOhm ESR..
Also twisted the heater wires in the power supply a bit.... Hum decreased, (I assume the twisting) but tube hiss is louder at very extreme amplification - I assume the voltage. (At volumes well above listening levels -- it would clip the amp if turned that loud....). Previously, hiss was less at that volume....
Is it possible that the new caps have raised heater voltage too much? The value of R in the CRC is 0.39 Ohms (wirewound Dale).
I can't find a set of parameters for the Duncan amps PS simulator that make sense for the old values... since I am away from the unit, I am assuming all heaters are in series. Of course, they could be in parallel, or in series between with channels in parallel too or any other arrangement. All will change the load. Will clarify Tuesday night.
I'm just paranoid, as I left the house and left the unit ON.
I have a pre with 6 12ax7's in it, and have swapped the caps in the CRC filter heater supply to mundorf, low ESR. Originally, 5000 uF sprague atoms (unknown ESR and 35 year old), now 6800 uF Mundorf with 55mOhm ESR..
Also twisted the heater wires in the power supply a bit.... Hum decreased, (I assume the twisting) but tube hiss is louder at very extreme amplification - I assume the voltage. (At volumes well above listening levels -- it would clip the amp if turned that loud....). Previously, hiss was less at that volume....
Is it possible that the new caps have raised heater voltage too much? The value of R in the CRC is 0.39 Ohms (wirewound Dale).
I can't find a set of parameters for the Duncan amps PS simulator that make sense for the old values... since I am away from the unit, I am assuming all heaters are in series. Of course, they could be in parallel, or in series between with channels in parallel too or any other arrangement. All will change the load. Will clarify Tuesday night.
I'm just paranoid, as I left the house and left the unit ON.
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If the excellent discussion here has piqued your interest, then some links below are to a few period technical papers on the topic of noise and trying to sort out the influences.
In order: a link to a 1950 paper by Harris discussing models for induced grid noise. It refers to measured induced grid/shot noise levels versus anode current, and cites Bell from 1950 (which I don't have, and Bell seems to have published a few other papers on the subject). The next link to a latter 1955 paper by Taply and Macnee, with a bit more insight on influence of anode current. Last is Llewelyn from Bell Labs who does a nice comparison in 1930 of thermal and shot noise contribution versus 'filament heating power'.
http://dalmura.com.au/projects/A note on induced grid noise and noise factor.pdf
http://dalmura.com.au/projects/The nature of the uncorrelated component of induced grid noise.pdf
http://dalmura.com.au/projects/A study of noise in vacuum tubes and attached circuits.pdf
Ciao, Tim
In order: a link to a 1950 paper by Harris discussing models for induced grid noise. It refers to measured induced grid/shot noise levels versus anode current, and cites Bell from 1950 (which I don't have, and Bell seems to have published a few other papers on the subject). The next link to a latter 1955 paper by Taply and Macnee, with a bit more insight on influence of anode current. Last is Llewelyn from Bell Labs who does a nice comparison in 1930 of thermal and shot noise contribution versus 'filament heating power'.
http://dalmura.com.au/projects/A note on induced grid noise and noise factor.pdf
http://dalmura.com.au/projects/The nature of the uncorrelated component of induced grid noise.pdf
http://dalmura.com.au/projects/A study of noise in vacuum tubes and attached circuits.pdf
Ciao, Tim
Very interesting papers!
My own feeling is that reducing cathode temperature a little may reduce shot noise a little, provided that there is still plenty of spare emission to provide space charge smoothing. Beyond that it gets worse again. Remember that what is called shot noise really comes from the temperature of the cathode, although this is hidden inside the 2.5 of the 2.5/gm formula.
In real life, audio performance is dominated by 1/f noise anyway so the small reduction in shot noise will not be noticed.
My own feeling is that reducing cathode temperature a little may reduce shot noise a little, provided that there is still plenty of spare emission to provide space charge smoothing. Beyond that it gets worse again. Remember that what is called shot noise really comes from the temperature of the cathode, although this is hidden inside the 2.5 of the 2.5/gm formula.
In real life, audio performance is dominated by 1/f noise anyway so the small reduction in shot noise will not be noticed.
Hello,
From looking through the old papers and books to current. It looks to me that not much has been added to the thinking or documentation of tube noise since the days tube radio receivers were king, 1946 give or take some. Tubes like 6AK5, 6J4 and DF96 were the top of the game.
Not much is documented for low noise tube audio circuits.
This looks to be a challenge to use the current computer aided FFT noise analysis to sort out the fact from fiction, also toss in strong onion.
My thinking is that reduced heater voltages are aimed at the magic of reduced noise and / or extended tube life.
DT
All just for fun!
From looking through the old papers and books to current. It looks to me that not much has been added to the thinking or documentation of tube noise since the days tube radio receivers were king, 1946 give or take some. Tubes like 6AK5, 6J4 and DF96 were the top of the game.
Not much is documented for low noise tube audio circuits.
This looks to be a challenge to use the current computer aided FFT noise analysis to sort out the fact from fiction, also toss in strong onion.
My thinking is that reduced heater voltages are aimed at the magic of reduced noise and / or extended tube life.
DT
All just for fun!
My guess is that any further work on valve noise would probably have appeared in the context of instrumentation (e.g. naval sonar?), not audio. RF issues are almost a separate subject, as grid noise is worse and 1/f noise irrelevant when compared to audio.
You have to remember that people do research when they have funding. Funding comes from either industry or government. Industry are happy once they understand something well enough to make money from it. Government are happy once they understand something well enough to keep the military happy. After that they both lose interest. Further refinements either never happen, or are left to the amateurs.
I did see a paper (from the 1950s?) which suggested that partition noise in pentodes has a small 1/f element, whereas the standard model says this should be purely white. There are occasional theoretical papers on things like whether shot noise is a classical or quantum phenomenon etc.
You have to remember that people do research when they have funding. Funding comes from either industry or government. Industry are happy once they understand something well enough to make money from it. Government are happy once they understand something well enough to keep the military happy. After that they both lose interest. Further refinements either never happen, or are left to the amateurs.
I did see a paper (from the 1950s?) which suggested that partition noise in pentodes has a small 1/f element, whereas the standard model says this should be purely white. There are occasional theoretical papers on things like whether shot noise is a classical or quantum phenomenon etc.
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