Effect of Regulated DC Filament Supply

[DavesNotHere]

the term rf is loosely defined comes from it being higher than the audio region.


inrush current is the cause of the voltage foldover, but if you supply the regulator with 2-3 times the regulated voltage the foldover goes away because the series regulator doesn't have to work as hard. thats why its better to use the proper transformer instead of fidgeting with something that wasn't design to do in the first place.

and yes, the transformer has to be chosen wisely, at least 25%. but putting on a separate transformer does has its advantages in physical tests.
And that is if the transformer has a heater secondary winding voltage 2-3 times higher than what you need.

btw I don't build cheap stuff either.

 

[DavesNotHere]

Not buying it, Dave, sorry.

You are technically correct - there must be thermal noise from a regulator transistor, and internal zener-breakdown noise from the voltage reference. Vnoise(z) is on the order of low millivolts, typically. Schott noise from the transistor similarly low. A capacitor on the output squelches both very effectively, if it is of concern. Alternately, small series inductors would do. However the whole idea really would benefit from a simple A/B switch test. I would put money on "can't be heard" by anyone, no matter how golden the ear.

Now, whether high-gain tubes have their own noisy plate dynamics is a different matter. One nominally tries to avoid the effects of Miller capacitance, so the idea of ADDING plate-to-grid low-value capacitors is a bit ... counter-intuitive. Yet, if the goal is just to ensure that the tube has a natural roll-off above say "50 kHz" or something, then I can see the utility of clamping the higher frequencies through such an added capacitor. Its one thing to have to deal with a parasitic capacitance that one can't do much about (Miller), and its another to take it by the horns, and decide to engineer a particular solution.

PS: you've made a good argument for using multiple stages instead of fewer, with the problems of very-hi-mu tubes being stated thus. Moreover, using multiple stages diminishes the distortion effect of non-cascode connected triodes in the front stages. The only reason to use fewer tubes are [cost] and [appeal to audiophiles who desire simplicity]. I prefer more tubes, lower amplification factors. I must be an old fashioned fuddy-duddy, I guess.

GoatGuy

no you not a old fuddy duddy . the trick is balance of parts count.
and achieving how should I dare say "efficiency" in each stage weather it is one or a thousand tubes in the stage.

 

[DF96]

all high gain tubes have natural oscillations on the plate due to the acceleration of electrons hitting the plate. when the rf energy gets too great it causes a decrease in gain.

historically the case of the 417A preamp tube when used as a high gain amp like a phono stage has a natural tendency to cause poor gain because of this phenomenon. It is usually fixed by injecting positive feedback from plate to grid by a small value capacitor.

You may be talking about secondary electrons. Not sure. Nothing to do with RF energy. Unlikely to affect audio as the frequencies are so much lower. A capacitor from plate to grid would probably be negative feedback.

You may, alternatively, be talking about RF parasitic oscillation. Nothing to do with the heater.

A sensible reference might be helpful, otherwise I might assume you are merely repeating an audio myth.

 

[DavesNotHere]

I don't do audio myths, sorry Mr. Sceptic

its in the rf region. its was also called tube hash a long time ago.
do you ever really look to what goes on with an oscilloscope?

and the noise is caused by several factors. not just one.

here is some historical data from 1947 RCA Engineering papers.




http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=34&ved=0CFMQFjADOB4&url=http%3A%2F%2Fwww.radiomuseum.org%2Fforumdata%2Fusers%2F4942%2Ffile%2FRCA_PapersOnElectronTubesVol2_288-300_noise_TEXT.pdf&ei=gTxNUdKVCIKm8gT2l4C4BA&usg=AFQjCNHyewjyY8ZeiLsdnoVVhf_jVOrALQ&sig2=D2ex2oIypcN8AVtJqWuXng&bvm=bv.44158598,d.eWU&cad=rja


Of course I could find Drs. Johnson, Nyqist, Fry and Schottky data on that subject as well.

of course the research done at the Institute of Radio Engineers in 1980 that retouch on the subject:

http://www.google.com/url?sa=t&rct=...=wKMtASPP8iHL81huHHAE5Q&bvm=bv.44158598,d.eWU

you'll have to look next time.

 

[DavesNotHere]

and yes it is a form of negative feedback going to plate to grid.

 

[hpeter]

Well, it is neither to uncork champagne.
Not if my eyes or the distance, but this here is called ripple.

not sure if actual circuit noise, or just related to digital scope himself

 

[Johan Potgieter]

To my experience we are discussing a non-problem here. Yes, I do look with an oscilloscope (20MHz) at a dc heater supply and have never found anything there but dc. The normal regulators are analogue devices. They are occasionally used as CCS devices in the tail of some phase inverters (although I prefer a resistor) and any audible effect they might possibly have would have been orders more - er - audible there.

I really think that any 'interference' to possibly have any audible effect, would come through the 'raw' ac heater supply which has served us all these years, far more readily than whatever coming from an LM*** or similar regulator, correctly used. I repeat that my main use for such a regulator is that it is a more effective ripple filter than an equivalent large expensive capacitor. In addition it regulates .....

GoatGuy, again I agree with your views as to constant power supplies (in the exact sense of the term) for heaters. But again tubes seem to generally give us good service with good old 6,3V heater supply, or a smidgen lower for pre-amps. I am lazy. I heard it said that lazy people make good engineers. Then I was a very good EE.)

(And yes, I have been active in the r.f. field up to 150MHz and am sensitive to effects up to there.)

 

[Johan Potgieter]

first factor is thermal noise from the series historically the case of the 417A preamp tube when used as a high gain amp like a phono stage has a natural tendency to cause poor gain because of this phenomenon.

This can occur in most tube RIAA stages using feedback but has nothing to do with r.f. effects. At h.f. the loop gain simply becomes to high for stability requirements to be met. This is simply cured by a small resistor in series with the h.f. cut capacitor in the RIAA network (effect outside the audio range) or as said by a Miller capacitor on the second tube, cutting h.f. loop gain which is not used anyway.

 

[hpeter]

or as said by a Miller capacitor on the second tube, cutting h.f. loop gain which is not used anyway.

big enough grid stopper has similar effect

 

[TonyTecson]

i was given a Broskie CCD 6sn7 preamp with hum issues to repair.....yes it had hum issues despite dc heaters, so what i did was to remove the filament dc supply and just hooked-up the ac filament leads, lifted the filaments to approx. 1/4 B+ and viola, hum is gone.....

i also found out that the filaments were actually running 5.2 volts ac, yet the sound is still very good....

 

[DF96]

here is some historical data from 1947 RCA Engineering papers.

I am aware of that stuff. It is talking about transit time phenomena, which only have effect at higher RF frequencies. Completely irrelevant for audio, as you would know if you have read and understood the papers.

 

[overmind]

I changed the Regulator to Motorola 7806 with a diode, MBR160, connected between adj pin and ground. Everything else is the same.

I like this sound better than LM317, much better even thou no burn-in yet.

the order of my reference of filament supply:
Unregulated DC
AC
Regulated DC using 7806+ Diode
Regulated DC using LM317

 

[Jay]

The heater supply imo is the bottleneck of most tube preamp implementations. For me, choke (CLC) input is a must. But finding the right values are difficult

 

[trobbins]

DC regulation on heaters. Its not a bad concept. but the things that have to be taken in account.

Perhaps for completeness:
4. High crest factor current transient drawn from AC supply through PT when heater circuit diodes conduct. Coupling in to HT windings of this di/dt, and also of any dV/dt disturbance of the heater winding wrt 0V - all lumped as rectification noise - and the likely ability to couple some rectification noise in to input stage grid or cathode in some manner.

 

[overmind]

The heater supply imo is the bottleneck of most tube preamp implementations. For me, choke (CLC) input is a must. But finding the right values are difficult

I agreed. Maybe C-L-C-R-C...
My concern: The choke required for this
for filaments of 5 tubes, 0.75A @ 12.6Vdc or 1.5A @ 6.3Vdc.
with some headroom, 1A @ 12.6Vdc.
Then if I consider the space constraint, then I cannot go for big choke.., I do not know how well a choke with inductance of a few mH will work here..

 

[overmind]

Another question: Passive filtering is ok.
How to achieve good line regulation?

 

[hpeter]

I agreed. Maybe C-L-C-R-C...
My concern: The choke required for this
for filaments of 5 tubes, 0.75A @ 12.6Vdc or 1.5A @ 6.3Vdc.
with some headroom, 1A @ 12.6Vdc.
Then if I consider the space constraint, then I cannot go for big choke.., I do not know how well a choke with inductance of a few mH will work here..

dude, you will have way better quality filament supply, than most people anode supply have.

 

[MelB]

Overmind...in your first example with LM317 with Duncan amps PSU II sim I get the voltage dropping to ~8.85 volts with a mean voltage of 9.8 volts so the LM317 will be below it's 3 volt spec. for regulating for part of the AC cycle. Of course I could have your transformers secondary resistance wrong. Could be making some nastiness there. (?)

 

[overmind]

Overmind...in your first example with LM317 with Duncan amps PSU II sim I get the voltage dropping to ~8.85 volts with a mean voltage of 9.8 volts so the LM317 will be below it's 3 volt spec. for regulating for part of the AC cycle. Of course I could have your transformers secondary resistance wrong. Could be making some nastiness there. (?)

I am using 6.3V for the filament.

 

[DavesNotHere]

which means you need at least 11.6VDC going into the voltage regulator.