Hello, I'm currently running four parallel 6.3V filaments of 12ax7 on DC. I just added two components: a diodes bridge and a 10000uF capacitor. A small resistor, which I calculated to be about 0.47-1.0 ohm was NOT required after measurement because the DC voltage drops below 6 volts.
I tried this in order to kill the AC noise but with NO success....
Questions, to understand if keeping the DC mod:
1) why the resistor was not required passing from 6.3V AC RMS to 6.3V DC? Is it because I'm doubling the power consuptions on a weak transformer?
2) do the filament last longer on DC?
3) is the thermoionic emission the same, so the filament temperature, if I power on DC? It looks that the tubes run hotter if i touch them
4) the tube amp sounds better and cleaner but AC noise is still the same when I put my ear close to the speaker. Is it a placebo effect?
5) one leg is grounded, so before (in AC) I had the filaments running with sine wave average on zero, and in DC I have the filament running 6,3 Volts above zero: I saved any diode effect on cathode which is running in autobias 1.5 volts above zero?
Thanks
I tried this in order to kill the AC noise but with NO success....
Questions, to understand if keeping the DC mod:
1) why the resistor was not required passing from 6.3V AC RMS to 6.3V DC? Is it because I'm doubling the power consuptions on a weak transformer?
2) do the filament last longer on DC?
3) is the thermoionic emission the same, so the filament temperature, if I power on DC? It looks that the tubes run hotter if i touch them
4) the tube amp sounds better and cleaner but AC noise is still the same when I put my ear close to the speaker. Is it a placebo effect?
5) one leg is grounded, so before (in AC) I had the filaments running with sine wave average on zero, and in DC I have the filament running 6,3 Volts above zero: I saved any diode effect on cathode which is running in autobias 1.5 volts above zero?
Thanks
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A DC power supply only gives 1.414 times the AC RMS input voltage when it isn't loaded, or with a lossless transformer and infinite value smoothing capacitor and zero voltage drop diodes.
I think I read somewhere that heaters may last slightly longer on AC, as DC can cause metal migration. However, far too small an effect to worry about.
Cathode temperature and hence emission will be exactly the same on AC and DC.
The heater-cathode insulation in a 12AX7 is good for 180V (from memory) so a few volts either way will make no difference.
DC heaters are almost never needed, except in sensitive situations such as some phono or microphone preamps. Look elsewhere to solve your hum problem.
I think I read somewhere that heaters may last slightly longer on AC, as DC can cause metal migration. However, far too small an effect to worry about.
Cathode temperature and hence emission will be exactly the same on AC and DC.
The heater-cathode insulation in a 12AX7 is good for 180V (from memory) so a few volts either way will make no difference.
DC heaters are almost never needed, except in sensitive situations such as some phono or microphone preamps. Look elsewhere to solve your hum problem.
DF96: Regarding metal migration; yes, but DC supplies usually have a soft start. The capacitor absorbs the inrush current, not the heaters. This might have a greater effect on longevity.
ygg-it:
5) Not a good idea. Instead of grounding one end, reference it to +40VDC that is bypassed to ground with a capacitor.
ygg-it:
5) Not a good idea. Instead of grounding one end, reference it to +40VDC that is bypassed to ground with a capacitor.
There are several potential sources of hum so you really need to discover which is causing a problem. Ideally you should try running the heaters on a regulated dc supply. If the hum goes then the heaters are the problem. If not then it is somewhere else. If you still have hum then it will either be predominantly at mains frequency or twice mains frequency. If it is at mains frequency then the likely source is magnetic coupling from the mains transformer. If it is twice the mains frequency then it is probably an HT supply problem.
Cheers
ian
Cheers
ian
I never use DC filament power. The guitar amps I make have no AC hum what so ever. At full volume. I normally ground the 6.3 (filament) power center tap. I super glue (tack every 3 inches or so) a well drill twisted pair of 18 gauge wires into the corners of the chassis. I use a .02 uF high voltage cap across the neutral & hot (125vac incoming power) wire. The B+ ground is the same as the input signal ground as is the same with the incoming power ground. always use one of the studs of the power XMF for your main ground point. I use mini coax grounded on one end for all the small signal paths and account for the added capacitance in my coupling cap value.
Have you preformed the very initial steps in testing for where the stray hum is coming from? Is the hum present with the volume up or all the way off, both? with the input grounded?? With certain supplies turned off? With the preamp or final tube removed. always attempt one of these tests at a time. The gurus will tell you that a properly constructed amp will never hum (I'm lucky). Please post a pic or two of your work as so we all can critique your style & get you going in the right direction to a no hum output.
Even the best are always fighting the hum deal & always makes for a good thread, so reply away brother.....~~~~~~~
Have you preformed the very initial steps in testing for where the stray hum is coming from? Is the hum present with the volume up or all the way off, both? with the input grounded?? With certain supplies turned off? With the preamp or final tube removed. always attempt one of these tests at a time. The gurus will tell you that a properly constructed amp will never hum (I'm lucky). Please post a pic or two of your work as so we all can critique your style & get you going in the right direction to a no hum output.
Even the best are always fighting the hum deal & always makes for a good thread, so reply away brother.....~~~~~~~
A dime from me:
In addition to the above: One must also keep in mind the 'cleanness' of the dc. If simply a diode bridge + capacitor for a large low voltage load (6,3V) there might be such charging transients that the result is sometimes worse than clean ac! With just simply rectifying 6,3Vac to get 6V dc, results can be disappointing.
Because it is relatively easy these days of semiconductors and suitable power ICs, the temptation exists to overdo matters. I myself came from the days when such was not possible; one could design an RIAA tube stage with inaudible hum, with careful layout and other precautions. Nowadays when I want dc, I start with some 9V or two 6V windings in series plus a regulator to get rid of ripple on dc, or some other arrangment. This can be more economical than a huge filter capacitor. (It is also more efficient to use heaters in series where tubes allow for such, for a lower current arrangment.)
But as said: Audible hum in amplifers seldom comes from heater supplies. There is often another cause. Good hunting!
In addition to the above: One must also keep in mind the 'cleanness' of the dc. If simply a diode bridge + capacitor for a large low voltage load (6,3V) there might be such charging transients that the result is sometimes worse than clean ac! With just simply rectifying 6,3Vac to get 6V dc, results can be disappointing.
Because it is relatively easy these days of semiconductors and suitable power ICs, the temptation exists to overdo matters. I myself came from the days when such was not possible; one could design an RIAA tube stage with inaudible hum, with careful layout and other precautions. Nowadays when I want dc, I start with some 9V or two 6V windings in series plus a regulator to get rid of ripple on dc, or some other arrangment. This can be more economical than a huge filter capacitor. (It is also more efficient to use heaters in series where tubes allow for such, for a lower current arrangment.)
But as said: Audible hum in amplifers seldom comes from heater supplies. There is often another cause. Good hunting!
Hum is high only when Volume pot is in the middle. I have no hum when it is at zero or full volume!
In any case the tube amp sounds better and cleaner with Dc filament, so I'm going to keep.
In any case the tube amp sounds better and cleaner with Dc filament, so I'm going to keep.
Ah! Wish we had known that earlier!
That is where the resistance from slider to common is highest. (The 'live' end of the pot is probably fed from a low impedance source, so impedance to common there also low.) Wish there was a schematic here, but go trace the lead from the pot slider to wherever it goes to a tube grid. Somewhere on the way it either runs close to an ac lead, or if a screened wire, the screen has become adrift. (If of any length it should be screened.) As MaicoDoug asked, a diagram or a photo would help; that for now.
(Comments on your work will not be criticism, only advice!)
That is where the resistance from slider to common is highest. (The 'live' end of the pot is probably fed from a low impedance source, so impedance to common there also low.) Wish there was a schematic here, but go trace the lead from the pot slider to wherever it goes to a tube grid. Somewhere on the way it either runs close to an ac lead, or if a screened wire, the screen has become adrift. (If of any length it should be screened.) As MaicoDoug asked, a diagram or a photo would help; that for now.
(Comments on your work will not be criticism, only advice!)
Yes, fixing a fault requires first finding the fault. Finding the fault requires noting the symptoms, including when it happens and when it doesn't happen.
As Johan says, the symptoms point to capacitive coupling to the circuit from pot slider to valve grid. A poor DC heater supply, with spikes and ripple, could be worse than a clean AC heater supply - although the heater wiring may not be the source of the hum.
As Johan says, the symptoms point to capacitive coupling to the circuit from pot slider to valve grid. A poor DC heater supply, with spikes and ripple, could be worse than a clean AC heater supply - although the heater wiring may not be the source of the hum.
ygg-it,
As described the hum is coming from the wire feeding the center wiper on that volume pot. Going to DC heaters helped, but that's only treating the symptom. The real cause is a too close wire an AC source. Please let us know what you come up with, because when it's fixed "Molto Bene" Amp yes?
Also it's always a good idea to use a .02 uF high voltage cap (600v or better) across the neutral & hot (125 or 240 vac incoming power) pair of wires. Even better, tied to ground and also across (3 total). this takes some of the hum out, but not all. Just actually takes off the peaks slightly & protects from stray spikes from other stuff for example light dimmers & compact florescent lights.
Schematic & a picture or two will help not only you, but many others when you get a chance.
Doug
As described the hum is coming from the wire feeding the center wiper on that volume pot. Going to DC heaters helped, but that's only treating the symptom. The real cause is a too close wire an AC source. Please let us know what you come up with, because when it's fixed "Molto Bene" Amp yes?
Also it's always a good idea to use a .02 uF high voltage cap (600v or better) across the neutral & hot (125 or 240 vac incoming power) pair of wires. Even better, tied to ground and also across (3 total). this takes some of the hum out, but not all. Just actually takes off the peaks slightly & protects from stray spikes from other stuff for example light dimmers & compact florescent lights.
Schematic & a picture or two will help not only you, but many others when you get a chance.
Doug
Putting a DC-rated cap across AC mains is never a good idea. Putting an ordinary cap from mains to ground (or chassis) is even worse - it is potentially dangerous.MaicoDoug said:
Caps across mains should always be X-rated. Caps from mains to ground should always be Y-rated. This means they are failsafe; they won't kill you or burn down your house.
Such caps do not remove peaks, but they may reduce spikes.
AC noise can get in a few ways.
1/ From the heater and heater wiring if AC is used.
2/ From the power supply if it isn't smoothed enough.
3/ Magnetic radiation from mains transformer.
1/ From the heater and heater wiring if AC is used.
2/ From the power supply if it isn't smoothed enough.
3/ Magnetic radiation from mains transformer.
Thank you DF96 for your illustrated concern. Safety is always first. The caps I'm using are from surplus 3 phase 480vac power input filters that are way too large, but upon researching the "X & Y" factor from Kermet indeed the issue is well published.
Is this a pre-amp or phono amp? If it is line level then you should not get any hum from AC filaments... really.
If it is a phono amp, then there are MANY possible sources of Hum.
What I did (ages ago) with one Phono amp where I thought the residual Hum was from the Heaters, was to use a car battery to run the heaters (correctly wired in series). Lo and behold, the hum was NOT from the heaters.
I eventually found the found the source of the residual hum (solved with smarter layout and shielding). I Hope this gives you some ideas.
Ian
Silicon bridge - large cap - 3 pin reg. for filament supply = big loss of "magic". I only use DC for filaments where I absolutely need it (input stage of phono pre). And then only with real attention to diode switching noise and reg. noise. That sh*t will couple via heater/cathode capacitance into the signal chain. Ferrite beads in series and a shunt ceramic cap near the tube can help with "grunge" from DC filament supply also.
There is a potential issue with capacitors in the power transformers' primary circuits (beyond the known and workable safety issues). Coupling from the primary to chassis, and so (usually!, not always) to signal ground can inject noise into a place from which it's hard to remove.
Ground is really only "ground" if the piece of electronics is sitting on a piece of glass and connected only to AC power.
A modern way to isolate chassis from signal ground is to connect them only by a back-to-back pair of really big diodes. I've also heard of using thermistors. Whatever it is needs to be robust enough to be able to blow whatever fuses or breakers exist fast enough to protect a human from failure to line. Another modern way is double insulation - no single failure can cause a dangerous condition or cause another dangerous condition. Modern TV's and such seem to be done this second way.
All good fortune,
Chris
Ground is really only "ground" if the piece of electronics is sitting on a piece of glass and connected only to AC power.
A modern way to isolate chassis from signal ground is to connect them only by a back-to-back pair of really big diodes. I've also heard of using thermistors. Whatever it is needs to be robust enough to be able to blow whatever fuses or breakers exist fast enough to protect a human from failure to line. Another modern way is double insulation - no single failure can cause a dangerous condition or cause another dangerous condition. Modern TV's and such seem to be done this second way.
All good fortune,
Chris
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