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Old 24th September 2015, 05:45 AM   #1
ygg-it is offline ygg-it  Italy
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Default Running tubes' filament on DC

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?


Last edited by ygg-it; 24th September 2015 at 05:59 AM.
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Old 24th September 2015, 09:18 AM   #2
DF96 is offline DF96  England
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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.
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Old 24th September 2015, 10:27 AM   #3
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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.


5) Not a good idea. Instead of grounding one end, reference it to +40VDC that is bypassed to ground with a capacitor.
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Old 24th September 2015, 11:51 AM   #4
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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.


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Old 24th September 2015, 11:56 AM   #5
AJT is offline AJT  Philippines
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to date i have never used dc on filaments of tubes.....
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
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Old 24th September 2015, 01:44 PM   #6
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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.....~~~~~~~
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Old 24th September 2015, 02:02 PM   #7
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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!
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Old 25th September 2015, 04:47 AM   #8
ygg-it is offline ygg-it  Italy
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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.
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Old 25th September 2015, 06:36 AM   #9
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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!)
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Old 25th September 2015, 09:40 AM   #10
DF96 is offline DF96  England
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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.
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