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DC Filament power supply

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Hi, there

As a beginner of Tube amp, I have a generic question regarding filament power supply. Is it true that I can always replace the AC supply with DC to reduce the noise? I bought a kit online. It uses a rare tube 11ms6, which require 12v filament power. The original design is AC power, can I change it to DC?

Can I just simply use 12vDC adapter?

Thanks in advance
diyAudio Senior Member
Joined 2002


Try Regulating your DC... This will give stability and consistency to the tubes and a benefit of much lower tube noise level.

I'm sorry to say but done properly AC heating the power tubes is often to be prefered.
I fail to see how the tubes are going to "see" more stabilty and consistency by supplying DC to the heaters?
If you have hum coming from the heater supply,something is not layed out properly,or you may have a short between heater and cathode on one of the tubes.
Moreover the tubes will last longer when fed by AC.

diyAudio Senior Member
Joined 2002


I wasn't stopping anyone from doing so,was I?
Still,as I said, when executed correctly, AC heating IDH powertubes is no problem whatsoever.
Sure they mustn't be starved on current:use of a correct transformer rating should take care of that.
You should always provide for a reasonable upwards margin for heater consumption.(I usually go for 30% at least,it may cost more but I never skimp on transformers. )
A CSS?Unless you go for special heater types designed for a specific current consumption (regardless of voltage) it is not a wise idea to use a ccs for this.
In current audio tubes manufacturers design for voltages,not current on heaters.
Take a good look at the databooks and you will notice it will severely limit you in experimenting with so called "equivalents"
E.g.:6dj8,6922,7308 and so on.
DHT are trickier but can still be done.
Hum should be both measurably below the noise floor of the circuit and inaudible.
Have you considered the situation where you would have an amp with 10 and more output tubes each consuming a couple of Ampere to heat up ?
And you're missing an important factor:AC heating is simply better for the tubes involved.After all they were designed for it and the part about AC prolonging their lifespan may not be important to you but is definitely important to most of us??
:( :(

My 0.02 Euro's worth:

Generally filaments can last longer with AC than DC. If you look at the specifications given by some bulb manufacturers, they quote a shorter lifetime for DC. This is apparently due to a phenomenon called "notching". If you examine a bulb filament near the end of it's life, there are small notches near to one end (can't remember which end).
Whether this has any relevance to tube filaments, at their lower temperatures, I don't know.

From another angle:
With AC the average emission of each part of the filament is more even. So possibly the filament emission lasts longer.
diyAudio Senior Member
Joined 2002

Well done Dhaen!
This is a little known secret.
Indeed the AC's alternating behaviour prevents notching.
It has sort of a cleaning effect preventing heater and cathode to stick together.
Notching occurs most often at the top of the inverted V filament,which seems logical to me since that's where it is closest to the cathode.
Some special winding of the heaters (such as in the 300B) was especially developped to heat the cathode as evenly as possible.
Other constructions exist,just to say that the heater to cathode relationship is VERY important to the end result you can expect from a tube.
Most change their characterics completely when changing parameters here.(current starvation over- and undervoltage etc.)
Those of you having the gear to do plots can eat their heart out experimenting with this.
I'm not familiar enough with those software programs such as Sofia,Pspice and such to say if they provide options to simulate this.


I wonder if the reason is that the mean voltage for AC 6 volts is actually lower than a DC 6 volts? I skimmed over an article once about this but forgot much of it. I will look it over again in more detail. For 6 volt heaters, that is.

As for DHT, I have keenly been interested in this because of my antiques. With DC the emission tends to be higher from one end, as far as what I read is concerned. For DHT's using AC and center tapping the filament to ground connection (the bias resistor etc.) is ideal for even distribution of emission.

However, with the DC filaments I do not know if this is an issue. Perhaps the filament is specially designed for uneven emissions. I do not recall reading such.

diyAudio Senior Member
Joined 2002


You're right,but the difference is not great.In this case 6.3 VAC would amount to roughly 6VDC.

DHT:in fact if you use DC on a DHT ,such as the ones with especially irregular filament windings ,you will throw the whole design out of balance creating "hot spots" on the cathode.
Go for AC heating and trim the hum away with a rheostat and you're done.Simple.:D
Your tubes will love you for it.:) :)

theoretical and longevity issues aside heater circuits have direct effect on the sonics. in DHT the heater is directly in the signal path and even in indirectly heated valves the sonics change drastically depending on whether heating is ac/dc/regulated/filtered. quality of regulating circuits and capacitors is paramount. generally, ac sounds best to me on all valves. if hum is an issue, current sourced heating is second best.

diyAudio Senior Member
Joined 2002


heater circuits have direct effect on the sonics

I couldn't agree more.
Heating affects the characteristics (curves) hence operating points of the valve.
All manuals I know of show curves with the valve AC heated,you change that and you're off on the wrong track.:att'n:
That's one more reason to do some curve plotting before you choose you're operating points!
Good thing these valves are good natured and usually abuse resistant...;)

Make sure your transformer can handle it tho, using DC will probably require 30-40% more current, and there will be enormous inrush current to fill the big reservoir caps you will need...

Go AC, its easier for power tubes!

I run my PP amp with all AC filaments except the very first preamp section and its dead quiet, no hum at all.

The best way to get rid of hum IMHO is to separate your grounds- run all signal and speaker grounds to one point, all cathodes to another, all power supply to yet another, and join them all at only one point. I call it multiple star grounding :D

It fixed the hum in my amp!
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