tube heaters

[AdamB]

Hey,

Still doing research for my low voltage preamp build...

Is it right that on 12AX7 tubes I run the heaters on +12V to pin 4, then pin 5 to ground?

What would happen if I applied more than that, would the filiment just burn out? Say 15V or 24V? What would happen if I ran the entire valve circuit off of 24V? Would I need to regulate that to 12V for the heaters still?

EDIT: All voltages DC

[markr04]

Yes on the polarity.

Running higher or much lower than 12.6v will shorten the life of your tube. I'm using 18vdc in my builds and a 1-watt 220 Ohm in series to the heater to get it to 12.6v.

[AdamB]

OK. I read somewhere that it is beneficial (sound wise) to run the tubes at higher voltages. If this be true is it a linear relationship? So if I was to run them at 24V (and the heaters ovbviously regulated to around 12V) would it be better? Or is this only true when you get up into really high voltages (like 200V)?

[markr04]

The heaters are set at 12.6v. Now you're talking plate (anode) voltages. You're right about the 200v. 24v won't give you any benefits over 12v. From my experiences (and some reading, too) there is no audible difference between 12v and 150v at the plates.

I should add that I'm using 18v only because I bought a case of wallwarts on the cheap, not because there's a benefit to them sound-wise.

[A.S.P.]

no problem turning the filament polarity around (+ to pin 5, 0 to pin 4)...

above 12.6V,the tube will glow brighter (for a short time - then they`ll dim forever...)

12AX7s come to life well above 80V-120V of B+...

[AdamB]

OK, it's just I have a 24V DC supply already from a power amp.

My project is basically to come up with a valve preamp that runs from this supply. It's easier if I can just run everything at 24V, but as I have to regulate the heaters to 12.6V I might as well run the whole thing through a 12V regulator, yea?

I'm planning on also running that 24V through a couple of regulators so that I can tap off 18V and 9V (18V for a built in supasustain compressor and 9V for a mini-mixer on the input and a reverb unit).

Is it a good idea to add filtering caps after each regulator stage? (from the output of the regulator to ground or the input of the regulator to ground?)

[markr04]

I don't know at which point (number) filtering caps offer diminishing returns. But if you have room for filter caps at each stage, it won't hurt to add them. Actually, ff you can experiment on a breadboard prior to your build, might try doing all of your filtering prior to the regulators.

[AdamB]

Actually, I might go with your way, markr04...

I could just put in a 1 watt, 220ohm resistor in series with the heaters and power supply. That'd save me adding an extra regulator, as the compressor runs on 18V too. So, run the preamp at 18V and the heaters at 12.6V via the use of a resistor (I might put in a trimpot before the heaters rather than a resistor, if there is one rated to 1Watt? so I can fine tune it to exactly 12.6V, as my regulator running at 18V will be slightly off no-doubt), and then only 1 more regulator down to 9V for the reverb.

So it'd look something like this:

Power Flow:
24V DC --> 18V regulator
                     |   
                     | -->Tube Preamp.
                     |
                     | --> 220ohm 1Watt Resistor --> Heating Filiments
                     |
                     |----->  9V regulator --> Reverb Effect
                     |
                     |-----> SupaSustain Compressor

Input Flow:
Valve Preamp. --> SupaSustain Compressor --> Reverb Effect --> Power Amp.

EDIT: Diagram was all messed up, erk!

[markr04]

The required resistance may vary as you're limiting current with the series resistor. I'm actually using a 1/2-watt to drop my supply to 12.6v. The 1/2-watters I use get hot, but don't brown. I suggested 1-watt because it is the better way to go, heat-handling-wise. Heat will also change the resistance value... whether it tends to go up or down, I've forgotten.

I created and posted a layout here in the forum last year for a rectified and filtered PS regulator. With 12VAC, it put out 1v-30vdc, limited by a standard 16mm 5kohm pot.

Take what you want from what I said here. It may mean you can use a plain ol' pot. It's the DIY way of learning.

[AdamB]

Yea, I've been looking at trimpots on smallbear, banzai and musikding and they're all more like 0.25W rating. Hrm... Maybe I'll just use a 16mm alpha pot inside the case, fine tune it until I get teh desired 12.6V and then hot-glue the shaft so it can't move.

-Adam

[gez]

Running higher or much lower than 12.6v will shorten the life of your tube.

As far as 'starving' the heaters at low plate voltages - around 12V - I'm not so sure it would shorten tube life.  At this sort of plate voltage you get a self biasing effect and the grid is often negative of earth.  This suggests that one or two electrons are 'surplus to requirement' and perhaps starving the heaters isn't such a bad idea.  The fact that there is all this negative charge floating around the grid also suggests that cathode stripping would be unlikely at low plate voltages.

I've been messing around with starving the heaters in these sort of circuits.  Interesting stuff happens (no comment yet).  Whether the life is shortened only time will tell...

[A.S.P.]

7812...

[A.S.P.]

gez:http://www.elixant.com/~stompbox/smfforum/index.php?topic=23875.msg153807#msg153807
http://www.takamine.com/?fa=cooltube2...

[markr04]

Running higher or much lower than 12.6v will shorten the life of your tube.

As far as 'starving' the heaters at low plate voltages - around 12V - I'm not so sure it would shorten tube life.  At this sort of plate voltage you get a self biasing effect and the grid is often negative of earth.  This suggests that one or two electrons are 'surplus to requirement' and perhaps starving the heaters isn't such a bad idea.  The fact that there is all this negative charge floating around the grid also suggests that cathode stripping would be unlikely at low plate voltages.

I've been messing around with starving the heaters in these sort of circuits.  Interesting stuff happens (no comment yet).  Whether the life is shortened only time will tell...

I stand corrected.

And on the Takamine Cool Tube tip, I demoed some Vox valve pedals at NAMM '06 which run on 9vdc. Whether the heaters are starved, properly fed in parallel, or what... I don't know.

[gez]

I stand corrected

Well, you don't really.  What I posted is just my hunch and makes sense to me.  Whether it's true or not is another matter entirely (though in tests everything seems ok so far). 

The problem is all the data given in data sheets is gathered from circuits running at high(er) plate voltages and isn't necessarily relevant at 12V.  Starved plate circuits are unknown territory in a sense so it might not be such a good idea to use standard ways of doing things - conventional wisdom for conventional circuits.

[Ge_Whiz]

You could try running two 12V heaters in series to a 24V supply. Or even put one heater in series with a small 12V bulb. But a 7812 regulator would be the best way, as suggested.

[AdamB]

OK, to hell with it, I'll just stick in another regulator to get 12V. I'm sure it'll be handy to have a 12V source to tap off for other things inside the box.

That should make available 24V, 18V, 12V and 9V.

I'm thinking of also adding in a noise gate circuit, anyone know any good ones?

-Adam

[brad]

Just a note: I'm using a 7812 on the heaters of my tubes, and it actually puts out 11.8V (from 17V).  Not that this is a bad thing of course!  Check out this article:

http://www.tubecad.com/july2000/page10.html (Right column)

Running heaters slightly lower than required has some pretty beneficial effects on tube life.

[A.S.P.]

a 1N4001 in series with the GND-leg of a 7812 will get you very close to 12.6V

[Eb7+9]

... actually puts out 11.8V (from 17V).

good voltage limit imo ... if you drive large signal into a tube that's running below proper heater threshold it's conceivable you could get cathode stripping happening - yielding shorter tube life ... that's one extreme, otoh running not too much below rated fil voltage does give longer filament life but you also get less noise from the devices the lower you go ... some recent high gain Marshall guitar amps have their front-end triode stage filaments running on a DC voltage converted to around 6vdc from the AC voltage feeding the power and PI tube filaments ... I run my hifi and fx preamp tubes at 5.9 vdc without any transconductance problems - they plain stop working somewhere around 5.8vdc ...

jc