Minimum Voltage For 12AX7

[Paul Marossy]

Does anyone know what the minimum voltage a 12AX7 needs to operate?

I'm really only familiar with preamp tube configurations in tube amps, but, I'm fiddling around with a tube distortion idea and I am having a little bit of trouble getting it to work. I was experimenting with running a little cascaded circuit at +12V/-12V and a 12V heater supply using one 12AX7 in a similar manner to the Shaka Tube, but without the opamp in front of the tube. Will that work, or does it need a higher voltage to work? I'm currently getting no signal at the end of the circuit, and I think that all my wiring is correct, but maybe I made a mistake somewhere...

I've looked at quite a few tube overdrive pedal schematics, and most of them use a B+ voltage anywhere between 50V and 240V, and then there are others that use something very similar to the Shaka Tube, around 12V-24V.

In the Shaka Tube, if you were to take away the opamp in front of the tube, would the circuit still work, or does it need to drive the tube?

[puretube]

as a matter of fact: 0V;
saw a circuit for an oscillator once with no B+, just the filaments glowing to throw electrons from the cathode
- but that doesn`t help you here -

Me thinx s.th. wrong in your circuit... at least s.th. should be heard (be it overdriven, distorted, clean or weak...

I`m not familiar with Shaka Tube (LINK?), but generally spoken, those 12AX7s will "work" with any input, but to be "overdriven", of course ask for more than say 100 to 200mV from the PU; (in HV circuits, they start crunching above ~1.5V input).

On the other hand, with a B+ of 12V ("starved plate"), these circuits benefit from operating in the non-linear region to get a tubey sound, rather than per se being overdriven.

I`m not into lo-volt tube-circuits, but some others will be able to give you practical assistance.
"Slajeune" (now on vacation, but soon be back) and "smoguzbenjamin" had such a project going on about a week ago... :

http://diystompboxes.com/sboxforum/viewtopic.php?t=19554&postdays=0&postorder=asc&start=0

[Peter Snowberg]

No B+??  :shock:

Wow! Is there anything on-line about this design? I would love to see more!

Paul, I made a two stage 12AX7 fuzz a few years ago that ran from 12 volts (using a car battery for portability), and while I'm not generally a big fan of starved plate designes either, it definitly did work. What voltages are you seeing on the plates, cathodes, and grids?

The schematic for the Shaka Tube can be found here:
http://www.diystompboxes.com/pedals/schems/shakatube.gif

Take care,
-Peter

[puretube]

in fact I saw that 2 times (different articles/schems) in HAM-magazines from the 50s.


B.T.W.: Paul: read the Jason-thread to the end:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=19794

maybe you just got the same top/bottom view like in above link...

P.S.: that top/bottom phenomenon by accident led me to the basic "Tube-Zipper" circuit in 1971...

[Paul Marossy]

peter-

I thought the same thing, I should be hearing something, a weak signal at least... When I tried the audio probe, I just got a hum anywhere I poked around. Does that mean anything?

I'll double check my circuit tonight and measure all the voltages. Maybe I will find my mistake just by doing that.

puretube-

Thanks for those links. I checked out the ruby tuby schematic, and I have something similar to that, but without the LM386. The whole thing is run on DC, huh? Isn't that harder on the heater filaments to run them on DC?

I knew that stuff could be run without a B+ voltage. The Seymour Duncan Convertible amp had several modules with no B+ voltage supplied to them at all - like the "normal" module, for instance. (For more on the Convertible, check out the link to my website and look under "guitar amps")

As far as that jason thread goes, I know I didn't wire the tube up backwards. I drew up a lot of PCB layouts for the Convertible amp, so I had to get all of those pinouts right, I practically have them memorized!

[puretube]

nice site you got there, Paul.

DC is perfectly OK. Tubes started out as pure DC devices...

AC heating only gives more hum-problems to take care of...

[Paul Marossy]

Thanks puretube.  

I've heard that DC was bad for the heaters, but the Seymour Duncan appears to run all of the preamp tubes on DC, and it doesn't seem to be a problem. I think I would have to agree, running them on AC just creates more problems with hum. And I have heard that using DC heating increases the bass response, too.

[puretube]

mo`bass ?: don`t see why...

that funny 6.3V for heating derives from old times, when the accumulator cells max. voltage (D.C.) for the tubes was exactly that...

[Peter Snowberg]

mo`bass ?: don`t see why...

I don't see why either, other than that maybe people have correlated the more bass sound with designs that don't have to filter out all that nasty 50/60Hz hummmmmmmmmm (or avoid it in the first place by smaller cathode caps).

Take care,
-Peter

[Paul Marossy]

I think the lack of that filtering is probably the reason why. I read that on this page:

http://www.tubecad.com/july2000/index.html

[brett]

Hi.  I know bugger all about tubes, but you might be interested in a 12V DC 12AX7 design published in Silicon Chip magazine last year.  Any big library would have a copy.  It's intended for audiophile use, but there might be some ideas there that you could translate across into your designs.  (That design is available in Australia as a kit (no wall wart) for about US$70)

cheers

PS I have a MusicMan amp with a 12AX7 driver.  Unfortunately, the pre-amp is solid state and includes anti-parallel 1N4148s on the inputs to the op-amps.  So you can't push more than about 0.5V into it without getting seriously bad-sounding diode clipping.  So I'd be quite interested in any design that you come up with that would let me overdrive a tube *before* the pre-amp gets hold of the signal.

[puretube]

cross-link for 12 volt-tubers:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=19854

[Ge_Whiz]

There is no problem using DC for filament supplies, provided that you equate DC voltage with RMS (root-mean-square) AC voltage. If you were to use an oscilloscope to measure a peak AC voltage across a heater, and then offer it that amount of DC volts, you will seriously shorten the tube's life. Other than this, DC is better.

[Paul Marossy]

OK, I checked all of my voltages last night and found one wrong value resistor on the power supply, so I changed that, and things have improved, some. (The power supply scheme I used is the same as the Shaka Tube, but the rest of the circuit is different)

At least now I can hear a signal, and it sounds pretty darn good, actually, but the effected signal is much weaker than the bypassed signal...
It seems to have to do with my two connections to V+, one measures around 14 volts and the other one around 2 volts, same size resistors.

Any ideas why that may be? I can't figure out what the deal is. Maybe my tube is bad? Would too low a value of resistor on one of the cathodes cause this?

[puretube]

play with the cathode resistors (in case you have a grid-resistor (1M?) to ground or Ub/2).....

[bobbletrox]

Are you guys using something like a 12V 500mA wallwart to power your tube circuits...or are you using mains power?

[Paul Marossy]

The one I have built (Shaka Tube) and the one I am designing is using a wall wart type transformer that puts out ~12VAC/~500mA. That in turn is input into a voltage doubler circuit which bumps it up to around 25V.
I suppose one could power it with a 24VAC wall wart and use a full wave rectifier to convert it to DC and filter it as well.

Running preamp tubes on such low voltage is referred to as "starved plate". Normally B+ voltage on the preamp tubes in a tube amp around 300V and Fender (vintage amps) seems to like to run their tubes at the max. the tube can handle, or even a little more...

Things like "The Real McTube" and the Matchless "Hotbox" use relatively high B+ voltages of around 140V to 240V.

[brett]

Just a wild guess, but it might be worth checking the current capacity of the voltage doubler.  Given that it's AC your talking about, you could use a simple tramsformer, I suppose??

[Paul Marossy]

Current capacity of the voltage doubler circuit?

Hmm... that could possibly be a factor. I am running my Shaka Tube on a 12VAC/800mA wall wart, and it works fine. But this circuit I am messing with is temporarily using a 120VAC to 12.6VAC 300mA transformer. My voltages seem to be OK. But, maybe there isn't enough current for the tube heater filament and the rest of the circuit... I'll do some more checking.

BTW, I tweaked the circuit a little bit more last night and have more or less what I intended, but the output is still too low.  

Well, I guess I'll be back at it again tonight. If I get this thing working, I'll post a link to the schematic.

[Roland]

My experience has taught me that low voltage operation of 12ax7's yeilds workable tones but with no headroom. If your looking at clean tones only any voltage will work. If you want fantastic cleans and lots of headroom for the best overdrive and distortion tones then your looking at 140 - 300 vdc.  
I find lower voltages seem to squish the high end out of existence as well as lowering the gain of a tube that was designed to give you > than 50db. With low voltages the gains are like < 20db.