Minimum Voltage For 12AX7

[Paul Marossy]

I'll agree that it really kills your headroom, but the Shaka Tube works satisfactorily. I'm just trying to figure out why I have such a low output compared to the Shaka circuit. It probably comes down to I don't really know what I am doing...  :oops: , well, not totally.

I'll figure it out. I guess that's how we all learn sometimes - trial and error.

[Mark Hammer]

What I know of tubes could fill a thimble.  I understand the idea of tube-starving but something I've never really received a good answer on is where tubes make the transition from being described as "starved" to "non-starved".  I realize it's not as if there is some magic voltage like 83v where it changes, and that different models/brands of 12A-7 tubes (AT, AU, AY, AX) may "go hungry" at different points.  Still when I see something like the various PAiA circuits that use invertor-based voltage multipliers to kick up a 12vAC supply input to in the neighbourhood of 45Vdc, would adding another stage of voltage multiplication to kick things up another 15v or so make a difference in whether it "sounds starved" or not?  Will preamp tubes powered by multipliers *always* sound starved even if you go nuts and tack on 10 stages of voltage multiplication?  What would be the minimum supply voltage you'd want/need to use so that the trained ear would NOT describe it as starved-plate.

I ask because 12VAC wall-warts are in great abundance where I live at very modest cost.  $5 will get you an amp and half, no sweat.  Of course, it will NOT get you a higher-voltage transformer.  So, fundamentally, my question is "What could I expect to achieve with such a wall-wart and voltage multipliers?".

[Paul Marossy]

"Still when I see something like the various PAiA circuits that use invertor-based voltage multipliers to kick up a 12vAC supply input to in the neighbourhood of 45Vdc, would adding another stage of voltage multiplication to kick things up another 15v or so make a difference in whether it "sounds starved" or not? Will preamp tubes powered by multipliers *always* sound starved even if you go nuts and tack on 10 stages of voltage multiplication? What would be the minimum supply voltage you'd want/need to use so that the trained ear would NOT describe it as starved-plate?"

I think to answer that question, you would have to look at the tube characteristics/curves. These starved plate circuits bias the tube(s) in their non-linear region to give them their characteristic sound. They need to be within a specified voltage range for them to operate in their linear region. The exact point where you go from linear to non-linear depends on who manufactured the tube, and then it varies from tube to tube. I think it's just the physical nature of tubes. When designing low voltage stuff with transistors and opamps, it seems to me that it is a lot easier to acheive a workable effect.

The best you could hope to achieve is getting a nice sounding overdrive pedal, similar to a Shaka Tube or others. With a 12VAC wall wart, there isn't much room to work, or I should I say headroom?  :wink:

[Doug H]

Just a few side comments, don't know if I can answer Paul's questions...

First, for a starved tube pedal to work "right" IMO, you have to scale down the gain of ea tube stage, commensurate with the supply voltage. The BKButler Tube Driver, Shaka Tube (I believe), and the Pentode Driver I built with 6ak6 pentodes a few years ago all do this. This is part of the reason they are driven with a high-gain/low-Z source like an op amp. (Note how the grid-shunt resistors in these circuits are in the 10k range.) Otherwise you won't get reasonable gain and overdrive without a lot of stages.

There are some quirky things about starved-plate tubes, but I think one of the things the PD, at least, excels at is low-gain, touch-sensitive, "cleanish" tone. So headroom is not a problem with a circuit like this if the tube stage gains are scaled properly. I did have some problems tightening up the bass in a 12ax7 circuit I breadboarded, but that could have been lack of understanding on my part at the time.

I don't understand some of the 9v starved-plate circuits I've seen with big-ass grid-shunt and/or plate resistors. I tried the big 1M grid-shunt resistors in my PD on the breadboard with 30v B+ and ended up with a "tube fuzz box". Lots of clipping but no dynamics or headroom. No point in fooling with tubes in that case (IMO), get a couple transistors and build a fuzz face.

For that matter I don't understand some of the mojo-promising
high-voltage tube pedals that have similar sounds. If you want rip-roaring fuzz tone, save yourself the construction headaches and go solid state. The tube thing is all about subtlety IMO. Sorry if this is heresy to some...

OTOH, I have heard a lot of good things about the hotbox, so I'm curious about that one. Would love to hear some soundclips of it.

Doug

[puretube]

don`t mix up multiplication factors like 20x or 50x with 20dB or 50dB...;

[Roland]

One thing about doubling or tripling voltage levels is you also divide your current availability . Everytime you double your voltage you cut your current supply in half.

One promising method for deriving high voltages is the mc tube method of using back to back transformers , using step down and step up tranny's in series. There are some back to back circuits out there that can deliver 280 vdc which is ideal. There again if you don't take current consumption seriously the tranny's will fry, or at least one will anyways.

[puretube]

my rule of thumb is 1mA @ 250V  ("availability") per triode section of 12AX7.

[Roland]

Holy transconductance! :shock:  That site of yours puretube is something else. I don't think I ever seen so many tube effects in one place. Your power bill must be thru the roof.

[Paul Marossy]

Some cool stuff you have there puretube.  

Do you have to custom build an enclosure for each one of those things?

[puretube]

one fine day there`ll be one box for 1-tubers with 1 pot,
1 box for 1-tubers with 2 pots;
1 box for 2-tubere with 3 pots;
1 box for 2-tubers with 4 pots and 2 stompswitches;
1 box for 4-tubers with 6 knobs and 3 slide-switches................