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Old 21st February 2011, 05:06 PM   #1
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Default 9V tube drive pedals - worth it?

Hiya all,

I was keenly plugging my way through my first exploration into tube amp design (a - hopefully - rack mounted tone machine project for my bass gear :P), when my brother and guitarist mentioned he'd be interested in a custom overdrive pedal... if I would be so kind!

So I have been nice and said I would look into building such a device. It would be something based around a 12AX7 running with a 9-24V (ie very low) plate voltage.

We both agreed it might be a better and safer start than going straight into building the 0-375V amp I've gotten down on paper.

SO!

- Are there are out there that are any good?

- And what are wrong with the ones out there? Because I've read alot of grief about the marketing scam which is the low volt tube device; this I know not to be entirely true, as the tubes offer great tone shaping with the correct design.

What are people opinions???

If anyone could enlighten me about the heating circuits needed for such a thing, that would be greatly appreciated!

B
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Old 21st February 2011, 05:16 PM   #2
DF96 is offline DF96  England
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If you are specifically looking for distortion then the low voltage makes distortion impossible to avoid, so it seems ideal for this purpose. Just don't expect it to amplify like a straight preamp.

12AX7 heater requires either 6.3V at 300mA, or 12.6V at 150mA - depends on how you wire the base.
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Old 21st February 2011, 05:24 PM   #3
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On the heating issue, I realise this - my point was that I have read projects that ran such low voltage without heating supply at all. This struck me as odd! Would I be able to draw enough current to supply both the B+ and heating supply from a normal 9V adapter?

On the distortion - does this imply that I won't get any kind of suitable range of tone? Just distortion? With the right resistor choice could I not get the overdrive I am after?

B
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Old 21st February 2011, 05:39 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by snowballfight View Post
On the heating issue, I realise this - my point was that I have read projects that ran such low voltage without heating supply at all. This struck me as odd! Would I be able to draw enough current to supply both the B+ and heating supply from a normal 9V adapter?

On the distortion - does this imply that I won't get any kind of suitable range of tone? Just distortion? With the right resistor choice could I not get the overdrive I am after?

B
There is much more to this than you think. Tube heaters are nearly short circuit on start up! So without measures to allow a soft start you will trip your supply!

The 12ax7 is not the best tube for your project if you want to run on low voltage. Check the current required by the heater.

The overdrive you speak of is harder to get right. Problems with heater to cathode leakage can be a problem also!

This is a good starting point to read!
NP-100v12: DIY 12AU7 (ECC82) Tube / IRF510 MOSFET Headphone Amplifier

Another link:

Small Tube/SS hybrid amplifier design 9V

This one is not low voltage, however it is interesting!

http://www.ampbooks.com/home/amp-tec...F86-power-amp/

Regards
M. Gregg
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Last edited by M Gregg; 21st February 2011 at 05:49 PM.
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Old 21st February 2011, 07:07 PM   #5
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Cheers; that is a good link. So the MOSFET is to tackle the soft start up, but what is the roll of the LM317 regulator? I am confused by the position of the component in the circuit.

Without meaning to sound too thick, why is this a problem with the low voltage application and not for other, fully saturated plates around 300V mark?

For the tube, would the 12AU7 be suitable? I know its behavior is particularly linear compared to the AX, and its not great for driving - would I get the right levels of distortion? Or perhaps should I look into small tubes?

B
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Old 21st February 2011, 08:06 PM   #6
DF96 is offline DF96  England
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At low voltages a valve is on a very curved region of its characteristics. It also is likely to be drawing grid current, so needs a very low impedance input (unless you are seeking distortion). I'm not sure what you mean by "fully saturated plates".

12AU7 draws exactly the same heater power as 12AX7. It may work better at low anode voltages. In a conventional circuit the 12AX7 has much higher gain and, potentially, lower distortion.
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