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Author Topic: Low Voltage Tube Preamp  (Read 8751 times)

jaylow

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Low Voltage Tube Preamp
« on: June 03, 2013, 02:55:23 AM »
Hello! I'm looking for a low voltage tube based preamp. I've search around the internet and found a few schematics but some of them are schematics of both the preamp and power amp sections and in some cases, I'm unsure of where the preamp ends and the power amp begins. I'm also looking for a no frills design, so something clean (as clean as can be hand under low voltage) and no tone shaping. Also, I'm working with 15V from an old power supply that I salvaged and would like to use.

So far, this is what my search has shown me:
Sopht amps - http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37
Soul-o-tube - http://www.avwz35.dsl.pipex.com/soul-o-tube.gif
Tube Cricket - http://www.beavisaudio.com/Projects/TubeCricket/TubeCricket_Final_2008.gif

So far, the sopht amp seems to be the best option since it uses tubes that are actually meant to be run at low voltages but I'm unsure where the preamp ends and the power amp starts. Does anyone have any experience with any of these circuits? And how would I go about modifying them to be clean and have no tone shaping.

My final option is to just go completely SS and do the fetzer valve but I'd really like to go with tubes (partly for the aesthetic value and partly for the challenge of learning how to work with tubes). I appreciate all the help you can offer! Thanks everyone!
« Last Edit: June 03, 2013, 03:03:23 AM by jaylow »

phatt

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Re: Low Voltage Tube Preamp
« Reply #1 on: June 03, 2013, 03:56:18 AM »
Hi jay,
the 12u7 pair (V1 and 2) is the preamp but you may need a buffer stage to drive long lines.

FWIW,, don't waste time with low voltage stuff. OK it passes signal and it WILL distort.
Which (as You have said) You don't want.
So You are going to fight an up hill battle.

Plenty of High voltage stuff around and can work as you ask.
Heck I use a hybrid Preamp implementing only One AX7 and it can come out very clean.

The High voltage is what scares most folks but it's no big deal and simple to implement.

Look here for my latest effort using a nixie psu for high voltage.  D load the sample for an idea.
Goes way clean to way Distorted.
Runs from an old printer supply delivering 18 VDC.
http://www.ssguitar.com/index.php?topic=2895.0
Have fun,, Phil.

jaylow

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Re: Low Voltage Tube Preamp
« Reply #2 on: June 03, 2013, 03:56:39 PM »
I know that the 12U7 is the preamp section. I'm just wondering if it ends before or after R7 and R9.

And Phil, your design looks impressive! It's a bit much for what I'd like to do but looks like something fun in the future.

Kaz Kylheku

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Re: Low Voltage Tube Preamp
« Reply #3 on: June 24, 2013, 09:07:22 PM »
I know that the 12U7 is the preamp section. I'm just wondering if it ends before or after R7 and R9.

R7 and R9 are part of the input stage of the power amp tube. The reason is that they bias the grid.  The signal is superimposed onto the bias via the C2 capacitor.

The division between the stages runs down the middle of the capacitor, so to speak. If you split the amplifier into separate devices, then C2 ends up being cloned: the preamp will end with an output coupler, and the power amp will have an input coupler.

Without the output coupler the preamp will produce a signal with a big DC offset in it.   

Without the input coupler, the power amp's grid bias will be disturbed by DC offsets in the input.

If things are split into separate devices, you may also want some DC return shunt resistors to ground, outside of the capacitors on both sides (relative to looking into the circuit).-

If a device with a coupling capacitor is connected to another device, and that other device produces a bias current, and there is no return path for the bias current, then that current goes into the capacitor. When as capacitor charges, the current slows down and stops. When you block the other device's bias current, it will probably stop working. This could happen in either direction.  If you know that the other device doesn't have a bias current, you can omit the DC return resistor.

You can even omit a coupling cap from one device if you're sure that the other device will always provide one.

Definitely, R4 is part of the preamp. It's a necessary V2 anode load resistor which develops the output voltage as the current across it fluctuates.

I hope I haven't over-complicated things and that you now know better how to find the division between stages.

« Last Edit: June 24, 2013, 09:09:39 PM by Kaz Kylheku »
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Roly

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Re: Low Voltage Tube Preamp
« Reply #4 on: July 19, 2013, 11:21:12 AM »
Beats me.  There is a very old saying that there are "horses for courses", that some things are better suited to some tasks than others - you open a tin can with a can opener, not a pair of bolt cutters, but you don't try to cut steel rod with a can opener.

Valves (toobs) are high voltage devices; transistors are low voltage devices.  That some valves can be made to work at very low voltages is a result of luck, not intent, and that should be evident by the fact that they don't work very well, e.g. low gain and high distortion.

If you want to learn how valves work and can't cope with high voltages, then build a FET amp, they are very similar apart from the voltages.  But even a solid state amp requires anything from 20 to 60 volts to be useful.

High voltage supplies are easy to build, and literally thousands of people have built valve amps using high voltages without mishap.  It's a lot safer than amateur rocketry or trail bike riding.

"Low voltages like 12 volts are safe".

Oh really?



Having worked around electronics (including power control) all my life I'm much more wary of high currents than high voltages - a 300 volts supply may give you a very alarming bite, but it can't take your eye out like an exploding twelve volt battery, nor is it as likely to start a fire.

If you can cook something on a stove, or even handle a soldering iron, without giving yourself third-degree burns then you can handle high voltages safely.
If you say theory and practice don't agree you haven't applied enough theory.

Jack1962

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Re: Low Voltage Tube Preamp
« Reply #5 on: August 12, 2013, 07:12:49 AM »
to use low( under 160 volts) you will have to use tubes that where used in things such as car radio's . However on this subject I agree with Roly , the voltage isn't your worry current is and "most" tube amp circuitry is low current. Not to offend anyone on this forum but Hoffman's form http://www.hoffmanamps.com/ would be a better source of info on this subject.