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Old 3rd February 2012, 07:53 AM   #1
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Default Guitar amp first stage voltage 100 or 250 volts

What is the difference I will see if any by choosing to run a 12ax7a at 100 volts rather than 250 volts DC? According to the tube manual, both still have an amplification factor of 100. I am confused at which voltage I should choose. I am also deriving my voltage from a 12 volt source in which I am using a 555 timer running a mosfet to get my B+. Any suggestions? I would like to use both half's of the tube. Can a 12ax7a overdrive the second half of itself on purpose. I am looking for both clean and overdrive by just using one 12ax7a. It will then go on to an external amplifier of quality or to a board for recording. I am interested in a small space to put it all in. I have already built the power supply portion and can output 250 volts at 1.8 ma or about 150-175 or so at 5.5 ma. I can get a tad bit more but trying to keep it cool as the transformer is not that big.
So which is it I should use 100 volts or 250 volts on the plate?
thanks
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Old 3rd February 2012, 08:36 AM   #2
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a difference in available output swing is the first issue, but really, you've grabbed the tiger by the wrong end - reach for the tail...

By this i mean, you design the voltage amp stages to be able to drive the output stage. So you need to know what the drive requirement of the output stage is first before any reasonable and useful statement can be made about voltage gain design
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Old 3rd February 2012, 09:24 AM   #3
DF96 is offline DF96  England
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Do some reading on load lines and valve characteristics. Then you will be able to understand the valve data sheet.
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Old 3rd February 2012, 05:16 PM   #4
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thanks for the input, good points. The output is really only going to go to computer line level as that is what I have intended for it. So line level output is what I need. It is for a project I want to try out. I want to run the output to a computer audio input to record with it. If I cant overdrive the second half of the tube I will most likely be able to simulate it with computer program. I recall there is an overdrive simulator. So from what I gather from the post, for my purpose probably not a lot to worry, 100 or 250 volts for the swing, as I don't really have to drive anything. just output to, I think 10k load. I think 10k is the usual load for line level, I could be wrong. Is that correct then that 100 volts will do just fine?

Last edited by yourownfree; 3rd February 2012 at 05:18 PM.
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Old 4th February 2012, 12:49 AM   #5
kevinkr is offline kevinkr  United States
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Since this is guitar amplifier related I'm moving it to the Instrument & Amps forum per forum policy. It may also be of interest to the group there.
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Old 4th February 2012, 03:13 AM   #6
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if you overdrive both sections of the ax7 as voltage amps you will blow the input of your soundcard (and possibly more...) through the middle of next week. You will be outputing a signal around 60+ V rms...

Time for a rethink.
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Old 4th February 2012, 04:29 AM   #7
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I'm not not that noob as it may seem. lol.. I will of course take steps to reduce that amount after I run it through the tubes and match it to the line level input of the computer. The whole idea behind this is to give the person the thrill of believing that they can get a tube sound since it went through a tube and using a name type of tube. Most of it is in the head above the shoulders in what they believe, of course. I have seen one in a magazine or online, dont have any details when or where though. Am I suppose to go to the musical part and post now and not here? I am a noob here as I am not here that often but would like to be as much as I can. I find all the info fascinating and rewarding. Besides the only place I know where I can actually talk electronics, nobody else to talk to. So let me know if I need to end this here and move to music section. Thanks.
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Old 4th February 2012, 08:46 AM   #8
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Hi, a moderator has moved the thread for you: I suspect you'll find you're unable to move it yourself.
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Old 5th February 2012, 02:13 AM   #9
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Using a 12AX7 gives you two stages of gain. The first one will not get overdriven to any great extent by a guitar. It might be a good idea to have a FET as a gain stage before it to give the first tube something to work with. Maybe a gain of ten with a pot to adjust the amount of drive. While the tube can run on lower voltages it sounds better fed around the 250-300v range. You would want a pot to reduce the voltage going to the second stage also and then one at the output. Along the way you may want a toggle switch with three positions (on-off-on) with the center position with a 2.7k resistor on the cathode. Then you switch in a 3.3k which will give you 1.5k, or the other position with a 1.2k which will give you 830 ohms. This gives you a cold bias, center bias, and warm bias. They each have a different sound when overdriven. The cathode on the first stage should have a 0.68uF cap and a 22uF on the same type of toggle switch. This give an increas in gain but the 0.68uF cap limits the bass. Coupling caps of 0.22uF or less, 100k plate resistors. This should give you some variety. The 1.8mA is marginal but if it drops a few volts it should still be ok.
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Old 6th February 2012, 02:26 AM   #10
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The 1st stage of a guitar amp needs 2 things, fairly clean amplification and low noise.
That low noise part dictates running at higher current (say 0.8mA to 1.0 mA) for a 12AX7. At that current you need plenty of anode volts such that the bias voltage required is above the -0.6V below which grid current becomes a problem.
You will definitely want the 250V rail rather than the 100V rail. For 100V you would want a different tube.
Cheers,
Ian
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