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Old 2nd March 2007, 11:08 PM   #1
fresno is offline fresno  United States
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Default Tube Preamp as a Guitar Effect

I have been warned against posting as a nūb in diyaudio forums, but here it goes anyway. I have built my own audio equipment in the past, but this is my first try at designing something. As such, I have little knowledge of designing circuitry and electrical laws. I do know ohm's law, but not much else. I'm capable of figuring out what something does by the schematic, but not why.

I want to make a Tube-based distortion effect pedal to place between the guitar itself, and a non-tube amplifier. I know that tubes are not vibration-friendly, so placing the tube on a stomp-box is probably not a good idea. If the tube and the stomp-box need to be separated, I'll just have to figure that one out. I think I'll use a 12AX7 tube as they seem to be the most readily available at a cheap price (I'm aware that you get what you pay for, but we're not going for pristine sound here).

Websites seem to recommend getting real tube sound by having a tube based preamp as the least-expensive way. As this will be going into an amp afterwards, it doesn't need a tone stack or significant volume increase really.

The proposed design is as such:

Guitar Input -> Volume Pot (my guitar lacks one) -> Tube -> Volume Pot -> Out to Amp

I'm debating how to use the two halves of the valve, and I'm looking for the simplest design possible, to minimise mistakes.

I have basic knowledge of how a tube works, but I don't know how to properly put one in a circuit.

My primary questions are:
  • Do I need an output transformer in this configuration?
  • Can I run this off batteries?
  • What position in the circuit does the "Gain" knob usually occupy?
  • How do I figure out what voltages need to go where, etc.
  • Can I make it glow purple?

I'm not good at getting my thoughts down on paper, so I tend to over-explain a lot. I've probably not made my idea as clear as I wish it to be, but that remains to be seen. I realise my spelling tends to be british sometimes, but I'm American. Chances are this is a lot simpler than I believe.

I apologise in advance for my noob-ness, and long post.
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Old 3rd March 2007, 11:42 PM   #2
kevinkr is offline kevinkr  United States
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I've never seen anyone beating up on a nubbie here who was honest and down to earth about his lack of experience - so you've got nothing to worry about on that account.

The 12AX7A is certainly a very popular tube in the MI marketplace in general, but some examples can be quite microphonic. Perhaps this can be dealt with via shock mounting the socket (rubber bushings and using flexible wiring from socket to a tag strip carrying the actual components.

Power supply from batteries is not practical with the 12AX7A, but the supply does not have to be very complex, just well filtered. Use a Sovtek 12AX7LPS (hum cancelling filament design) and you can probably get away with AC on the filament.

The circuit itself probably ought to be very similar to that found in the first two stages of a Fender or Marshall amplifier.

In you can fit it (doubtful) the power supply could go in the box itself, but more likely should go in a seperate box with an umbilical cable connected with a LOCKING connector (for safety) like an AMP CPC type.

If you use the usual instrument input on your amplifier you will not need a transformer, but keep the cable short (6') and bear in mind that this is not going to work very well as a DI box.

If your guitar has no volume control you can add one at the input should probably be either 250K or 500K depending on pickups. You might want to try both, and you might want to try alternate configurations that vary the loading on the pick up as well as the usual volume control configuration.. Experiment and see what you like and/or what works best..

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Old 4th March 2007, 12:23 AM   #3
teemuk is offline teemuk  Finland
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Everyone is a newbie once - no shame in that.

You do not need an output transformer as it is used for impedance matching between high impedance loads required by tubes and low impedance load presented by the sepaker. You might consider using one due to another reason explained soon.

Note that a tube in the signal path will not give you an outstanding tone unless you voice the circuit to do so. For example, if you never overdrive the tube stage it's existence is pretty worthless. In practice, you can pretty much mimic the soft clipping behaviour of a distorting preamp tube (it's transfer curve) even with a simple circuit that consists of some diodes and resistors so for "preamp distortion" the fact how much gain you use and how you voice the distortion tone is pretty much where all the magic lies. The equalizing you incorporate post/pre the distorting stage makes a great difference.

I'm pretty sure you have heard about the concept of "power tube distortion sounding better than preamp distortion". This phenomenom has a lot to do with complex interactions of power supply voltage sag, use of output transformer and high output impedance output stage together with an odd impedance of speaker load creating an odd frequency response. To recreate this behaviour you should design (likely a push-pull) transformer coupled circuit that would use an odd impedance load that mimics the impedance curve of a speaker. A saturating transformer will limit bandwidth so the interaction with it should not be neglected. Then figure out a way to make supply voltage sag even on low current that a circuit running on preamp tubes likely requires.
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Old 28th March 2007, 01:18 AM   #4
fresno is offline fresno  United States
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Default Space Charge

So, if it's impossible to run a 12ax7 from a battery, would so called "Space Charge" tubes be an option?

And I'm sure power distortion is better, and I'll probably get around to building a full amp someday, but this is more of a project to make people go "oooooh! it glows!" and to convince Greg to stop using his awful distortion.

What type of equalizing do I need?

I've recently learned that the tone I'm going for is called "Marshall Crunch" or something like that.

I think.

Sorry for my lack of knowledge but my time is split multiple ways as of late.
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Old 28th March 2007, 02:20 AM   #5
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Welll this is a big undertaking that you are involved in. The biggest problem with battery powered tube pedals is that the filaments will always draw lots of current. Let's say you use a 9V alkiline battery, well as 550mAH of life, and the heaters drawing between 150-300mA each... well that only leaves you with 1.5 - 3.5 hours of battery life. A compromise might be running the heaters off 4-5 AA batteries, which will give you 8-10 hours of life 9which is about the same as battery life for a chorus pedal--so that's not TOO bad), and then run the high voltage supply off a 9v or a pair of 9v in series for 18 volts. You'll only be drawing a few mA from the 9v's, so they will last a long long time... probabluy approaching twice as long as a tubescreamer (which draws around 7mA or so depending).

The second biggest problem is that you need to get the plate voltage up high enough to get the tube conducting decently. A 12ax7 will start to work well enough for this project around 40-50 volts, but you don't need a super high gain tube like that for a low voltage pedal, because you only need a few volts of swing to get it to clip. So you would probably be better off with the just-as-common 12au7 and treat it like a JFET preamp gainwise.... eg instead of swinging 50 volts of AC signal around, think more in the ballpark of 5 volts. A pair of 9v batteries is definately doable though, and with some tweaking, you MIGHT be able to get some decent sounds out of a 12ax7 even... definately not marshall quality, but close.

As far as tone controlls, just scale the standard Marshall tone stack down to whatever impedance you are feeding it. You can download Dave Cigna's tone stack calculator on Duncan Monroe's website, which will do all the dirty work for you...ie less math for you.


Here is my recommendation... use a simple Fet input stag (the input stage is not going to be clipping anyway), like a J201 or whatever you have lying around, and then follow that up with a 12au7 tube, and just scale down a Marshall JCM800 circuit, but with the tone stack hanging off the plate instead of a DC coupled cathode follower. That way you have one tube going, but still have the 3 fain stages that you need, because if you add another one, you'll be halving your battery life. Run the tube heater off 4 AA batteries and the plate supply off two 9v.
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