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Old 18th February 2011, 02:30 PM   #1
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Default Begginer questions about a guitar pre amp

Hello Everyone...

I have a quick question to all the guitar experts there. I wanted to make a tube pre amp to be used with my regular guitar amp (sort like a stompbox of sorts). Keep in mind the amp is not a power amp, just a regular guitar amp. Does that make sense? Will i be able to get that tube feeling with it? Or a tube preamp should be used only with a power amp. I am good with electronics so that wont be a problem.

Thanks for all the help
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Old 18th February 2011, 02:35 PM   #2
TheGimp is offline TheGimp  United States
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You can aget some of the harmonic distortion given that you have an effects loop you can feed into.

You will not be able to get the output stage clipping distortion, unless you simulate it by using a small PP output stage with something like 6AU6 tubes. By the time you do this you may only have saved (1) cost of output tubes and larger output transformer, and (2) cost of a larger power transformer and filter caps.
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Old 18th February 2011, 02:40 PM   #3
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Hello there. Thanks for the quick reply. I do understand the part of not being able to have the power tubes distortion, which i am fine with it. Now, the details, what should be the gain? Does it make sense to go full gain and get some distortion out it, or will it sound bad? Should is even bother attaching and tone stack to it? Or should i leave it up to my regular amp?
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Old 18th February 2011, 02:55 PM   #4
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It all depends on what you want to achieve. A relatively clean, but slightly colored jazz-type sound, a bluesy overdriven warm tone, a hard rock distorted sound, a metal-type shred sound, a chaotic fuzz...? Take your pick. There are lots of schematics out there on the net that accomplish different things, some with tubes, some with silicon devices. The easiest is to pick a pedal that you like the sound of and then see if there's a schematic available for it. Pedal schematics, especially the ones using tubes, tend to be relatively straightforward and easy to imitate in a DIY setting.
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Old 18th February 2011, 03:02 PM   #5
GoranB is offline GoranB  Poland
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You can look inside your guitar amp and find the point where is connection between preamp and power amp. Discconect it and add an extra input and pot on chassis directly to your power amp so u can connect and use any preamp you want. You can also add a switch so you can choose, original preamp or external one.
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Old 18th February 2011, 03:19 PM   #6
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i figure i should start slow...just doing a one tube (maybe two), just ti get things going. Assuming i will not use a tone stack, do i still have to use a cathode follower? I mean i will plug the output of the preamp direct into my amp input (with the appropriate volume pot in between). the reason i am asking is because my amp was design to see a medium impedance going into it, and that may not be the case with the tube correct?
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Old 18th February 2011, 03:31 PM   #7
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Depends what 'medium' impedance stands for. But with one tube (one or two gain stages), you'll get plenty signal into the amp even with bad impedance matching. Don't forget to add a coupling capacitor to prevent blowing up the sensitive silicon in your guitar amp.

If it's just for experimental purposes, I'd recommend building some gain stages on an experimenting bread board, so you can easily swap components and experiment with different topologies. Browse schematics of amps you like for inspiration, and definitely include the effects pedal schematics I suggested earlier in your search.
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Old 18th February 2011, 05:46 PM   #8
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The medium i meant the impedance of a guitar, which as far as i know is not too high, but not too low (please correct me if i am wrong). Thanks for suggestions....
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Old 18th February 2011, 06:52 PM   #9
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As far as I know, a regular guitar pickup is just about as high an output impedance as it gets. Well, save maybe for microphones, I have no experience there, but a whole lot higher than your average hifi line stage. I wouldn't call it 'medium' for sure. A typical input impedance of a guitar amp's first gain stage is 1 megohm.
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Old 20th February 2011, 09:55 AM   #10
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hpupo: Google "Anvil preamp", "Kitty Hawk Quattro" and "Bogner The Fish".

You'll find some nice schematics and ideas!
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