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Old 24th April 2004, 06:27 AM   #1
Kaiowas is offline Kaiowas  Colombia
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Default Guitar Tube Preamp


I want to build a Tube Preamp for my guitar, but this would be my first tube project.

The idea is to give some tone to my sound, because i have a solid state amp, but here's my question:

I've seen many preamp schematics (Marshall JCM 800 2203 or the Vox Berkeley Preamp, for example), but as they the Preamp of a Power Amp i don't know if i should use the same power supply or if they need some modifications.

I'll appreaciate any help!!!
Thanks in advance!!

(I've also posted this msg on Duncan's Amp's Forum, but no answer, i really need some help!)
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Old 24th April 2004, 08:38 AM   #2
svokke is offline svokke  Belgium
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I am sure some people over here can help you, but a lot of guitar amp gurus are at:
In illusion comfort lies
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Old 24th April 2004, 08:50 AM   #3
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Default You might check out

You might find some of the tube amp and preamp kits at worth a quick look-see. If you want to go straight to the tube section of their website, click this link:

Some years ago I built their Stack-In-A-Box. Their kit was pretty complete (includes a front-panel, but no case), and as I recall the directions were pretty decent. It's been a while, I forget some of the details.

I liked the sound that I got from the Stack-In-A-Box - it gives you a little tube warmth, but will not (at least with my guitar and playing style), give you loads of crunch or extreme distortion.

The single tube ( a 12 AX-7, if memory serves) died after a short life of a few months. That was my only bad experience with the kit, but better quality AX-7's are not too hard to come by.

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Old 24th April 2004, 09:11 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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I have built the guitar preamp at and it sounded very nice. It has a gain, 3 band eq, and output volume control. Uses 2 ECC83. Has a wide range of tones from clean to grinding.

One mod I did was change the output resistor from 22K to 10K to get a hotter output (line level!)

Also shielded cable, star grounding and possible DC filaments keep this one totally quiet.
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Old 24th April 2004, 07:24 PM   #5
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Default Transformer spec?

ShiFtY, what was the secondary voltage on the power supply transformer you used for that preamp?

Its easy enough to convert from the DC voltage at the output of the diode bridge to RMS rating of the transformer secondary, but not knowing the preamps current draw, I don't know how many volts are lost in the series (22k + 10 k) resistors in the positive lead, which in turn means I don't know the voltage at the diode bridge output.

This may be something that every real tubehead knows the answer to, but I have to confess that the majority of circuits I've built have been solid-state.

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Old 24th April 2004, 10:21 PM   #6
SHiFTY is offline SHiFTY  New Zealand
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From memory I think the transformer was about 300-0-300, giving 300V using a tube rectifier (5Y3) and 10uF capacitors.

You could just adjust the dropping resistors to suit, anywhere near 300V will work just fine (say +/- 10%)
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