so first the schematic:
http://i263.photobucket.com/albums/ii151/cameramanlink/GuitarPreamp-1.gif
I was looking for a preamp to make for a friend at my church for his electric guitar. His effects board makes his guitar sound well, not great. I wanted to help the guy out and make a tube preamp.
So I googled, guitar tube preamp and found this schematic. I liked how simple it is and I like that its a plate follower so that its high output impedance, so it could go into a DI box, to the house board.
I know the bypass caps are quite large and I added a 22k grid stopper resistor at the second grid, to take advantage of the Miller capacitance effect to remove the radio frequencies I was picking up. Plus there is ALOT of gain so it helped attenuate this design.
As well the B+ voltage is quite low as it suggests 110 VAC recitified into about 160 VDC. My preamp is slightly different at 123 VAC into 174 VDC.
I am impressed at how little hum there is with this preamp and how it sounds. It sounds really nice.
I am looking to make another one for the other guitarist and I was curious if there is anything you see that immediately is like, that isn't right.
Thank you for your help,
James
http://i263.photobucket.com/albums/ii151/cameramanlink/GuitarPreamp-1.gif
I was looking for a preamp to make for a friend at my church for his electric guitar. His effects board makes his guitar sound well, not great. I wanted to help the guy out and make a tube preamp.
So I googled, guitar tube preamp and found this schematic. I liked how simple it is and I like that its a plate follower so that its high output impedance, so it could go into a DI box, to the house board.
I know the bypass caps are quite large and I added a 22k grid stopper resistor at the second grid, to take advantage of the Miller capacitance effect to remove the radio frequencies I was picking up. Plus there is ALOT of gain so it helped attenuate this design.
As well the B+ voltage is quite low as it suggests 110 VAC recitified into about 160 VDC. My preamp is slightly different at 123 VAC into 174 VDC.
I am impressed at how little hum there is with this preamp and how it sounds. It sounds really nice.
I am looking to make another one for the other guitarist and I was curious if there is anything you see that immediately is like, that isn't right.
Thank you for your help,
James
The first stage of the preamp will have no headroom for clean signals. You won't be able to turn the Gain control down far enough because the signal will be clipped before it gets there. The output impedance of the second stage is very high, driving a DI box will be difficult. The schematic looks more like a stompbox fuzz tone than a guitar preamp.
looks to me that A little resistance is need on input from a brief search, I think 68k would be a good place to start...something to make your gain usable.
regards,Elwood
regards,Elwood
For the next pre-amp add a 68k resistor before the first grid. Would it be advisable to switch the second stage to a cathode follower design to better drive a DI box?
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IMO and IME, 680k is way too high for a plate load. It will make for a very steep loadline. You might want to experiment with values below 470k.
The cathode resistor of V1 is wrong (15k?). It will be biased almost at cold cutoff, leaving absolutely no room for the down going input signal. If 15k is actually what's used, I don't think it's fit as a preamp, but rather an FX box. 3.3k would bias the stage at 1V, leaving 2Vpp for the input.
The 1M output pot is wired in such a way that preamp's output impedance varies depending on it's setting. An output should in this case be buffered so the volume settings doesn't alter the tone. A cathode follower can do this.
I suggest adding another tube. You could even slap a simple tone control in the middle! Use one half for a more controlled gain stage, and the other half as a cathode follower output.
The cathode resistor of V1 is wrong (15k?). It will be biased almost at cold cutoff, leaving absolutely no room for the down going input signal. If 15k is actually what's used, I don't think it's fit as a preamp, but rather an FX box. 3.3k would bias the stage at 1V, leaving 2Vpp for the input.
The 1M output pot is wired in such a way that preamp's output impedance varies depending on it's setting. An output should in this case be buffered so the volume settings doesn't alter the tone. A cathode follower can do this.
I suggest adding another tube. You could even slap a simple tone control in the middle! Use one half for a more controlled gain stage, and the other half as a cathode follower output.
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One more thing, adding 68k in front of the 1M grid leak will do very little. It will only reduce the input signal by 0.57dB. If more headroom is what you're after, you could increase the B+, get rid of the cathode bypass cap and choose a more suitable bias point.
Change the power supply to a voltage doubler and then use more "traditional" values of plate load and bias resistors.
On the back to back transformers, connect the 12V widings together and ground the centre tap on the input transformer only.
Cheers,
Ian
On the back to back transformers, connect the 12V widings together and ground the centre tap on the input transformer only.
Cheers,
Ian
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