Fender Bassman and Sunn 200s preamp pedals.

[Pavlos]

Not sure how much this reply from Ciaran regarding the Sunn 200s would apply to the Bassman? I'm guessing that changing the 68K resistor would possibly have the same effect as acting as a pre-gain? but there is already a 47uF cap between the second FET's source and ground, so would increasing/decreasing that cap affect the gain?

Many thanks in advance


If you want to add gain you could try putting a cap between the second FETs source and ground...just like the first stages 250uF...maybe on a switch?  Theres room on the layout for that.

Reducing the 68k at input will allow more guitar signal it hit the first gate.  Maybe replace that with a pot and call it pre-gain?

[Ciaran Haslett]

Hi Pavlos.

Yes, changing the resistor to a pot would act exactly as described.  Increasing the cap value will increase the gain.  decreasing will lower the gain.  But there comes a point where it just starts sounding terrible so I would definitely socket it and try a few values.

Good luck.

[Pavlos]

Thanks Ciaran, much appreciated  been building for less than a year (Mostly successfully so far)and still haven't really got a clue what I'm doing, but thanks to the generosity of all those like yourself who are so willing to help and share their superior knowledge with the rest of us, I've managed to learn far more than I thought I could when I started......so now I know next to nowt, rather than absolutely nowt, and some day I might even now summat, until then simple questions and little steps

[velouriafx]

I built Bassman Preamp with ROG Condor Cab Sim and they worked very well together. The Cab Sim have excessive trebles to my ears and i will mod this. When i finish, i come back here to post a demo. Sorry my poor english.

[alexdestructor001]

So...
I just finished the Sunn200s, everything seems fine, no bridges, no cold solders, but when I plug I just heard the guitar like really really quiet, in the background, at very low volume. The tone and the volume control seems to work fine tho.
Is just the signal that's comin' out too low.
What could be the problem? Thanks in advance, and sorry for my english...

[holm]

Hello
Just finished this, and it sounds pretty good, however I find the high frequencies a bit hizzy and I tend to roll the treble all off. Is there a way to reduce the hizz? Also, why isn't there any diodes included in the layout?


In advance thanks
Erik

[traktop]

You could try a couple of caps to ground on a switch before the máster volume ala "bright switch"

[holm]

Thanks, I'll give that a go.

[SeaWitch]

Frank_NH wrote

According to my JFET spreadsheet, for a 1.5K source resistor, the optimal drain resistance for a "typical" 2N5457 is only about 7K!  So a 100K trimmer is overkill.  You can use a 20K (or even a 10K).  Try to get about 4.5V - 5V on both drains.

If I add a voltage doubler to either of these builds, is there a general rule as to what voltage I should re-bias the drains on the 2N5457's?

For a 9V source, the drains should be 4.5V - 5V, what should they be with a 18V source?

Thanks!

[rocket88]

Not really a rule of thumb, but what I’ve found with these preamp builds is typically if you multiply the bias by how ever many times you’re increasing the voltage from 9V. IE if you’re doubling the voltage and the bias at 9V is 4.5V, setting the new bias to 9V should get you in the ballpark, and may or may not need a little tweaking to get it perfect. What I’ve found from my mini amp builds is that since I’m running them at 24V, I keep the ratio of FET bias to voltage and it’s typiclaly spot on, and using trimmers makes it nice and easy to get it where I want.

[SeaWitch]

Thanks!  Exactly what I was looking for!

On 11/25/2018 1:48 PM, rocket88 [via Guitar FX Layouts] wrote:

Not really a rule of thumb, but what I’ve found with these preamp builds is typically if you multiply the bias by how ever many times you’re increasing the voltage from 9V. IE if you’re doubling the voltage and the bias at 9V is 4.5V, setting the new bias to 9V should get you in the ballpark, and may or may not need a little tweaking to get it perfect.

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[SaidBisher]

Hello all. I'm a bit of a noob and new to this forum. I'm planning on building a sunn 200s preamp but I am unsure about the voltage requirement of the capacitors and the wattage requirement of the resistors. My own amateur logic tells me that 16v to 50v capacitors and 1/4watt or 1/2watt resistors will do but it would be great to have a confirmation from a wiser head. I know this is an old thread but, fingers crossed, someone will help me out.

Edit: I'll power it using a 12volt power supply.

[rocket88]

1/4watt resistors are commonly used, and cap voltage is based on how much voltage your giving the board. You always want to make sure it’s at least a few bolts higher to decrease voltage issues.

[SaidBisher]

Thanks a lot. Very helpful

[SaidBisher]

Is it possible to build the sunn 200s preamp with BF245A transistors and 1N5819 diodes? These are the closest substitutes I can find in Kenya where I am. Do I need to make any changes to the circuit to accomodate the substitution?

[turnbuckle]

First off... thanks for posting these! I have built them both, and the Bassman sounds great! But my build of  the Sunn sounds extremely bright and brittle, and wayyy scooped. I built it to the spec, with the exception of the diodes. I couldn't easily find Bat85s, so I used some Bat46 that I had. I don't know the forward voltage expected of the Bat85s, and I didn't measure the Bat46s... could that be part of the problem? Or should I just try experiment with other value caps in the tonestack? Or elsewhere?  Or?... Thanks!