Fender Bassman and Sunn 200s preamp pedals.

[Beaker]

Found these over at diystompboxes. Originally posted by Dead Astronaut:

Would anyone like to have a go at them?

First the Fender Bassman:





Second, the Sunn 200s:




Here's a link to the thread, with verified PCB and perf layouts (but only for the Bassman):

http://www.diystompboxes.com/smfforum/index.php?topic=101898.0

And here is the perf layout for the Bassman:

https://dl.dropboxusercontent.com/u/7464107/bassmanlayout.pdf#page=1&zoom=300,-2,595

It's on a PDF and I can't work out how to post the image on it!

[rocket88]

I'll take care of these. I'll try and do them tonight while my new pup is sleeping. Funny thing is I'm still playing with a turretboard amp layout for the 2000s model, which is the big brother to the 200s.

[Beaker]

Thanks Zach!

[Ciaran Haslett]

Sorry Zach!  I was mid way through when you posted.

Here's the Fender Bassman


Beaker...I went with the final schematic in that thread that dead astro posted.  In this he removed the diodes which he thought were imperceptible on tone.  He also moved the switch to the treble caps as he thought the previous position provided way too much of a volume boost and didn't like what it did for the tone either.  This same schematic was used for the perf layout.

As always with my layouts....give it a good going over first

[rocket88]

no worries man. i just finished it, and wasn't really that happy with the layout, 19x10, so i'm still messing with it. i did it based on the schematic beaker posted, and was going to make one on the modded on in the thread. yours looks great man.

i ain mad, just means i can go back to working on the tube amp layouts. all i can say is stay tuned, i've got some real good stuff in the works. just to add to the anticipation i'm working on the following now: orange or120 (graphic mk2) and 1979 superbass, matamp gt100 (gave birth to the or120), sunn 2000s, and green MV120 to name a few. i recently finished the layout for the 1st gen sunn model t

[Ciaran Haslett]

Seriously can't wait to see those lad!  Those Sunn 2000s have a certain something going on that I just love

[Beaker]

Thanks Ciaran, that looks great. I can't wait to see the Sunn version!

Zach, my son is really excited to see the Sunn 2000s when you have it ready.

[rocket88]

I'm going to upload a few at a time instead of one at a time, this way there's more amps available to build. The 2000s is a little tricky because of the way the switch is wired and the amp design is pretty unique. That and I kinda have my own way of doing amp layouts. Instead of doing everyone on one big board I have strips so you can see the amp chasis through the components, which I think just looks better/cooler then looking at a big piece of empty board, and I really try to keep everything with as few cross over wiring as possible. One thing to keep in mind is they some of the components are not to scale, so when we all go to build them we may have to adjust the spacing of some of the turrets to get it right. Some of the electros use for tube amps can be massive.

I'm also trying to figure out how to modify the 2000s to give it more gain, maybe have another preamp tube that's switchable, and I'm going to add a master volume since the original didn't have one and the amp breakup is when your at like 6 or 8. So I'm thinking with a master volume the original volume ends up controlling the gain, so you can drive the amp at lower volumes.

Leave it to me to always tinker. Lol

[Ciaran Haslett]

.....and the Sunn shines bright (hopefully)

[Beaker]

Wow! Thanks Ciaran. I had better get busy.

I can't wait to find out how different they sound.

[rocket88]

Should be very different. I know both schematics pretty well now, and the sunn is very unique in it's preamp. It's part of the reason it's taking me awhile to get the amp done. It's so different it's not to my liking as far as keeping all pot wiring on one side and tubes on the other. Its what really makes it a real highfi special amp.

Hfe, can't wait to hear what you think of the sunn beaker.

[Ciaran Haslett]

So I threw the tone stacks into TSC and got the following...

Fender Bassman


Sunn 200s



By the looks of things they will sound massively different!!!

[skeletonghost]

Planning on building one of these very soon, maybe the answer to this is obvious: Preamp pedals like these are meant to be played in the FX loop to bypass the preamp stage of your amp? Is there any sense in playing them through the instr. input?

[rocket88]

my understanding is that you they should be put in line with your instrument, at the end your your pedal chain. just set the preamp tone controls flat and you should get the tone of the preamp pedal rather then your amps preamp controls. at least this is my understanding of how to use them, so i could be wrong.

[Ciaran Haslett]

I've used a few preamps live before and always put it at the end of my chain as Zach said.  I remember putting one in the loop and almost putting my back out as my tone controls were now on the floor instead of the amp (I tend to tweak quite a bit playing live).  Up and down like that water sipping bird

[SonusFluxa]

Not sure if anyone has verified this yet, but I built the Sunn last night, and it sounds amazing! Definitely putting this at the end of my chain.

I currently have it set for an awesome clean tone. However, the left trimmer acts like a gain control for about a 1/4 of the rotation before sputtering out. Would taking this offboard make any sense, maybe as a 20k or 25k pot? Or is it best to leave it where it is?

Thank you Ciaran, this really adds a new dimension to my guitar in exactly the way I was hoping.

[Ciaran Haslett]

Thanks for verifying Sonus

That trimmer just sets the bias of its JFET.  Its function is to find the voltage your FET works at best and keep it there.  You may find that swapping the FET might yield better results (they are notoriously all over the place in terms of spec).  I wouldn't bother taking it off board.  Reducing to a 25K might be more accurate finding the sweet spot if it only works for the first 1/4 turn...but again that might only be due to the particular FET you're using at the minute.

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?

Anyway...thanks once again lad.  I really should build this...my bro has been after a preamp for ages and for some reason I never got round to it.  I can now build in confidence lol.

Cheers again

[Frank_NH]

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.

[Ciaran Haslett]

Cheers for that Frank.  Didn't you share your spreadsheet at one stage?  I'd love to get a look at it.  Sounds like a great resource.

[Ciaran Haslett]

Serves me right for not looking.  Found it!