Aclam Guitars Windmiller/Grampian 636 preamp

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Sesh
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Post by Sesh »

Hi there

I asked the good Fuzzhead (dirtboxlayouts.blogspot.com) to see if he could come up with a layout for a vero version of the preamp of the legendary Grampian 636 spring reverb unit. Its claim to fame is that it was used by Pete Townshend as a distortion unit and for dub goodness by King Tubby.

I really want to build it for it's awesome saturation properties (to use with synths, drum machines etc.). Plus, of course, I'm curious to try it on guitar as well. I thought others might, too!

So as he was working on it, I found out that Aclam Guitars has just released the Windmiller preamp which is exactly that.
https://www.aclamguitars.com/thewindmillerpreamp

Fuzzhead was trying to figure out how much of the circuit should be included if it's supposed to work only as a distortion unit, like the Windmiller.
Here's what we've got so far:
it has pointers and question marks for the things Fuzzhead is unsure about.

Here's the original schematic: https://soundgas.com/wp-content/uploads ... ST4328.pdf
Technical docs: https://soundgas.com/wp-content/uploads ... ure_p1.pdf

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Frank_NH
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Post by Frank_NH »

Thanks for the schematics! Here is a link for more info on Pete Townshend's Grampian reverb unit:

https://www.thewho.net/whotabs/gear/gui ... everb.html

From the photos, it looks like Pete plugged into the low signal (10 mV) Aux input (no need for the mic input part of the circuit). Looks like the PNP transistors are running off a -9V supply, which you can get from a charge pump IC. Like Aclam, I would experiment with a tone control and perhaps add a master volume as well.

[edit]
The output stage is actually at -12V and a resistor brings it down to -9V for the earlier stages. Also note that the original had germanium transistors. Don't know how much difference that will make. I'm playing around with the circuit in LTSpice and can report what I find later.

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Post by Sesh »

Yeah, I thought as well about the mic preamp. Would be great to add for a larger rack unit clone, tho - transformers add sweet saturation in themselves.

Fuzzhead also suggested the master volume by making the last resistor a pot.

Awesome.. let me know what you find out. I'm certain Fuzzhead would love to do a vero layout if it pans out great. Should definitely keep the germanium transistors of the original - love the "imperfection" they bring to the table.

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Post by Frank_NH »

One issue as mentioned previously is getting the -12V voltage. You could use an LT1054 charge pump for this and simply create a voltage inverter to go with a +12V wall wart power supply. There may also be a way to achieve this it with a normal +9V power supply.

My LTSpice analysis seems to indicate that the circuit is very bass heavy for an overdrive, probably because of all the large bypass and coupling caps. You can tame this by reducing the values of these caps. It can also use a treble roll-off as it has a pretty flat frequency response above 1000 Hz. It doesn't seem like the transistors matter too much (Si or Ge) but then it would be good to breadboard this before creating a vero layout just to confirm that it works and to tinker with the stages to dial in a good sound.

Interestingly, I don't think Pete ever used the reverb itself although I could be wrong. I wonder if it would be worth adding the reverb part of the circuit, maybe using a Belton brick style reverb as a substitute.

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Post by Sesh »

Frank_NH wrote: 04 Aug 2021, 12:27 One issue as mentioned previously is getting the -12V voltage. You could use an LT1054 charge pump for this and simply create a voltage inverter to go with a +12V wall wart power supply. There may also be a way to achieve this it with a normal +9V power supply.

My LTSpice analysis seems to indicate that the circuit is very bass heavy for an overdrive, probably because of all the large bypass and coupling caps. You can tame this by reducing the values of these caps. It can also use a treble roll-off as it has a pretty flat frequency response above 1000 Hz. It doesn't seem like the transistors matter too much (Si or Ge) but then it would be good to breadboard this before creating a vero layout just to confirm that it works and to tinker with the stages to dial in a good sound.

Interestingly, I don't think Pete ever used the reverb itself although I could be wrong. I wonder if it would be worth adding the reverb part of the circuit, maybe using a Belton brick style reverb as a substitute.
Right, we have considered the charge pump+inverter option.

Thanks for taking the time to analyze the circuit. Fuzzhead has an encyclopedic knowledge of tone stacks and controls, so he might find a good one to use with this one. The Aclam has hi- and low-cut. From what you describe, that's probably a good idea to incorporate.

What did you do about the node leading to the reverb driver in your simulation? Did you delete the 25 uf cap?

Honestly, it may be worth recreating the whole thing, with spring and all... look at how much money Soundgas' modern reproduction costs: https://soundgas.com/product/soundgas-t ... ng-reverb/ Make the circuit work and make PCBs for everybody!

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Post by Frank_NH »

"What did you do about the node leading to the reverb driver in your simulation? Did you delete the 25 uf cap?"

I just removed it as it's just part of the reverb mix. Let us know if you breadboard the circuit!

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