The Beatdown: A True MIAB

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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Ripthorn
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Post by Ripthorn »

My main amp for many years was a Marshall 5210 solid state from 1983. These were the first solid state Marshalls (along with the Lead series amps) and have always been renowned for sounding particularly good for a solid state amp. I got rid of mine because I got a tube amp that I like and the cleans on the 5210 had a kind of honky quality that i didn't really like too much. But the gain channel was amazing!

So what do we do? Well, we go and take the dirty channel preamp and turn it into a pedal! There are a couple of small changes to it to make it a little more convenient for a pedal build, but it works quite nicely. It does a great classic rock crunch, and I used to get into higher gain territory by putting a Guv'nor in front. I really quite like it, so I thought I would share with you fine folk!

Here is a quick little demo I made of it. I used S-Gear for the clean base and the dirt is from the circuit on the breadboard, so some noise from that, for sure.

https://soundcloud.com/user-45568666/beatdowndemo1

Also, here is the schematic, broken up into the power and signal sections. I'll be working on a two layer board layout soon and will add it when completed. Let me know what you think!

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peterc
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Post by peterc »

Sounds glorious. Thanks.

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EddieTavares
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Post by EddieTavares »

Ripthorn

It sounds really amazing!
When I tried to breadboard some guitar preamps I never get close to the tone of that as real preamps connected direct to the power amp when i connect it in another preamp tube or SS, It sounds a little better when I change the tonestack to baxandall or big muff style tone control. You did without bigger changes! Great!

Something what you don't need in your circuit is the 78l15 it is just draining current and the max supply is 36v.

Ripthorn
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Post by Ripthorn »

Eddie, thanks for the confirmation about getting rid of the voltage regulator. I had that thought earlier, and that would allow me to get rid of the regulator, a cap, and a diode. I was a little concerned that I might need the regulator in there, but I guess the +17V or so would be as stable as the 9V input is, which shouldn't really be an issue in most cases. I worked up a perf layout (not a great one, but then again, I suck at them), but I'll get rid of those components and maybe it will allow my layout to be slightly less sucky.
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bmxguitarsbmx
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Post by bmxguitarsbmx »

Sounds killer Ripthorn! Thanks for the scheme and the clips! Some of those early Marshall solidstate amps did sound awesome 8)

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Post by Ripthorn »

I did some more tinkering today and made the following changes (I will update schematic shortly):

1. Remove 78L15 with its accompanying 10 uF capacitor
2. Increase R17 from 100R to 1k
3. Increase gain pot to 50kB
4. Put a 2k2 resistor between gain lug 3 and junction with C11 and R19

The reason for 1 is that it wasn't necessary, so it reduces part count/complexity

The reason for 2-4 was to change the behavior of the gain pot. With 25kB there wasn't quite as much gain as I wanted, but going to 50 kB without increasing R17 meant that you could get nasty squealing at max gain due to the very low resistance along that path. The addition of the 2k2 resistor prevented the signal from completely disappearing at minimum gain. Tweak this value to taste, as the larger it is, the more gain you will have at minimum. Please note, however, that simply increasing it doesn't give more gain, it interacts with R17 and the gain pot. However, i found that the settings above resulted in a very wide and very useful gain range.
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Ripthorn
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Post by Ripthorn »

Alright, here is the updated schematic. Please note that I went through and cleaned up the component names to make more sense. This includes the tweaks in my previous post, but the component names no longer match.

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Cub
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Post by Cub »

Hello lads, nice work ! Do you think scaling the component values in the tone stack would help bring the noise down a bit ?
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Post by Ripthorn »

Cub wrote:Hello lads, nice work ! Do you think scaling the component values in the tone stack would help bring the noise down a bit ?
Totally possible. Ing I get a chance I may try it today.
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Ripthorn
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Post by Ripthorn »

I tried the tone stack scaling on the breadboard and didn't notice a drastic difference in noise, but it's a breadboard and it was being just a little weird anyway, so that's not definitive. Still trying to hunt down the weirdness...
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bmxguitarsbmx
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Post by bmxguitarsbmx »

Scaling the tonestack is a cool idea, but, there is only a gain of 4.7 after the tonestack so it will be small.

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Cub
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Post by Cub »

Good luck with that! I wish I was more knowledgeable so I could offer any help.
Thank you for taking the time to share this project with us.
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Post by Ripthorn »

After some significan thought and breadboarding, I have changed things up some. I changed to a bipolar power supply so that VREF becomes ground. This greatly simplifies things like layouts. I also made a change to how the volume control works. There is a ton of volume/gain on tap, so I changed it to work more like the gain channel and master volume in the Marshall 5210 amp, rather than the gain channel/gain volume in the amp. Here are the updated schematics. Still sounds as glorious as ever!

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