Merlin's Practical Serial Effects Loop layout

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xtian
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Merlin's Practical Serial Effects Loop layout

Post by xtian »

I'm going to build Merlin's FX loop, shown on p. 219 of Designing Tube Preamps for Guitar and Bass, Second Ed. Anyone done a layout? Here's my napkin sketch.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

Here is my draft. (BTW, Hi, Merlin, if you're reading this!)

1. The 2K2/120K resistor pair connected to the input grid (pin 2) is a voltage divider. Merlin says,
Add a potential divider immediately before the driver triode to ensure the signal passing through the loop is not clipped...the attenuation of the divider should be the inverse of the gain of the recovery stage to give overall unity gain through the loop.
Earlier, he says, "The gain of a typical fully-bypassed ECC83 gain stage is close to 60." The inverse of 60 is 0.0167. So I chose 2K2/120K resistors, which is a ratio of 0.0183. Am I understanding this correctly?

2. The 10K resistor connected from the Send jack to ground represents Merlin's 10K Send level pot, set for max level out. Can someone tell me if this is unnecessary and I can remove it?
Last edited by xtian on Fri Jul 04, 2014 10:05 pm, edited 1 time in total.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

No comments, mates?
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Re: Merlin's Practical Serial Effects Loop layout

Post by cbass »

Does it need coupling caps?
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

No, don't think so. See schematic from 1987 Marshall attached. Goes where red X shown.
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Re: Merlin's Practical Serial Effects Loop layout

Post by Merlinb »

The layout is fine. You might want the attenuator to have higher resistance though, to avoid loading the tone stack, say 1M+18k. It doesn't have to be exact- this is rock 'n' roll! When you solder it together, make it easy for yourself to de-solder the divider in case you want to make adjustments.
You might also want to think about whether you want a send-level control (which would also serve as a pre-PIMV). Or maybe you don't have the space?
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Re: Merlin's Practical Serial Effects Loop layout

Post by cbass »

xtian wrote:No, don't think so. See schematic from 1987 Marshall attached. Goes where red X shown.
Oh I see now.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

Thanks, Merlin! I've got plenty of space in the chassis--I'll add that 10KA send pot.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

Loop kit. My kind of fireworks package!
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Re: Merlin's Practical Serial Effects Loop layout

Post by Merlinb »

xtian wrote:Thanks, Merlin! I've got plenty of space in the chassis--I'll add that 10KA send pot.
Might be a bit late now, but if it were me I'd put the pot (1Meg) in place of the input divider, just like a regular MV. That way it has no effect on output impedance and if you ever wanted to eliminate the FX the loop, you could still use it as... a regular MV! It's not a big deal though.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

Merlin, I'm already finished. Took four hours, including drilling, soldering, testing.

Absolutely brilliant!

No adjustments needed. Perfect gain staging. No added noise. No change in tone. Excellent!

I put a TC Electronic reverb in the loop, maxed the gain, dialed the Master Volume way down low for bedroom levels. Wonderful. This is a sound you absolutely cannot get by using a reverb out in front of the amp's input.
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Re: Merlin's Practical Serial Effects Loop layout

Post by xtian »

Updated layout attached, error corrected, input voltage divider values updated.
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Re: Merlin's Practical Serial Effects Loop layout

Post by Merlinb »

Yay! :P
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Re: Merlin's Practical Serial Effects Loop layout

Post by arcturus »

Elegant implementation of an fx loop! i'm willing to copy this to my own amp but i'm curious how do you connect the B+ rail. have you created a dedicated node on the power supply with its own cap and resistor?
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Re: Merlin's Practical Serial Effects Loop layout

Post by norburybrook »

Excellent. I've just put a metro zero loss Fx loop in my JTM45 and it works great but there's something not quite right about seeing PCB's and IC's in a point to point hand wired guitar amp.


This would have been the more in keeping option. Might have a go at this one.

One point I'd make to anyone though is FX loops work best with master volume amps. My jam 45 isn't master volume so I'm not really sure yet of the sonic benefits.


Interesting that Robbin Ford now uses everything in front of his amp an no longer uses the dumbelator and fx loop.


Marcus
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