Passive FX Loops (in RobRob Micro Amps) - Your Experience?

Lancer X

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Hi folks. My back is finally feeling better, so feeling motivated to resume building my @robrob Micro Bassman EF80 combo.

My question here is in regard to Major Rob’s “Poor Man’s FX Loop”. What has your experience been with any negative tone impact from shoving your time-based effects into this loop? (This is pretty much a follow-up to @King Fan ’s prior thread here.)

I’m building this amp specifically because I love in-tube-amp overdrive and tone, and I keep hearing that passive FX loops are the black plague of tone choices (internet forum-goers are rarely hyperbolic…). Purists like the folks at Vertex Effects warn that passive loops will ruin the tone of our lovingly hand-built amps. Since I do love clean, time-based effects, I am torn.

Micro amp builders, what has your experience been? If your first pedal in the loop features a high-quality buffer, do you still need an active loop output? The VE people assured me that one does (and were not recommending any of their products as a solution).

Several companies sell both SS and tube-based active loop kits, and if necessary I’d like to plan it into my build vs modding it later.
 
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Scottcurry13

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I tried it in two different builds and it didn’t sound any better than running a reverb or delay before the amp. I think it could also add potential for noise. I don’t have very high gain preamps though, so maybe it helps with those.
 

Annies Dad

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I've had successful experiments with this type of setup. I tried a Pod by itself and with a small Mackie mixer. I kept the send cable very short. This was on a 50 watt Marshall style build. there may be a slight compromise in the amp's core tone but was worth it for all the other sounds. I really liked using the mixer for parallel returns and wet/dry control.

I installed a Metropolous buffered loop in a friends 100 watter. It worked very well with a TC delay pedal. Definitely worth the money.
 

pavel

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I too only have (limited) experience with the Metropoulos loop. But presumably, even with a passive loop, you should be able to completely bypass it with a switch no?
Also, could not add an extra 12AX7 for an active tube loop?
 

Lancer X

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I too only have (limited) experience with the Metropoulos loop. But presumably, even with a passive loop, you should be able to completely bypass it with a switch no?

Bypass yes, but impact on tone when in use is my concern?

Also, could not add an extra 12AX7 for an active tube loop?

Yes, potentially. I’m just playing at home though. If a buffeted pedal could preserve any coloration / losses, that’s fine for me too.
 

Ten Over

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The need to attenuate the signal for the EF80's gives the opportunity to combine this with the need to attenuate the signal for effects.

Fx Loop EF80 Micro 1.png
 

NickK_chugchug

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Agreed - that's the natural breaking point between the pre and power sections and the position I was thinking about putting an effects loop in.
The fender standalone reverb uses a cathode follower after the effects to provide a buffer to the next tube stage. Most modern effects pedals will end their internal chain with an output buffer which has low impedance to drive the signal to the amp through a long effects connecting cable.

I also note that rob's designs have also used both a 12ax7 LTP and a 12at7 LTP. So depending on the attenuation required for the effects loop the 12ax7 isn't particularly linear in the sound passing through it. So that's another contributing factor.

All depends on what you find acceptable to you - effects-pre-power, pre-effects-power will each have a different sound from a mathematical perspective.. but in practice what you find great may be different to someone else!
 
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Lynxtrap

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I would say it depends on where you want to generate overdrive in the amp. The loop "should" be placed after the overdrive. That's why they are often used in amps where overdrive is generated in the preamp. That way the delay or reverb won't be distorted.

Since this is a Bassman, I guess most of the overdrive will come from the output tubes. I don't see much of a point with an FX loop there.
 

Lancer X

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Since this is a Bassman, I guess most of the overdrive will come from the output tubes. I don't see much of a point with an FX loop there.

Oh man - yes! Duh, I was totally missing this point - thank you.

OF COURSE the whole reason for building these micro amps is to enjoy that juicy power tube saturation at reasonable SPLs. Which will, of course, color the time-based effects regardless of their placement.

Granted, the loop would bypass any preamp saturation, but as you say this is probably a minor issue (and all the greats back in the day sounded great with no FX loops). The potential signal loss or tone coloration likely makes it not worth the effort.

Very appreciated - I was totally spinning my wheels on this passive/active FX loop thing. I'm gonna leave it in as a preamp out option (and pre-PI gain control), but you've just vastly simplified my pedalboard build.

@King Fan , I think this fact may further explain some of your experiences as well.

TL;DR: The RobRob Micro builds aren't intended as pedal platform amps. Rather, the point is to generate overdrive via both preamp and power amp saturation without blowing your eardrums. Therefore, little need for / impact from a complicated FX loop.
 
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Ten Over

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What if you didn't parallel the 12AY7 and instead used the extra stage for an effects return recovery? Maybe something like this:

FX Loop EF80 Micro 3.png

There is distortion in the pre-amp mainly from the cathode follower even with a 12AY7. You can use the effects loop for other purposes like an adjustable pre-amp out with or without the power section. Or a power amp in that you can use for different pre-amps with the power section. Also, effects are going to run quieter when they are in the loop as opposed to in front of the amp. The loop can also properly accommodate line level devices.
 

Lancer X

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What if you didn't parallel the 12AY7 and instead used the extra stage for an effects return recovery?

That would be a cool way to do it - will keep that idea in my pocket for future builds. In my case though, I am using both sides of V1 to create a second "Lead" channel (see below).

Thank you very much for all your explanations and suggestions!

G-man Micro LTP EF80 MV Layout Final LRM 12-8-22.png
 

Annies Dad

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Another option is to add a line out tapped off the speaker jack. It is a pot and a couple of resistors. Could be built into a pedal box. Take the line out and run the effects into a second clean amp or two for wet/dry/wet.
 




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