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Can an effects loop be added to any guitar amplifier?

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Can an effects loop be added to any guitar amplifier?

Postby jellyjim » Mon Oct 15, 2018 9:41 am

Hello

Can an effects loop be added to any guitar amplifier? I'm guessing it's just an insert point between the pre-amp and the power-amp. I have a Supro Tremoverb (re-issue). Is it something a decent local tech could do?

I've been having all sorts of fun processing guitar with some Eurorack in the studio and now I want to take it out live. I don't really want to rely on going through a PA and in any case some of our songs need a cranked up amp sound. An effects loop seems like the correct solution.

Thanks
Jim
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Re: Can an effects loop be added to any guitar amplifier?

Postby Sam Spoons » Mon Oct 15, 2018 10:22 am

Depends on the amp, a hand wired on turret board or similar job should be no problem for an experienced valve tech (and fully reversible too), A modern PCB with SMCs probably not doable. I'd think the Supro would be worth getting a tech to have a look at but find someone who has experience of valve kit.
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Re: Can an effects loop be added to any guitar amplifier?

Postby jellyjim » Mon Oct 15, 2018 10:25 am

Sam Spoons wrote:Depends on the amp, a hand wired on turret board or similar job should be no problem for an experienced valve tech (and fully reversible too), A modern PCB with SMCs probably not doable. I'd think the Supro would be worth getting a tech to have a look at but find someone who has experience of valve kit.

Cool, thanks

The Supro marketing blurb suggested it was pretty much identical to the 60s original. Though I am a succor for such things so there could be a Raspberry PI in it running an amp sim for all I know!
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Re: Can an effects loop be added to any guitar amplifier?

Postby Sam Spoons » Mon Oct 15, 2018 10:31 am

I suspect the internal construction will be a fair bit different...

http://www.marshallforum.com/threads/current-supros-for-jimmy-page-led-zeppelin.96125/

Image

Not entirely sure the pic is pukka but, if it is, the modern version will be much more difficult to mod successfully than the vintage. Still worth taking to a good valve amp tech though.
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Re: Can an effects loop be added to any guitar amplifier?

Postby jellyjim » Mon Oct 15, 2018 10:33 am

Ooh, nice find. Interesting photo

No Raspberry PI. Phew!
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Re: Can an effects loop be added to any guitar amplifier?

Postby Sam Spoons » Mon Oct 15, 2018 10:35 am

:bouncy:
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Re: Can an effects loop be added to any guitar amplifier?

Postby Hugh Robjohns » Mon Oct 15, 2018 11:30 am

jellyjim wrote:I'm guessing it's just an insert point between the pre-amp and the power-amp.

It may be that simple, but probably isn't. There are many things to consider such as where in the signal path you want the insert point to be (before of after EQ, etc), and then what the nominal signal level is at that point in the circuit path and whether it is appropriate for an external FX pedal. There's also the impedances of the circuitry at that point to consider in terms of whether the preceding amp stage is suitable to drive an external loop, and whether the following stage is suitable to receive a signal from it with appropriate RF rejection etc. It may be necessary to add an additional output drive and/or input receive buffer stage(s) so sort out the signal levels, impedances, and RF protection.

H
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Re: Can an effects loop be added to any guitar amplifier?

Postby jellyjim » Mon Oct 15, 2018 11:34 am

Hugh Robjohns wrote:
jellyjim wrote:I'm guessing it's just an insert point between the pre-amp and the power-amp.

It may be that simple, but probably isn't. There are many things to consider such as where in the signal path you want the insert point to be (before of after EQ, etc), and then what the nominal signal level is at that point in the circuit path and whether it is appropriate for an external FX pedal. There's also the impedances of the circuitry at that point to consider in terms of whether the preceding amp stage is suitable to drive an external loop, and whether the following stage is suitable to receive a signal from it with appropriate RF rejection etc. It may be necessary to add an additional output drive and/or input receive buffer stage(s) so sort out the signal levels, impedances, and RF protection.

H

Never is, is it? :)

Somebody on another forum suggested using a remap instead - for my purposes, obviously it's not an alternative to an effects loop

electric guitar > doepfer a119 external input > eurorack processing > output at eurorack level > reamper > guitar amp

Jim
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Re: Can an effects loop be added to any guitar amplifier?

Postby BigRedX » Mon Oct 15, 2018 12:38 pm

The return for the effects loop needs to come before the master volume control otherwise you won't be able to change volume once you've set up the gain structure of your external effects.

I had a Carlsbro guitar amp in the late 80s whose effects loops came after the master volume control which entered it pretty much useless.
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Re: Can an effects loop be added to any guitar amplifier?

Postby ef37a » Mon Oct 15, 2018 1:33 pm

As Hugh says, it is not a trivial piece of design as evidenced by the fact that so many amp manufacturers have got it wrong for years!

There is also a valve amp topology that is problematic. If the preamp stage has an odd number of inverting stages this gets put to rights by feeding them onto the two grids of the Phase Inverter. Where then do you put the FX loop?

Regarding levels and impedances I commend folks to "Tube* preamps for guitar and bass" by Merlin Blencowe. He includes good circuit explanations and many circuits of FX loops.

*Do not worry my fellow Brits! He reverts to "valve" in the actual texts. A bit of pandering by the marketing men I shouldn't wonder? (at it everywhere!).

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Re: Can an effects loop be added to any guitar amplifier?

Postby Humble Bee » Mon Oct 15, 2018 7:19 pm

Look into trying one of these Fryette Power Station PS2 devices... It does fx-loop and attenuation with any amp. I've played a plexi head with some strymon and it worked really nicely.

Just a thought as you dont have to modify the amp itself.
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Re: Can an effects loop be added to any guitar amplifier?

Postby jellyjim » Mon Oct 15, 2018 7:38 pm

Humble Bee wrote:Look into trying one of these Fryette Power Station PS2 devices... It does fx-loop and attenuation with any amp. I've played a plexi head with some strymon and it worked really nicely.

Just a thought as you dont have to modify the amp itself.

nice, thanks!
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Re: Can an effects loop be added to any guitar amplifier?

Postby resistorman » Tue Oct 16, 2018 3:56 am

Back when I was a kid and didn't know any better, I would just reroute the signal from the top of the last volume pot and wire it to a TRS switching jack and send it back to the pot. It always worked fine. The joys of ignorant youth...
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