Fx loop in a vibro champ

[Reimer]

Can anybody tell me if it is possible to make a fx loop in my fender vibro champ, an silverface one.

[olddawg]

I'm sure it can be done. The question is why bother. There is no advantage in doing it on low power amps like Champs IMHO.

[Reimer]

I have a low powered siegmund amp, single ended amp, that have a fx loop, I put a delay pedal in there, it sounds awesome.

I have a vibro champ that I have made my self, it runs a 6L6 tube, bigger irons, sounds also good with a KT66. Thatīs the one I will try on first, then Iīll make it on my fender vibro champ )

The question is not why, the question is how?

[Chuck H]

Well... Here's sort of a why and how answer...

If you run the amp into distortion then the effects loop becomes useless since that amp will be clipping at the power stage first. Meaning that if you run effects in an effects loop you will be distorting the effects rather than effecting the distortion. If you get my meaning. At that point you would do as well to put the delay at the input and forget the loop. So that's the "WHY?"

The how is a matter of padding the signal down to about a volt right behind the power tube input. Low impedance is desireable for the "send" so that your fidelity wont suffer any loading effect from cables and effects unit inputs. Usually the impedance will be low enough once you pad down to a volt but sometimes it's good to use a low impedance output amplifier circuit like a cathode follower for the "send". Now you need to build a new amplifier stage or stages for the return so that the effects output can be brought back up to drive the power tube. You want the same voltage you had before padding down the signal. You would need to add a tube or at least use the trem triode (disabling the trem) to do it.

[Reimer]

Hey Chuck

Very fine explanation, I see it is nok that easy to make, if it has to be good.

[Chuck H]

It's actually not that hard either. But the details get in the way. If you can measure the signal through the amp, understand the principals of decoupling, impedance and know how to layout and ground circuits so that they stay stabil it's pretty basic. If your not experienced at these things it's still do-able. But you'll need someone who is experienced to help.

If you'd like to try it post some gut shots and a schematic for your amp.

[olddawg]

All of this padding a reamplification adds a lot of noise. I'm just curious as to what you think the advantage is to have an effects loop in a single ended amp. I doubt if you could tell the difference a/bed with a delay on the input blind except that if you overdrive the preamp with the effects loop, it will sound like crap. It's all subjective of course, but makes no sense IMHO. That's why it usually isn't done on small amps.