Amp question, adding an effects loop

[Luke51411]

Sometimes I like to do things just because I can... I have a fender frontman and I found the schematic here http://support.fender.com/schematics/guitar_amplifiers/Frontman_25R_schematic.pdf
Would it be difficult to hack in an effects send/return? From my limited knowledge it seems like you would just need a buffer/splitter before r55 by the power amp and send and return jacks. If it is that simple, I'll probably do it. Its not something I want to spend a lot of time on but I'm just curious if it can be done fairly simply or not.

[mth5044]

I'd imagine you would want it before the reverb? You could try taping it right off the end of the tone control. That way you'd have some level reduction from the tonestack as there is a chance the signal might be too hot to run into pedals elsewhere. I'm sure PRR will come in and do a bunch of ratios to say what the approx level is at that point, but we all can't be PRR    You can take the time to figure it out or just take a tap between the bass control and U3-B and see what happens

[Luke51411]

I'd imagine you would want it before the reverb? You could try taping it right off the end of the tone control. That way you'd have some level reduction from the tonestack as there is a chance the signal might be too hot to run into pedals elsewhere. I'm sure PRR will come in and do a bunch of ratios to say what the approx level is at that point, but we all can't be PRR    You can take the time to figure it out or just take a tap between the bass control and U3-B and see what happens

Good points. I'm not sure what all this would accomplish but I like to take any opportunity I can to learn something new. Is U3-B a buffer? Couldn't I just take it from where it is splitting to send to the reverb? Or maybe that's what you are saying. Wait, then it would be past the reverb... I think I'm understanding a little. Wouldn't I want the send to be in parallel with the tonestack/drive channel etc so maybe I would take the send from somewhere towards the beginning?

[mth5044]

A (the?) purpose of an effects loop is to put effects that some people think don't sound great before clipping (delay, modulation, reverb and whatnot) after a clipping source. The clipping source is usually what ever clipping might happen in the preamp. So effects loops usually got between the preamp and the poweramp. It wouldn't go after the poweramp as that's where the speakers are and it wouldn't go before the preamp because that would be just like putting them into the front of the amp.

You mention 'shouldn't the effects loop be in parallel with the tonestack/drive sections'. An effects loop needs a send and a return, so basically insertion points in the amp. You could put it the send before the tonestack/drive as you mention, but then the return would be where? Before the tonestack/drive sections? That would defeat the purpose of the effects loop, you might as well just plug in. Would the return be after the tonestack/drive? Could be, but then whatever signal you send to the effect would not be affected by the drive/tone shaping. Whatever effect you use would be in parallel with the preamp. Could be interesting to mess around with, but not a typical effects loop as I understand them.

An effects loop is meant to break the flow of an amp in order to insert the effect downstream instead of in the front. As I said, between preamp and poweramp is typical. I like reverb near the end of the chain, so that's why I suggested before the reverb. Preamp 'ends' after the tone control.

Pedals run on somewhat lower voltage compared to amps, so you want to make sure you are feeding your pedals with a signal they can handle. Looks like, up until the poweramp, the amp runs at +/-15V. Might be creating signals too hot to directly feed into a pedal, but you can take some measurments or simulate to find out if you need to lower the voltage before you send it out.

[Luke51411]

Ok thanks, at this point it is an academic exercise and you've helped aid my understanding. An effects loop is usually parallel with the dry signal though isn't it? It would just typically be parallel with the signal after the preamp/tonestack etc. So I was incorrect in assuming it would typically be parallel with the preamp/tonestack. My line of thinking was that if you plug directly into the return you would be bypassing the preamp and essentially inserting your own front end. For some reason that got me thinking that it would be parallel to the whole thing. I'm not sure if it is becoming more clear or more muddy at this point haha...
I guess I was thinking the effects loop would be similar to the reverb and you would have a mix control to determine how much of the sent/returned signal comes in.

[mth5044]

It is very much like the reverb; however, the reverb is not parallel with the tone controls/drive section. The reverb driver and recovery section comes after the preamp and is in parallel with dry signal, otherwise you'd just have a very wet reverb signal. This 'parallel' part of dry signal is just part of the reverb circuit. Take a look at the stage center reverb circuit:



You can see that parallel with the reverb pan is the dry signal with the 50k pot.

An effects loop can have the same configuration, except when you plug into the effects loop, it disconnects the dry signal between the preamp and power amp. If you don't have anything in the effects loop but keep the plugs in, you won't get any signal. Take a look at the Peavey Classic 30 schematic in this PDF:

http://www.bustedgear.com/images/schematics/Peavey_classic-30.pdf

The effects loop is on the right side under the tone controls. It uses switching jacks to control signal flow. When there is nothing in the return jack, Q2 and the supporting circuitry from C24 to the send jack isn't doing much. The switch is closed in the return jack as there is no plug, so signal goes right from the treble pot to C31. Now, when you plug your effects into the effects loop, the switch in the return jack is broken and signal can no longer go right from the tonestack to the poweramp. Instead, it goes into Q2, out the send, through pedals and back into the return jack.

Maybe you consider that parallel? I don't, but maybe it is? Either way, that's a pretty simple example of how an effects loop. You can use the same switched return jack trick as the peavey uses on your fender. You might need to pad the send and boost the input depending on the swing of the signal where you make your tap.