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Tube Guitar Amp Effects Loops

Effects loops are often added to a tube amp. Almost as often they are either unused or misunderstood. This month we’ll take a look at how effects (FX) loops work, the two different types of loops and the pros and cons of each.

What is an effects loop? Simply put, an FX loop allows you to insert effects into the amp’s signal chain. The loop will, at a minimum, have a send jack and a return jack located (typically) on the back panel of your amp. There may be other controls associated with the loop, but we’ll talk about those later.

Why have an effects loop in an amp? Why not plug effects into the input like most guitarists? There are a couple of reasons.

First, it is often advantageous to have some effects like reverb, delay, chorus, flanging and phasing, work their magic on your guitar tone after it has gone through distortion pedals and the like (that reside between your guitar and your amp’s input) AND your amp’s preamp. This is particularly true if your amp creates preamp distortion. At that point in your amp’s signal chain your gear has worked much of its mojo on your tone and it is often the best time to slam it with a time-based effect like those mentioned above. This is, of course, a subjective decision. Some guitarists prefer putting time-based effects in the same chain as their other pedals between their guitar and amp.

Second, the FX loop’s signal is better suited for rack mount type equipment that likes to see “line level” signals instead of the “instrument level” signals that exist between guitar and amp. We won’t dive into the technical definition of line and instrument level signals other than to point out that they are different and some ‘fancier’ gear likes it better when it’s driven by a line level output. This does NOT mean that you cannot put good old stomp box pedals in an FX loop. If your loop has a line/instrument switch or a Send Level control, you should be just fine. Even if it doesn’t and you want to use a stomp box in a loop – go for it! It may be that the signal levels in the loop won’t be matched perfectly, but you won’t hurt anything – depending on what’s in the loop you may hear more ‘noise’ (hiss), but that again is a subjective decision: is it good enough or not? Remember, this is a guitar amp we’re talking about, not a hi-fi amp!

How does an FX loop work? To answer that question we’ll have to discuss the two types of FX loops: series and parallel.

Series FX Loop

A series loop is one that disconnects the signal path of your amp between the preamp and the phase inverter and sends it to your effects via the Send jack. The signal from the effects in the loop are connected back into the amp and fed to the inverter.

That means that your entire signal goes out the back of the amp, is processed by your effects gear and is then returned to the amp. There are a couple of things to consider with this type of loop.

First, since your precious tone is being routed completely through the effects in the loop care should be taken to ensure that the effects are as transparent as possible – in other words you want good quality stuff in the loop.

Second, if you are using digital effects in the loop consider that your signal is being digitized (turned into a digital signal – effected by the software in the effect – turned back from digital signal into an analog signal for reinsertion into the amp). This analog/digital and digital/analog conversion process technically messes with your tone. Again, you need to listen to it to determine if this is a concern.

Third, because the entire signal is routed through the loop you can experiment by putting effects normally found between guitar and amp in the loop. Effects like compressors, EQs, wahs, etc. can sound awfully good when plugged into a series loop. Experiment!

So how does it work? Take a look at the diagram. You’ll see that after the preamp a switch of some sort is present that either allows the signal to connect to the amp’s phase inverter as normal or to the loop. Note that almost always this switching action is accomplished simply by plugging into the loop. As mentioned above, it’s an either/or proposition: either the signal goes to the power amp or it goes to the loop.

Series Effects Loop

Also note that the effects loop shown in the diagram uses one half of a preamp-type tube to send the signal to the loop and to return it to the amp. The loop signal is ‘tube buffered’ meaning that the signal going to the effects can handle long cable runs and it ensures that the effect will not ‘load’ or unnecessarily degrade the signal from the amp. Return buffering does essentially the reverse: it will boost the signal if necessary after a long cable run and it isolates the loop signal from the rest of the amp so that the signals are well matched and noise is kept to a minimum.

One thing to watch for is that many tube amps use solid state components for the send and return circuits. Transistors or what are called op-amps are used to buffer the send and return signals. This is not necessarily a bad thing, but for those of us who like pure tube tone it is something to consider. Solid state circuitry used in this application will actually result in a more noise-free FX loop – that’s a good thing. However, a tube loop is essentially another preamp stage and that means your signal gets further shaping by the natural characteristics of another preamp tube – and that’s also a good thing!

If your loop has a Send Level and Return Level control, you have signal level tweaking capability. Here’s the way to set up your loop for best operation:

  1. Adjust your amp for the desired volume and tone.
  2. Set the Send level to maximum.
  3. Plug the effect into the Send and Return jacks.
  4. If your effect has an input clipping light or LED set the effect’s input level control so the clipping light just blinks when you play most aggressively. Note that you may have to turn the Send Level control down a bit to get the clipping light to ‘just’ blink while playing aggressively. If your effect doesn’t have an input clipping light, don’t worry. Set the Send level at max and play on – it is highly unlikely that the input of the effect will be sensitive enough to be overdriven by the loop send. However, if you the effect sounds distorted turn down the Send Level.
  5. With the Return Level control set mid-way, set the effects output level so that the volume level from the amp is the same as it was without the loop working.
  6. The Return Level control provides additional tweak-ability of the signal level from the effects unit to compensate for long cables or weak effect output.

Bottom line: Series loops are versatile in that ANY effect can be plugged in – if it sounds good use it! They require care in matching signal levels so that when you are using the loop the volume of the amp doesn’t change. Remember that a series loop sends your entire signal to the effects so consider what negative effect that may have on your tone.

Parallel FX Loops

A parallel loop works like a mixer effects buss – the FX loop allows you to blend or mix the effected signal with your amp’s signal.

That means that your amp’s signal is ‘tapped’ and sent through the FX loop and the loop’s return signal is mixed with your amp’s signal just before it goes into the phase inverter. Like the series loop, there are a couple of things to consider with the parallel loop.

First, effects like EQ, compressors, tremolo, noise reduction, etc. don’t work well in this type of loop. They work properly when they see the ENTIRE signal.

Second, you should set the effects in the loop so that it produces a 100% ‘wet’ signal. Some effects blend the signal from their input with the effected signal in such a way that the two are ‘out of phase’. That results in degradation of your tone. For example, when I use my old Boss DM-2 analog delay in a parallel effects loop, I set the “Echo” control for the delay length I want, I adjust the “Repeat Rate” to produce the number of echoes I want to hear after each note, and I set the “Intensity” control to maximum to provide a 100% wet signal. The controls on your effects will of course be labelled differently, but you get the idea.

Parallel Effects Loop

So how does it work? Take a look at the diagram. You’ll see that the preamp signal is permanently routed straight to the phase inverter – your amp always produces an unaffected tone. After the final preamp stage the signal is tapped and sent through the Send buffer circuit and on to the effects in the loop. Once again the loop is typically switched on when you plug in.

A parallel loop ideally requires at least a Return Level control. Adjusting the Return Level subtly or dramatically adds the effected signal to your amp’s tone. This provides a TON of flexibility. You can use the output level control of your effect to blend or mix the amount of effected signal with the amp’s straight signal. However, that may lead to a problem if, as with my old delay, your effect doesn’t have one! A Return Level control provides the best way to mix the effect in with the amp’s straight signal.

Bottom line: I love parallel loops! For time-based effects it provides total flexibility by allowing you to mix in just the right amount of effect. However, if you’re into surf music and need MAXIMUM reverb, for example, a series loop is the way to go. If you have a loop with a series/parallel switch you’re in the driver’s seat!

As always, I would love to hear from you. Just send me an email!

Don Mackrill
Mack Amps