“Can I add an FX loop?” is another common question. The answer is very similar to that for adding reverb.
The short answer:
Yes, but it’s not trivial, and it’s difficult to get it sounding good.
The long and detailed answer:
The main reason for having an FX loop in an amplifier is to add time-domain effects, such as delay, chorus and/or reverb. However, all of these sound best when placed AFTER compression and distortion. If you look at the typical FX chain in a multi-FX pedal, guitarist’s pedal-board, digital modeller or backline+PA rig, you will almost always find that these are the last effects applied. The reason is that it sounds natural and musical.
If time doman FX are applied to a signal BEFORE distortion and/or compression, the result is messy. The compression ‘interferes’ with the delayed element: echo repeats and reverb tails get increased in volume in relation to the dry guitar signal. So instead of the echoes/reverb being able to tail off naturally (as they do when played through a clean amplifier), the repeats/decay get boosted, and it can sound boomy. It becomes hard to mix in with the dry signal without smearing the dry signal. Typically, to solve this problem, you find yourself reducing the amount of reverb and/or distortion levels to try to stop this mess from ruining the dry signal.
What has this got to do with the WF-55, SE-5a, PP-18, SL-18, PP-36, etc? Well, all of these amplifiers are designed with power valve distortion as a significant part of the amp’s overall distortion tone. So, if you were to place an FX loop within the amplifier’s preamp signal path, you get the problem mentioned above – the power valve distortion and compression messes with the natural sound of the chorus/echo/reverb. This is true of many vintage-style amplifier circuits, too: Tweed Deluxe, Vox AC-30, JTM45, etc.
So why not keep the power amp clean?
That’s usually the best approach, but – of course – you lose the power valve’s contribution to your overdrive and distortion tone. In most vintage-style amplifiers you end up with a very clean amplifier tone, because there’s little distortion in the preamp circuit itself. It’s no coincidence that the best sounding vintage reverb amplifiers are like this: the Fender Blackface and Silverface models, such as the Super Reverb and Twin Reverb.
To get good amounts of distortion and a good-sounding FX loop in an amplifier circuit, you need:
1) a preamp circuit that can provide plenty of distortion, running into…
2) the FX loop circuit, running into…
3) a clean power amp.
There are plenty of commercial examples: Soldano SLO, Diezel VH4, most modern Marshalls and many Mesa Boogies. But these all sound and respond differently when compared with an amplifier that uses lots of power valve distortion. It’s also quite a bit more expensive and significantly more complicated to build as a project.
Back to the Amp Maker amplifiers: we’d be adding at least two extra valves:
a) to achieve the same sort of distortion levels in the preamp alone
b) to drive the FX loop
c) to recover FX unit’s output signal back up to a decent level
d) all to mix it back in with the dry signal
e) to drive the output valve(s) which we’d now run totally clean
That’s such a big change that it’s better to start with a clean sheet of paper (IMHO!). Which I will do.