In the early days of popular beat combos it was normal for the whole band to use a single
amplifier. And volumes were a lot lower too.
So that little amp could have
bass, two guitars, and a transistor organ all plugged in together. Or three guitars, an
accordion, or whatever.
Two channels meant that there was some flexibility,
bass and rhythm guitar on the normal channel, lead guitar and keyboard in the lead
channel, for example. And the earliest FX pedals, like treble boosters, were to some
extent a way to get around those limitations.
When you look at the inputs on
each channel you will find that they are wired in parallel. And that is entirely for
practical reasons, whether you use one input, the other, or both together, it WILL work.
There are downsides when you plug two instruments into one channel, each guitar will place
a load on the other so changing the volume, tone, and pick-ups on one can change the sound
of the other. Really just the same as using both pick-ups on a Les Paul which can do
unexpected things when you start juggling the controls.
A useful by-product of
the pairs of inputs being wired in parallel was that they also functioned as a passive
signal splitter. And once those inputs were freed up by everyone getting their own amps it
became possible (and often desirable) to drive both pre-amp channels together for more
It works great on the WEM and the 1974x (a lot of people would
claim it is EXACTLY what makes the Marshall a classic). But on an AC30 it is amazing. Not
only do you have 3 channels to play with instead of 2, but it makes the Vib/Trem channel a
lot more useful because you can mix it in rather than just on/off.
And there is
another hidden pleasure with cross-linking amp channels like this. You have a spare input
socket left over, perfect for linking into a second amp... and then a 3rd, and then a 4th
Largely, but not entirely, dysfunctional.