The ShadowMusic Web Community

Some time ago I used to work on music amps including AC30's. I am currently working on several and did not notice the difference in the available power output when using the front and rear inputs for any of the input types.

For example the Front top boost socket is quieter than the rear one. One of the users pointed this out to me.
I have looked on this forum for information on this - to no avail at this time.

So in analyzing schematics (many of which are hard to read) and actually measuring the manner in which the input sockets react - I have come up with an explanation.
Seems that when using either socket individually there is input attenuation to the first amp stage by the input resistors/socket-switches - with the rear socket being reduced by approx 5% whilst the front socket is reduced by 50%.
When using two guitars the volume setting on the guitar plugged into the Rear Socket will affect the volume of the other guitar.

Further research on the internet - I have found one schematic that actually indicates this situation.

This could be a nightmare - for example adjusting the volume on one guitar will affect the other.

Does anyone know why the amps have been designed in this way?

Almost all amps which have two inputs per channel have this feature.

To my certain knowledge (born out of practical experience), that includes Fender, Vox, Selmer, Marshall, Gibson, WEM, Kelly, Alpha & Omega, Carlsbro and Sound City.

The reason is that not all guitars have the same output levels and less powerful instruments need less attenuation whilst high-powered units like DiMarzio Super Distortions may need to have their apparent strength reduced in order to contain the level of distortion.

As a feature, of course, it dates from the days when distortion was seen as an unwanted by-product of having powerful pickups. Selmer amplifiers used to be supplied with a leaflet which explained that full power might well be reached at a point before the amp's volume pedal reached "10".

With Peavey amps (at least the older models) the two inputs are marked High and Low, so the volume levels are different, but when used together they are equalised in strength(?).

Looks like John has provided the answer to the design.

For one guitarist there is the choice of low or high (for whatever reason) - but when there are two guitars the amp is then equal at the higher level for both inputs.

I suppose also that a low strength input would be useful when using a guitar with active pickups, and would help prevent unwanted overdrive of the pre-amp section of the guitar amp by high level pickup output (which would occur with a guitar with a battery powered pre-amp)

If I play my simple Yamaha (passive ) bass into my Peavey bass amp, using the 'HI' input is fine, but when the bass player in my band once used my amp, he put his(active) Fender bass through the same input, and it made the amp distort. I mentioned to him afterwards that he would have been better using the LO input..

Mind you, all this is a bit moot, because apart from reissues of old 'classic' amps like AC30s, Fender '60's models etc, most guitar amps for many years now have only had one input, even when the amp has two or more (switchable) channels...which I suppose is what 'the market' wants..

I suppose all young kids these days want and get their own individual amp when joining a band...whereas amps of any kind with extra inputs were highly valued and shared by other band members...sometimes the whole band...for mics, bass, guitars!