Amps with high and low inputs

NelsonP

Why are there so many amps that have high and low inputs, but which offer no ability to switch between them. Surely most people will just plug into the high channel, so what's the point? Is it just a legacy from the old days?

Surely most people would want to be able to footswitch between a clean and gain sound. How hard would it be to create a circuit on the input that allows you to do that?

Or am I missing something.

fretmeister

It is a bit weird - especially as some of them even cut off the other input so an A/B pedal can't work either.

But then again something like the JCM needs vastly different settings depending on which input is being used so it wouldn't be that much use to have anyway.

There should be a ban on shared EQ, gain and output!!!  

Cirrus

In my experience of AC30s, Tweeds and Marshalls with High and Low inputs, there's basically no time I'd ever want to be able to switch between them mid performance because it's such a minor tone change & the volume change is totally dependent on how the amp's running - clean, the typical 6dB would be too much for a solo boost, distorted, the extra gain from that 6dB extra input wouldn't be enough to make the difference I look for.

It's just a legacy of old amp designs. The choice can be useful - if you have hot pickups and are running out of headroom too early the low input can be good. If there's a bright cap on the channel vol & the amp is too bright at the volume you need, you can reduce its effect by using the low input and turning up to get the same overall volume. If there are multiple channels with two inputs, you can use the spare input to jumper the channels and blend to taste.

Yorkie

fretmeister said:

There should be a ban on shared EQ, gain and output!!!  

Hear hear!

Yorkie

Guitars with humbuckers -> high input.
Guitars with single coils -> low input.
Guitars with both -> panic attack. We did not plan for that. Please burn your guitar. Or buy two amps and a switch. Yeah, two amps and a switch will do, thank you. Actually, it works like that by design. It's a signature feature of the amp, and we'll sue your arse if you try to copy it. 

Flanging_Fred

The difference in the hi and low inputs on my JCM800 is huge, both in term of volume and tone. It’s like having two separate amps and the approach to using them is totally different.  

On the low input Its a very clean flat sort of sound which is excellent for taking pedals. I used this exclusively for a long time with a pedal board full of goodies. 

The high input gets you that classic JCM800 crunch but in doing so limits your effective pedal choices and for me it encourages you to work the guitar volume control far more to achieve semi clean type sounds. 

Both brilliant ways of doing things, just different.

Cirrus

IIRC on the JCM 800 the low input actually bypasses the first gain stage, so yeah gonna be pretty different all round

NelsonP

Cirrus said:

IIRC on the JCM 800 the low input actually bypasses the first gain stage, so yeah gonna be pretty different all round

Yeah but why not just have a switchable, different first gain stage, and a single input. You could even eq it differently too.

The high and low inputs thing just feels like a legacy from yesteryear that is done because 'we've always done it that way'.

Jetfire

NelsonP said:

Cirrus said:

IIRC on the JCM 800 the low input actually bypasses the first gain stage, so yeah gonna be pretty different all round

Yeah but why not just have a switchable, different first gain stage, and a single input. You could even eq it differently too.

The high and low inputs thing just feels like a legacy from yesteryear that is done because 'we've always done it that way'.

Guitarists fear change.. 

mo6020

NelsonP said:

Cirrus said:

IIRC on the JCM 800 the low input actually bypasses the first gain stage, so yeah gonna be pretty different all round

Yeah but why not just have a switchable, different first gain stage, and a single input. You could even eq it differently too.

The high and low inputs thing just feels like a legacy from yesteryear that is done because 'we've always done it that way'.

This is actually what happened with the 2205 and 2210 JCM800s...

ecc83

The idea may have started when peeps put keyboards through guitar amps as they are about 10dB hotter than a passive Strat. Some say it is for "active" guitars but I think the practice pre dates active guitars?

A few amps (Blackstar Artisans) do leave the second jack active and so you can use the 4 inputs as a crude FX loop.

Foot switchable? No, cost. Needs another jacks or/and an onboard switch and a switch is a lot more expensive than a couple of resistors.

Switch the gain of the first stage instead? How? unbypass the cathode? Yes but then that changes the output resistance of the valve and forks with the filtering. Also, switching a gain stage RIGHT at the front of an amp is very hard to do without thumps and clicks.

As with many things to do with guitar amplification...lot of theories...much lost in the mists of time, beer fog and Capstan full strength.

Dave.

randella

mo6020 said:

NelsonP said:

Cirrus said:

IIRC on the JCM 800 the low input actually bypasses the first gain stage, so yeah gonna be pretty different all round

Yeah but why not just have a switchable, different first gain stage, and a single input. You could even eq it differently too.

The high and low inputs thing just feels like a legacy from yesteryear that is done because 'we've always done it that way'.

This is actually what happened with the 2205 and 2210 JCM800s...

Yep. I guess with the original 2203 they were giving players what they wanted at the time. A pre-phase inverter master and an extra intermediate gain stage biased cold to reproduce the distortion that's lost by attenuating the signal before the PI.

Basically a dirtier, more controllable Super Lead.

Worth bearing in mind that the 2203 JMP Master/JCM800 circuit was designed barely a decade after Marshall started out making amps. It really is a 'legacy' product, electronically far closer to the 1959 Plexi than anything remotely modern.

monquixote

fretmeister said:

There should be a ban on shared EQ, gain and output!!!  

Not sure I agree. 
Most amps with two fully independent channels sound so different I struggle to use both and end up just sticking to one.

Musical_Mystery_Tour

Simple solution

rze99

My '79 JMP 50 has them. For me it was High=guitar low=out a bass through it. Low sounded good for bass or certainly good enough for me then.  I did that for years when I was doing both bass and guitar band work.

Modulus_Amps

One of my most popular kits has this, high gain side is a boosted plexi, low gain side is exactly the bright input classic plexi sound. So you can get classic rock and modern rock from one amp. It is for those that tend to use one core sound or to be used in a studio.

If you want an amp switcher then there are plenty of options that offer that.

ICBM

There are two kinds - on the older type (vintage Fenders, non-MV Marshalls, Vox, Hiwatt etc) it's simply a pair of jacks and a resistor divider network so the 'High' or '1' input has a higher input impedance (usually 1M) and the 'Low' or '2' has a lower impedance (usually 136K, ie two 68K resistors in series) with the signal to the valve taken from the junction of them, hence halved in signal voltage or a -6dB pad. The idea is to use 2 for louder or 'line type' signals - but the main reason for having two jacks is so you could plug in two instruments at once - if you do this, both become 'High' inputs. Remember that back then, it was common to plug multiple instruments - or even the whole band! - into one amp.

The other is like the Marshall 2203 or Mesa/Boogie Mk I, where the 'High' (or 2 on the Boogie) jack goes through an extra valve stage to create much more gain and overdrive. Depending on how the routing is arranged, you *may* be able to switch between them externally - at the expense of a huge volume jump when you use the High, unless the amp is already fairly cranked so it just overdrives further.... so still not that useful for most purposes. With the 2203 I've always preferred to use the High input with the guitar straight in - it just sounds better, even if you want a semi-clean sound with the gain down a bit (although I usually just set it at 10 and use the guitar volume) - but the Low input if I'm using an external pedal or multi-FX for overdrive.

Nerine

Ah. The JCM 800. My favourite amp of all time. I shall never part with mine. Nothing comes close to it. It’s about as perfect as an amp gets for me. Will literally work for any style or genre and is really transparent in that every guitar you use sounds like the guitar. It’s real good at that. Better than most I’d say.  

Mine is only an old reissue, so it has an fx loop. Crucial for me, that, though. 

It’s seen off Diezels, Friedmans, Bogners without even trying. The aforementioned stuff is like a neutered, polite facsimile of an actual 800. 

snowblind

I have one amp with a normal, bright and linked inputs. No switching available. Only ever use the linked input. Everything is done with pedals or the guitar controls.
I have another with one input with switchable clean/drive channels. Only ever use the clean channel and again do everything with pedals.
I have a third with high and lo inputs AND switchable between a clean and drive. Only use lo input, clean channel and yes you guessed it, pedals.

If you run two amps in the rig it seems to be the way to go.

pt22

ICBM said:

The other is like the Marshall 2203 or Mesa/Boogie Mk I, where the 'High' (or 2 on the Boogie) jack goes through an extra valve stage to create much more gain and overdrive. Depending on how the routing is arranged, you *may* be able to switch between them externally - at the expense of a huge volume jump when you use the High, unless the amp is already fairly cranked so it just overdrives further.... so still not that useful for most purposes. With the 2203 I've always preferred to use the High input with the guitar straight in - it just sounds better, even if you want a semi-clean sound with the gain down a bit (although I usually just set it at 10 and use the guitar volume) - but the Low input if I'm using an external pedal or multi-FX for overdrive.

I believe this is how my Magnatone is designed (or at least that’s the end result) and that’s exactly how I use it. It’s nice having the option.