help with ampeg b25 clone build

[embotone]

I am going to build a clone of an ampeg b25.  I want to keep the front of the amp simple, so I was going to put a switch in allowing me to have a single set (Bright and normal)of input jacks and switch the input between channel A, channel B, or both in parallel.  Inside the amp, I would have both channels built seperately, and would use dual gang pots for the volume, treble and bass.  I would then have only 3 knobs on the fornt of the amp, which is what I want.  I then noticed the the tone stacks and everything beyond are the same for both channels.  Could I just have one set of tone controls, and switch in channel A, B, or both in parallel feeding into a single tone stack?.  If so, should I  move the mixing resistors to a point before my switch feeding the tone stack? 
On a separate note, why are mixing resistors necessary?

[PRR]

> use dual gang pots for the volume, treble and bass

Then save a tube and three costly dual-gang pots. Build just one channel.

Two channels _slaved_ to the same settings doesn't really offer anything more than one.

The 2-identical-channel design goes back to when there might be more players than amps. Two guys can plug into one 2-input amp, use 2 sets of controls to get their volume and tone blend.

(Alternatively you could have two very _different_ channels, so one player could access 2 _different_ sounds instantly.... this is very common today, not so much in 1968.).

Mix-resistors avoid shorting-together two sources. In this particular amp, without mix resistors, the two treble-pot wipers would be shorted together. Say one player wanted a mellow rumble while the other wanted a bright pitz-solo sound. With treb wipers shorted together you could not get separate settings, and if you tried you'd kill both players' treble. In other amps you'd short two plates together, which "will mix" but overloads easily.

There is yet another trick with the mix resistors. Get two B-25s and run a jumper between the Ext Amp jacks. Now you can have _four_ players mixed into _two_ 60W amps and speakers. If all four play strong at once, it could be a muddy jumble; if they play mostly gently with one of them taking lead/solo it can work quite nice.