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Gain Stage Mods Lesson #1: ‘The Grey Mod’

1959 Style mods

First off lets see whats in the diagram. Channel II has been jumpered with Channel I at the input jacks, and the cathode is now shared. This means one channel uses both sides of V1, and should provide more gain and touch responsiveness. It has a unique sound of its own, so don’t expect this amp to sound 100% vintage accurate any more. The input resistors have 68k’s added once on each channel, which in parallel make 34k resistance. This changes the gain in one of the jacks, but be careful with values here as its easy to add noise if you bump up the gain too much.
A ceramic disc capacitor has been placed between the plate and input terminals of V1b – this was a feature seen on the ‘SIR’ or ‘Chip’ mod I reported on back in 2006, and I think the Roccaforte mod. There the value was 1000pf, try a few and see what you think!   
Wondering how to go gain boosts or reduction switches? Cathodes are a really easy place to start. This diagram has pink and purple wires leading to SPST switches on the back of 2 of the pots, which decide whether the cathode goes to ground after the first resistor, or after both. Note how each resistor has a capacitor on it – this creates a change in tone. Basically, a resistor and capacitor together form RC time constants (you’ve read George’s sticky on this, right!   ), and depending on the value of the capacitor you put in here, different frequencies will be boosted. While the resistors set the bias voltage for the tube, the caps work like a high pass filter; a larger cap will boost the gain in lower and lower frequencies, until effectively the whole frequency range is getting a gain boost when you reach values like 330uF. Use your ears for these, and be prepared to spend quite a lot of time trying different frequencies. There are so many possibilities with this switchable gain boost on both V1 and V2a’s cathodes, that you could spend ages here until you’ve got something you’re really happy with.
Looking at the signal caps, we can start with .022uF on the left and .0022uF on the right (I’ve labelled these ‘big’ and ‘small’). These aren’t the values used in the original mod this diagram was based on, but should give you a solid reference to start from.
The 470k mixers now BOTH have silver mica bypass caps – I’d recommend starting with 470pf here and then trying others when you’re happy with other parts of the circuit. You’ll probably want to go with higher values if you do change.
The stock 33k/470pf tone stack now has a switchable cap that you can add in parallel for tone shaping; to detirmine the value of this, I’m sure someone stickied a tone stack calculator on this forum somewhere, so have a play about there once you get going.
The blue metal film resistors on the tone pots show how people go about modifying the Marshall tone controls to make them more user friendly. Lets be honest, theres not that much sweep on a stock 1959 tone circuit, so you can change resistances by both putting resistors in parallel with pot lugs, or making them switchable. Ignore the switchable resistors on the mid pot though, I’ve not finished drawing them up (the left one should go to ground).
Whats really important with this mod is to use shielded cable, unlike my diagram. The mod I used to draw this up actually had the input resistors placed at the jacks, as thats where Marshall had put them on the reissue. Do not do this! The best place is right on the tube pin itself; get 2 conductor shielded cable from George’s store, and wire a 68k and a 34k to the two cables. Pull some heatshrink tubing over them, so that theres very little exposed resistor or wire between the input jacks and the tube pins. 

http://forum.metroamp.com/viewtopic.php?f=4&t=27315

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