Neil special

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hebaton
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Neil special

Post by hebaton » Sat Jan 30, 2021 7:41 am

CAn anyone tell me where the full specs to build the "Neil Speacial" can be found ?
Please ?

danman
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Re: Neil special

Post by danman » Sat Jan 30, 2021 6:05 pm

The only available info that I'm aware of is the layout on SDM's site. The amp can be built on a 50 watt or 100 watt chassis so just get the appropriate OT for either one. The PT you use is up to you depending on what wattage amp you want to build and what you want your final B+ voltage to be. Everything else is basic Marshall spec.

https://drive.google.com/drive/folders/ ... zd3STVYd3c

Explorerman
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Re: Neil special

Post by Explorerman » Mon May 03, 2021 5:46 pm

I was also looking into a build using this 2 in 1 a couple of years ago, ended up building a 5e3 as well as an 18w TMB over the past two years. Those builds worked out really great and now back looking at ordering parts for the for 1987 first, then once up and running the 2204 and 1987 in one. One question I have about this layout: what is the purpose of the 0.0022 cap connecting between the switch and the bright vol pot? The switch puts it in series with the existing 0.022 cap that both the 2204 and 1987 has, but both of them don’t have this 0.0022 cap.

danman
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Re: Neil special

Post by danman » Mon May 03, 2021 6:53 pm

Depending on the actual year of the circuit, both of these models ran a .0022uf coupling cap on the first stage of the bright channel. Some of the older circuits used a .022uf in this position but as the years progressed, Marshall switched to the .0022uf cap to help cut a bit of low end and tighten the sound a bit.

Explorerman
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Re: Neil special

Post by Explorerman » Mon May 03, 2021 7:19 pm

Cheers, yep I have seen that, however the 2 in 1 layout in the mods section shows a 0.0022 in series with a 0.022 (the 1987 and 2204 have on or the other, not both in series) - I don’t really understand the purpose of the extra 0.0022uf?

Pressed_Rat
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Re: Neil special

Post by Pressed_Rat » Sat May 22, 2021 7:37 am

This is a clever way switching between coupling cap values of the 1959 Bright Channel and the 2203 1st Gain Stage.

When the capacitors are in series with each other (switch in the 1959 position), the effective capacitance is the reciprocal of the sum of the reciprocal of the individual capacitances, or in this case:

1/Capacitance = (1/0.0022 uF) + (1/0.020 uF) = 1/0.0020 uF. The capacitance is 0.0020 uF/1 (or 0.0020 uF) which is very close to 0.0022 uF. This is same algorithm used to determine the effective resistance of two paralleled resistors.

So with the switch in the 2203 position, the 1st Gain Stage's coupling cap is 0.020 uF. With the switch in the 1959 position, the Bright Channel's coupling cap is effectively 0.0020 uF, while the Normal Channel's coupling cap remains at 0.020 uF.

If you want the bona fide 0.0022 uF value of the 1959 Bright Channel' coupling cap, you could place the 2203's 1st Stage 0.020 uF coupling cap after the switch (i.e. between point 2 and the normal input jack). I don't think any human could detect the 0.0002 uF difference in capacitance. The 2 in 1 method was probably chosen as an effective way to work with the existing layout of a 1959 amplifier.

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neikeel
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Re: Neil special

Post by neikeel » Wed May 26, 2021 3:43 pm

Sorry to be slow to the party!

It is a cool mod and I have mine on a 50watter.

I don't use PPIMV at all as the MV works well for me in both modes

Pressed_rat descrobes the effect of two caps in series precisely.

This mod on a Superbass with a 2200pF over the 470k treble peaker is my favourite Marshall at the moment
Neil

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