Plexi Mods - Marshall Amp Modding Secrets!

EVH Magic Marshall Spec (12000 series)

EVH Gear
From the Plexi Palace:

(1) By Mark Cameron from pictures taken in the early 80’S.

There ARE small tone altering and/or gain altering mods.
I do have pic’s so I CAN physically see that the amp has a split cathode arrangement.
…the cap on v2(330uf) cathode.
…its a 2.7k/.68 and the resistor looks like the stock part (but I don’t think it is) but the cap is one of those yellow square type from the 70′ Marshall’s. From the circuit card It looks like it came stock with this arrangement. The post that is pressed in looks stock too which tells me more than anyone could. In the pics the serial # is 12301.

(2) Cerrem 1980.
To try to wrap up is ED thing…
His head was a 67/68 and he had the first stage valve with BOTH cathodes tied together sharing the same 820 ohm resistor that was bypassed with a 330uF blue cap…

His treble cap was a round shaped ceramic that was a 250pF that said MURATA … With 56K on the tone circuit feed..

One of the 470K mixer resistors was bypassed with a round hollow tubular MURATA cap 500pF… If memory serves me right on the value, or it was a 330pF..I will check my notes….

The real kicker, his phase-inverter “get-rid-of-the-FIZZIES” cap was a 100pF instead of the normal 47pF ….and this my friends is how the “brown sound” with that added compression happens..

Oh, BTW those 820 ohm resistors were carbon-comp and drifted in value up about 1.1K and make the amp much more gainy and warmer, since these re-bias the 12AX7 valves in a bit more non-linear region..

I am pretty sure the feedback resistor was a 47K …I will have to check my notes…

The filter cap in the center of the board was a dual 16uF grey RS cap…

The screen filtering was 2 DALY 32uF light-blue caps in series… The voltage doubler were 2 100uF DALY royal-blue caps…

Rear cap on top of chassis was a royal-blue HUNTS 32uF or 16uF…need to check notes..

The value of the coupling cap between V1 and V2a….022uF.

At least that was what it was in 1980…

(3) Plaap (a friend of Peter Van Wheelden who restored Eddie’s amp.)
Edwards amp’s internal measurements were as follows:

1.The first 820 ohm resistor (carbon type) measured a little over 1K. It was bypassed with a blue 330uF resistor. can

2.His treble cap was a 250pF Murata flat ceramic one. The cap across his 470K was a Murata hollow round 330pF. cap

3.His second stage 820 ohm (which also measured a little over 1K, was also bypassed with the exact same type 330uF blue coloured cap that was on the first cathode resistor.

4.The filtering caps for the middle of the board were grey coloured RS caps that had dual 16uF values.

5.The screens were 2 blue caps in series (DALY 32uF’s).

6.The voltage doubler were two big blue caps (100uF DALY’s). The one on the outside was a blue Daly – and was a single 32uF.

7.The feedback resistor was a 47K of unknown origin.

8.His power transformer was the smaller one of that era. His OPT was also the smaller one with 1.5″ stack.

The amp was either left stock into a load resistor or, a big Ohmite (or other) power resistor was placed somewhere in the circuit to cut the power of only the output stage meaning Sylvania 6CA7’s were the only valves to hold up to this.

If this is true – doesn’t this kinda dispel the whole ‘special’ 67 slp 100 myth …as almost all amps of that year had pretty much identical configurations (with the small exception of the 330uf cap on the second preamp stage?

Dankuwell ! (dutch for many thanks)

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