I called up Ken Fischer several years ago, when he was feeling better ... Ken's the kind of guy who cares numero uno about helping other guitar players and didn't hold back in answering any questions. I was asking for tech secrets and he obliged beyond my expectations ... usualy guys tell you to fuck off when you call 'em up like I did out of the blue. The mod outlined below should be applicable to the following amps: Metalface Marshall coding 69-A, 70-B, 71-C, 72-D, 73-E.

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Proceedings of conversation with Ken Fischer about modding early 70's Super Lead 100's (model 1959) for more Plexi Tone ...

-Observations on plexis-

*primary output impedance levels: -very early Marshall Plexis with Geneflex KT66's had very high plate voltage and 3200 Ohm plate load through two 50watt Partridge trannys, chassis were made of aluminum which affects fields of trannys and therefore tone ... forget about reproducing the sound of those plexis.

-primary impedance on the output tranny was around 2500 up to 1965 on main versions,

-following 1967 it went down to around 1750 Ohms with smaller output tranny and EL-34's, this gave the response a more raw and bitting quality. Best sounding are Parks at 2500 Ohms.

Differences in output tranny and chassis material (steel) explain mainly why SL's can't sound exactly like real plexi.

*component values: -65-66 Aluminum plexis had double 32uF filter caps in both preamp and output

-early '68 super lead, 56k->33k, 0.02->0.002,0.1->0.02 in tone stack and 0.005uF on volume control, some have 0.002uF.

*tubes and voltage:

- early 67 plexis ran big KT (kinkless triode) tubes at over 600volts while metal face marshalls run about 505-515 volts at the plate. Plexis then switched to EL-34's in late '67.

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Ken's recommendations

(A) The primary of the power supply has the following color codes:

orange = common red = 120

-> disconnect them, use red as common and violet in place of red as 120 (you're using a different length of the primary winding in this case)

this drops the supply voltage to 460volts which achieves same results as EVH's variac method except you don't have to worry about blowing variac - verify that this voltage is correct in your amp if you do this. This makes metal face marshalls sound more like plexi;

(B) Plexi's had less filtering in power supply, so replace 50u/50u by 20u/20u or 30u/30u and cut100uF down to 50's in series voltage doubler. By doing this we're making the power supply less stable, so if subharmonics start creeping in and you don't like 'em then increase filtering in preamp back up a bit.

(C) leave 0.68uF in bass input (2.7k and 0.68uF together) change tone stack and volume control cap to '68 values.

(D) In US models, change to EL-34 from 6550.

------------------------------------------------------------------------------ Speakers used with plexis ...

with 100w -> Celestion H30's also 15watt Alnico Blue Vox Bulldogs, JBL D120/6@16 Ohm, Altec 417

Date: Sat, 6 Mar 1999 21:50:50 EST To: jc@lynx.bc.ca Mime-Version: 1.0 Subject: Marshall amp info

Hi JC,. About a year ago you put a post on the alt.guitar.amps newsgroup pertaining to Marshall amps that noted when you talked with Ken Fischer of Trainwreck Circuits. I know its been awhile since that post but maybe you can help clarify something you wrote. This is the segment I'm referring to:

-- early '68 super lead, 56K->33K, 0.02-->0.002, 0.1-->0.02 in the tone stack and 0.005uF on the volume control, some have 0.002 uF.

What I'm trying to figure out here is; what does " -->" mean? For example, are you saying in early 1968 the 56K resistor was changed to a 33K resistor on the Lead versions? Or, are you saying that in early 1968 the 56K resister was used on the Bass amps and the 33K resister was used on the Lead amps? I was guessing the first interpretation I gave ("the changed to") would be correct because of the way your post was worded. If you could clear this up for me that would be greatly appreciated.

Sincerely, Mark

> What I'm trying to figure out here is; what does " -->" mean?

>A friend of mine sent me a link to your discussion with Ken Fischer on Super >lead 100 amps. i got as far as the statement "chassis were made of aluminum >which affects fields of trannys and therefore tone" and had to respond. BASIC >PHYSICS: Aluminum is a non-ferrous material. It cannot be magnetized. >Therefore the correct statement is: "chassis were made of aluminum which does >not affect the magnetic fields of trannys and therefore the tone is >distinctive from metal-chassis units."

thanx, I guess that's what was meant ...

@ Viva Analog jc@lynx.bc.ca