Fabrication Marshall Superlead 1968 Replica

Généralement les mecs qui bricolent jouent comme un pied ...

Eddie’s Plexi # 12301 ( VH 1 to Carnal Knowledge ):

· Split cathode V1a 250uF / 820,V1b .68 / 820
· All coupling caps are 0.022uF, Bright channel coupling cap is 0.0022uF
· bright cap on vol pot 0,005uF
· 470k mixer resistors
· 470pF mixer bypass cap
· Bypass cap on V2a is .68uF
· 33k / 556pF tone stack combo
· 47k NFB resistor at 4 ohm tap
· 220k bias splitter resistors
· uF cap on presence control
· 47pf on PI


John Suhr ( v2 fat cap and other mods ) – 1st restoration:

· Split cathode V1a 250uF / 820,V1b .68 / 820
· All coupling caps are 0.022uF, Bright channel coupling cap is 0.0022uF
· bright cap on vol pot 0,005uF
· 470k mixer resistors
· 500pF mixer bypass cap
· Bypass cap on V2a is .68uF and a 220uf to 470uf
· 33k / 500pF tone stack combo
· 100k NFB resistor at 4 ohm tap
· 220k bias splitter resistors
· uF cap on presence control
· .022uf output couplers
· 47pf on PI


Dave Friedman ( latest restoration ):

· Split cathode V1a 250uF / 820,V1b .68 / 820
· All coupling caps are 0.022uF, Bright channel coupling cap is 0.0022uF
· bright cap on vol pot 0,005uF
· 470k mixer resistors
· 500pF mixer bypass cap
· Bypass cap on V2a is .68uF ( back to stock )
· 33k / 500pF tone stack combo
· 100k NFB resistor at 4 ohm tap
· 220k bias splitter resistors
· uF cap on presence control
· .022uf output couplers
· 47pf on PI


Here’s what “we think” we know:

· The Power Supply Sags …. In fact, Dave Friedman said that in Eddie’s # 1, the power supply “sags about 70+ volts with normal wall voltage dimmed, and 50 some volts with a variac on it” ….

· B+ and Plate Voltages drop …. Mark Abrahamson ( Rockstah ) said the B+ in the amps he’s measured were around 363 dcv, and while he evidently didn’t take the 120v measurement, he estimated that they would normally run between 410 dcv and 500 dcv ?

· Tube Bias drops …, which is why everyone who uses a Variac says to “crank the bias” ! Again Dave Friedman’s comment was: “Now I have said this before, the amp is biased to like 85ma with normal wall voltage and then dropped with a variac to 90 volts. This will bring the bias down to around 50ma.” Of course “the” amp he was referring to was Eddie’s # 1 ….

· Don't underestimate the contribution of the 6CA7 Power Tubes ....

The EL34 is a pentode, while the 6CA7 ( which delivers a similar range of power output ), is a beam tetrode which RCA referred to as a beam power tube. Although power pentodes and beam tetrodes are based on different principles of operation and have different internal constructions they are functionally closely equivalent.

That being said, they both sound different. The EL34 for the most part has a creamy sound with a little grit ( crunch ). The 6CA7 has a tight bottom end and good mids with articulate highs. A lot of people call them a small version of a 6550.

I’m guessing that the frequency response of the Power Transformer gets restricted somewhat too ? But I’ve never been able to find any specific measurements on that ?

* Finally, don’t forget that Ed used a lot of cord ( extra capacitance ) in front of the Amp, as well as an Echoplex EP-3 as a slight booster …

And both Dave Friedman & John Suhr have been quoted as saying, the Amp was "stock" ( ie; no Jose Mod ), but there was one thing that neither will comment on ?

It 'might' have been John Suhr that said the amp was a very bright beast with a little more gain than a stock 12,xxx Plexi.

Bypass cap on V2a is .68uF and a 220uf to 470uf
ça me dit kkchose

mon avis: vu ce que friedman fait dans ces amp (et le type en connais un rayon un les amp evh...) je chercherai pas a me compliqué la vie dans un premier temps je commencerai par ça ....
mais quand tu fait des essais fait si possible a chaud avec un switch sinon tu risque de passez a coté de truc sympa vu la dernière fois
pour simuler la variac voir le friedman phil x (de mémoire ^^)
très simple et aussi efficace et sure pour l ampli .