Here's some information that I've seen posted around the webz on the specs of Ed's # 1 ....
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 ) – 1[SUP]st[/SUP] 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.
IMHO- It all comes down to what the Variac contributes ? Here's what John Suhr had to say about it over @ HRI ....
Posted by: Suhr - 8/1/2013 @ 5:09 pm
Here is the thing though.
Lowering the plates is one thing but lowering the heater voltage also changes the tone and is the way Ed's was run. You can safely run the heaters on the low side it is a major component of the "brown sound".
There are other amps with low plate voltages like that, Fargen’s Marshally amp. That is only like 25W but uses two EL34's as well which equals low plate or screen voltage.
To my ears, as well as last time I talked to Dave Friedman though it isn't 100% the same unless the heaters come down a bit as well ( the SL68 does drop the heater voltage too ). Of course there is a voltage drop that is acceptable and in some audiophile circles believed to extend tube life, you can also go too far and hurt tube life. For that reason it should not be adjustable.
Posted by: Suhr - 8/2/2013 @ 12:51 pm
Heaters in a plexi are AC there are no diodes.
Take a variac and run a stock plexi amp with 500V plates at 120VAC at 95~90V, around 435 B+ is what I read on his, rebias the power tubes for the B+ you are reading and you are good to go. I wouldn't go lower than 5.5. You might be able to drop them independent of the B+ a little with a larger wattage resistor but it will get real hot and the value will be real small.
Remember though you cant variac an amp that has any switching or regulators in it.
I remember Ed's conversation clear as day when I picked his brain while working on his amp in 91’. He brought the original light dimmer he used too.
The amp was stock ( almost ) but the OT was replaced and there was a small mod to increase bass response.
The Marshall was a little hotter circuit than typical though, there were a few versions Marshall would use that had slight differences.
He used the variac to turn the volume down until the pilot light dimmed when he played a note on the G string. He said that was the main amp used on all records until F.U.C.K. He said everything else was engineer and the pedals he may or may not have used, the Echoplex, EQ and said he honestly just didn't remember and never knew it would be such an iconic elusive tone or he would have documented it all. IMO it had everything to do with his attitude and engineering. I heard him play through that amp with and without the variac and surprise ! ..., it all sounded sick and was Ed's tone in a box. In fact I doubt anyone could really tell a difference when he is playing it. It was more of a loudness and feel thing. When I played thru it, I sounded nothing like him, it just sounded like a freaking loud Marshall.