Debate on Eddie's Plexi

[pfrischmann]

So,
I tried some tweaks
It's basically a 12 series with a .002 bright cap. It didn't really work until I moved the NF to the 16 ohm tap. For MY amp this was the missing part. Now It's perfect. The amp just sounds right.

I'll live with it for a while but whether I switch between the .022 and .0022. I think I'm done.

I can get closer by pushing the bias up but It's not really needed.

[leadguy]

The chocolate cap is a great example of what Dave F. is talking about....That cap variety reacts in a differant way than the later orange drops and the tonal differance is generally agreed upon as real and some say the differance between a white face and black face fender amp.

BUT because it warms up the tone and cuts highs better than identical orange drops of the same value it makes total sense to place that type in a place where you want bass added. Bottom line is values matter but so do the construction of the part itself....like the zozo cap guy pointed out in some write up.....one alternative part (even of the same value) in the entire circuit path can make or break the tone of the entire amp....it's all true.

And it gets worse because of shelf life parts can drift in value up or down...it really takes training to able to tune things up properly.

It's all pretty obvious unless someone doesn't know anything about electronics and there are many players that don't and don't need to.

Recording studios all vary, transformers all vary, Jeez solder varies but in general someone can set a filter at a close enough point to the frequencies that are targeted and Marshall and other tube amp manufacturers do some of the filter EQ design of the amp by using different filter values ie like 820/.68uf on the cathode which is a filter that allows certain frequencies to pass but theory is also tempered with practical tests involving combined parts and past experience, so things vary from ears to designs to parts to etc etc.

Take all the cathode caps off a amp and guess what happens.

[StuntDouble]

...

[RACKSYSTEMS]

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
0.1 uF cap on presence control
.022uf output couplers

Here is the spec for the tone. Now this spec depends on the amp and might not sound right. Very important you also have to bias the amp to like around 80ma at normal wall voltage and then drop the amp to 90volts with a variac. You must have the 6ca7s for the tone and there ability to hold up to this abuse. But everything matters in the tone kind of wire in the amp caps transformer etc.

Look I have been inside of many of Eds amps they are all for the most part stock Marshalls with minor changes

[Mcewen4444]

Oh god...what did I start when I gave you guys the link to that TGP thread. 9 pages now, wow.

Thank you Mr Friedman, will try that receipe for the hell of it. Pretty much what I came up with experimenting but had a 330p across the mixer and was increasing negative feedback to tighten it up. Will play with V2 caps a bit and put it to early metal panel negative feedback (100 on the 4ohm). Have it biased to 49ma at 90v input.

My 2 cents, it is spliting hairs, for me, alot of it is NMV circuit dimed with some kind of treble blocking and or slight bass boost. I.E., recorded with an SM57 center of cone right on the speaker grill (sounds real real bright in the room). Listen back and it sounds kinda right. Or if not recording, put resistive load on amp and line out to eq to poweramp (cut high treble fizzies bring up bass a bit). Or send line out to a second NMV Marshall with mid treb and presence off, bass up bright channel volume at 9 o'clock....all fun for experimenting (important that mid and treb are off).



MC

[RACKSYSTEMS]

Bias the amp to 80 ma with out the variac. Then just use the variac at 90. The bias lies where it lies

[pfrischmann]

Gosh Dave,
What do we talk about now??
LOL!!

Thanks alot for posting this.
You know the journey has been the best part of this. I've learned so much.
My amp as been...
a super bass
a '67 era 1959
a standard '68
a 69'
split and shared cathode of each of these
a jcm-800
a jcm-800 with lower filtering
Rockstah's Plexi/Jcm-800 spec
Ralle Spec 1,2,3
Stan Spec
All with every conceivable combination of bright cap, slope resistor and mixer bypass cap.

and has had
Jose master volume
Lar/mar MV
Trainwreck 1-4 MV
one wire mod
cascaded gain mod ala' RR, Choke......

Funny enough, the original specs always sounded the most organic to me.

I absolutely loved the Super Bass spec.

[pfrischmann]

BTW,
What is that yellow, square .68 on the pic ofEed's amp. Is that an old Wima?

[vh junkie]

BTW,
What is that yellow, square .68 on the pic ofEed's amp. Is that an old Wima?

If you believe these guys, its on of these:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0413553810

EDIT: Well, this morning there were 5 available @ $3... I bought 3 just FTH of it... looks like someone else grabbed the other two... now they are $3.50 each!

[StuntDouble]

...

[vh junkie]

I called up my buddy who workds on a tone of vintage fender stuff and he said those chocolate caps are paper/mylar made by Arco Elmenco; supposedly pretty common in a lot of the silverface Fender stuff.

Does he have a notion on what value range we might have for them?

I have the 820 in on v1...trying to adjust for that as I described previously

I already had .1uf output couplers (I did a poll a while back and was surprised to see that they were the winner!)

I switched the PI input from .022 to .1(new, not broken in Sozo) as you suggested, and did not like it much! Hey people! Caps need 20 to 100 hours to take on there true form!

But! then I added a broken in .68uf in parallel to the 820/330uf I had on v2 and really started liking it!

[StuntDouble]

...

[vh junkie]

I'm having the most fun with this thing!

Some one w/ some decent fingers has to make some clips with this!

Pretty much a stock 12xxx (but I have .1uf output couplers), running in LOv (400v plates).
a) Change v1 from 2.7k/.68uf to 820/.68uf
b) Change v2 to 820/330uf/.68uf (Dave! You da man!)

Gotta make both changes at once in my mind. Play that for a while!

I'm not sure yet if it's VH... but if you want to hear really monster tone, per StuntDouble, add:
c) Change PI input coupling cap from .022uf to .1uf.

[plexified]

if you walk through the lineage of this amp being from the Rose Palace (which would have worked it hard on the weekends and during events if they had a rock band like Zep in town) it pretty much makes some sense .

a) Rose Palace modest play time , but may have been exposed to abuse if it was in a practice hall situation . If so it was exposed to abuse and repairs . As you get into the 73 era the 12 series couldn't keep up with a metal panel volume wise or balls wise . So maybe then it was worn or had high expense from service . And was sold

b) I believe Ed got the amp with the origional transformers . Mainly because he said Jose beefed up his amps . He was right . Because he blew up the origional ot in his experiments and went to Jose , right down the road . Here Jose put in a JMP output . Much lower impedence , physically larger and would absolutly define 'beefing up ' a 12 series 100 watter . It would be much louder and ballsier . A 12 series is more like a loud 50 watt plexi rather than a Metal Panel 100 watter or Ali chassis 100 .

c) from his relationship with Jose , the head evolved into basically a master volume JMP .

d) not too far off of an amp for VHII and the similarity with the live tone . When the VHII tour came out his live tone was still very stellar . Those JMP head were VERY similar to what his main head turned out to be !

Right down to the American shipped JMP's with 6550's . GE 6550's which were the closet tube to a Sylvania 6CA7 in tone and ruggedness . The mod to do so ? Beef up the bias and take the feed back from the 4 ohm tap instead of the 8 ohm tap . Sound Familiar ?

So on a final note . When is the earliest picture of Ed with the #1 and is it woodie ?

Here is an early pic from a backyard party in 1976.

VH_-_1976_77_.jpg (56.65 KiB) Viewed 3580 times

Is this the woodie head or his favorite white plexi he has always pined after ?

So are they bare ( tolex stripped ) or white with a black Marshall logo ?

or is this 1976 Starwood gig have # 1 ?

1195.jpg

(22.11 KiB) Downloaded 1284 times

My take is he was really impressed with what Jose gave him back upon a blown otx . A much bigger , louder mother of an amp . So he tried to help Jose ( a likeable old immigrant gentleman like himself ) and he believed Jose did great work .

[vanhalen5150]

BTW,
What is that yellow, square .68 on the pic ofEed's amp. Is that an old Wima?

If you believe these guys, its on of these:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0413553810

EDIT: Well, this morning there were 5 available @ $3... I bought 3 just FTH of it... looks like someone else grabbed the other two... now they are $3.50 each!