Plexi

[JC@]

As a newbe I'll try to be as concise as possible.
Would it really be possible to recreate a Plexi with today's components?
Mercury Magnetics trannies or whatever.
I ask this because I've worked and played with Soldanos, all kind of Marshalls etc etc etc untill I had on the bench a real Plexi.
Well, I've never ever heard something not even similar to it except on recordings!

What makes them so unique?

The aluminium chassis? The output transformer? The mustard caps.
None of them alone are enough to get the plexi sound, but which of their components would be the real essence of their sound?
I've heard a thousand times that Marshall was based on the Bassman, but man, this beast is a piece of art. Just trust me in case you haven't played through a REAL one.

[Chris / CMW amps]

Yep!

Altough there were lots of variations between '63 and '68, and later ones of course.
A nice set of MM iron and maybe a little research on certain circuit variations will get you very, very close.

Hope this helps/keep on building!

[Ray Ivers]

JC,

I agree with Chris - in fact, I'd go a step further and say that it's perfectly possible to create a clone of ANY Marshall plexi-panel amp - using (possibly custom) components made today - that almost all of today's players would think sounded better than the 'clone donor' amp in a double-blind listening test (where neither the test administrator nor the listener knew which amp was being played through at any given time, until after the test was over).

Sometimes when I tell people I have a '67 50W plexi with all NOS Mullards, they go, "oh, wow! - how does it sound?" I tell them "it's got great bass, the clean sound's OK, and I'm terrified of blowing the OT so I almost never crank it up". The two 100 watters I played through were deafeningly loud and pretty noisy by today's standards; Hendrix made 'em work, but I sure couldn't.

I really don't mean to down plexis by any means, but many times I think that people who really worship them (but have never owned or played one) should continue to neither own nor play one.

Ray

[andrew4566]

Sometimes I think the mojo factor of playing a 40 year old amp clouds peoples perception. I find it amusing when people say they want the old stuff 'cause it has an aged sound you can't get out of new components. I don't know how many times I have pointed out to people that Hendrix and others played essentially brand new amps for most if not all of those great recordings and concerts.

In fact I would bet that the amps made for them were built to an extra tight standard. I know if I was building an amp for someone famous I would take the extra time to measure each component for tolerance etc..



-Andy

[Ray Ivers]

Andrew,

At the very least, I would think that amps/guitars/whatever provided by the factory to big-name players would be gone over with a fine-tooth comb to insure reliability - and of course we've all heard about amps being factory (or non-factory) tweaked for this or that player.

There's a flip side to this, and if you ever get a chance to the 'Lifelines' Hendrix box set, you'll hear it clearly on certain cuts; the sound of amps that are barely being kept alive, perhaps during the final few dates of an extended tour or a last-minute studio session.

Ray

[JC@]

Sometimes I think the mojo factor of playing a 40 year old amp clouds peoples perception.
-Andy

I agree, but this is not the case, not at least *my* case.
I have played through quite a few vintage amps.
All of the four amps I keep for myself are homebuilt except one original that is now moded so much you can't consider it original anymore.

So this is not the case. I just cleaned a bit this plexi, installed new set of EL34's tubes, adjusted the bias (it came with a simple bias pot mod for all the tubes), new preamp tubes. Plugged a guitar to it, played a bit and the sound out of it surprised me.

Maybe right after that I magnified it a bit because it was a plexi and I never had one before, but the fact is that when I first turned on this amp I hadn't any special predisposition to be surprised because of the brand or model.
I just liked it a lot, and on top of that, it was a real 100 watt plexi.

Anyway, as I have used lots of tubes types and brands, cap types, etc, and know what they really apport to the final sound on different amps, I'm begining to think that the output transformer is a lot more important than I used to think. I mean I now Mullards, and mustard caps, I really know how they sound, and it is not that what I perceived in this amp that I liked so much. In fact I didn't install mullards in this amp

So I believe it is the output transformer. What else?
I'm not an expert in anything. In fact I am a newbe, with a little experience with good tubes, and I know very well mustard caps. That's all. So again it has to be the output transformer.

[famp69wv@yahoo.com]

It has to be the Output Transformer!

JC

I'm sure the OT has much to do with it. Don't overlook the remainder of the components though and especially the PT and, for a Marshall, IMHO the tone stack slope resistor. You've built before so you know the amp is the sum of all it's parts.

You may want to look at Obsolete Electronics' website (OEI) for, not only some very good OTs but, some pretty good info. A number of companies have the parts & a few can knock off cabs & chassis so thats not to big an issue.

Did you happen to take any voltage measurements from the amp you worked on? How about marking down all the numbers on the trannies for ID purposes? Do you know the year it was made? If so then I think you'd be in a good position to start the cloning process. If not get it back & get a comparable schematic off the web, mark any differences and get the above info.

Anyway good luck

Philip

[bob p]

I'd like to follow up on Ray's observations about how its perfectly possible to clone any original amp using modern components. I agree with his assertions whole-heartedly, and I'd like to add something else that may be worth considering.

There was A LOT of variability in those old amps. If you've ever had a chance to play two original plexis side-by-side in an A/B test, I'd be willing to bet that the two amps sounded different. They may or may not have sounded dramatically different, but I'd bet that they sounded different. Some of this is attributable to the changes in parts/suppliers that took place over the years, and some of this is attributable to the fact that there is a range of tolerances for every part that's used in the amp. Not surprisingly, variations in tolerances of many parts can add up to one amp rolling off of the production line sounding different than the next, or one amp having a "magic" sound quality that the one next to it on the production line didn't have.

To quantify this sort of phenomenon, quite a few people have dissected original marshalls and taken precise measurements of the parts to try to reverse-engineer the magic. The only drawback to doing this is that it allows for the "cloning" of one specific amp that may or may not be representative of the population at large. There are always going to be subtle variations from one specimen to the next. That might help to explain why one person's plexi clone doesn't sound exactly like someone else's. Given today's improved manufacturing techniques, I think that there is less variability in production today than there was before, and that today's amps probably sound more alike than different.

[Chris / CMW amps]

Hi all

Good posts! Lots of differences between all those old Marshalls (and even "newer" ones). As example: pcb '73 SuperLead with double 0.01s in the tonestack, 0.0022 for the bright-channel and everything else 0.1... Marshall was out of 0.022s that day

After posting something about plexi-transformers on a dutch vintage-amp-forums I got a lot of bashing but this is the new AmpageForum!

My 1st "real production" amp will be my "Plexi+" or "SmallBox" (please Rick E. let me know about it!).
I've heard/played/serviced "some old Marshalls" so to say, most were "ok" or "nice" and 1 or 2 were "almost amazing". Almost?: yep, still possible to make some tweaks to make it even "better" and +/- 50 Watts is still too loud for most gigs/sessions so I'll include a post pi mv.
I've been comparing various transformers/etc. and after spending lots of $/euros ( alt-128 doesn't work) I'm happy.

In a few words: always try to play an old icon but never doubt about your own toughts/hearing!

[Chris / CMW amps]

test(av./sig)

testing

[Chris / CMW amps]

It works!

[dai h.]

A lot of parts were different, a lot of the specs were different so there is no single truth when it comes to the old amps despite what some people believe or want you to believe. That's all pretty obvious though if you look at enough chassis pics (or actual chassis) and have read what owners have reported to be in their chassis. The idea that the mustards are the only cap type or the "correct" cap is not one that is substantiated as far as I know (unless someone has actually gotten enough of the various types to match values and scientifically A/B'd them all). Transformers varied, voltages varied, small circuit specs varied, cap brands, resistor brands, parts tolerances, wire colour, pot values, pot tapers etc. varied so the homogenized ideal that some people have and present as truth is in actuality a semi-truth. Some actually go so far as to change the original parts to the idealized "correct" types. But that very variation is what makes the old amps interesting though IMO.

As far as recreating the sound, I think it could be done since there are enough parts both new and NOS to pick and choose from to create your very own customized ideal (or clone). The big things would be choosing the right (as in right for the sort of sound you want out of the amp--and what you actually want as opposed to what you think you want--for example choosing a JTM45 when what they really want is more gain from a later lead circuit) circuit and circuit variations, good transformers (esp. the OT), then maybe the parts types as next in importance (don't forget the tubes!). Outside of the amp, speakers should be hugely important also.