parallel caps in Marshall curcuit

[dumbassbob]

I have a 1993 100watt Marshall "plexi"ri..A lot of the caps have parallel caps like the bright cap is a 4n7 and a 1n makes it 57000pf.. Why did they do it this way?,insted of just using one cap...If I was to pull some of these caps would it change the voltage like a resistor in series or parallel...That bright cap is like a on/off switch for the volume pot

[Chuck H]

Pulling caps will change AC voltage @ frequency relative to the knee change. In other words... It will only change the signal voltage. Which sounds like the goal. Pull the 1n cap if you like. It should improve the "problem". FWIW plexi Marshalls are notorious for coming up fast on the volume knob. Some people think this was done on purpose because it makes the amp seem louder to non tech players that rarely get the volume above four. Which was the probably the largest demographic. That this design was developed during the "watt wars" may support this idea. It's not that hard to manage. Still... 4.7n is closer to the stock value of 5n than 5.7n. So there's nothing to lose in trying it. Except that (IIRC) that amp uses a top loaded, bottom soldered board. So it'll be a PITA if you want to put the 1n cap back in.

[J M Fahey]

The 4n7 bright cap is the original value and what makes it a Plexi of that era.
Yes, it's already distorting with volume on 2 ... that's the way it works.

The 1 nF in parallel is not original, and probably an add on pulled straight from "the mod pages" .
Which would be confirmed even more if more caps have the same piggyback ones.

Not what a Factory woud do unless it were a critically tuned circuit (think graphic EQ, specially a 15 or 31 band one) in which there's no other way to fine tune.

Personally I'd put that amp back to Factory specs, but to each his own.

[MarkusBass]

I'm not sure whether 4.7n was the original value. I've seen old schematics where the capacitor was listed as: 0.0005 (this is old Fender style of marking capacitors on schematic). When you have that many zeroes you have to think if the value is 5nF, or 0.5nF. And in this case it is 0.5nF = 500pF. It is were easy to make such a mistake. As you can see below, even Bob made the same mistake. He calculated that 4.7nF+1nF=57000pF. But it is only 5700pF (ten times less).

I have a 1993 100watt Marshall "plexi"ri..A lot of the caps have parallel caps like the bright cap is a 4n7 and a 1n makes it 57000pf.

With 4.7nF the amp is too bright which makes it very difficult to use it with bright guitars like e.g. Fender Startocaster. I was fixing several amps with this problem. And the only fix was to replace 4.7nF capacitor with 470pF capacitor. In some cases I installed a switch to switch in the old capacitor (4.7nF) but later I was told that the users never use it. This is something to consider. We all know that resonable values of the bright capacitors are between 82pF and 330pF. 470pF is already too much. So you have to decide if you want to keep the amp to factory spec, or you want to make the amp sound like the owner would like it to .

Mark

[dai h.]

(from what I remember reading from a Ritchie Fliegler(sp?) book) the values were copied from a vintage Marshall the developers felt was a good sounding unit. They are in parallel (probably) because it was cheaper to achieve the values that way. So, 470pF + 100pF(IIRC) was probably to get the same value as a 560pF (or possibly a 500pF that was a bit off).

The 5700pF(4700pF+1000pF) was probably close to a value of a stock 5000pF disc ceramic on the volume pot. Whether you use this or that is greatly a matter of taste. Old Marshalls were all over the place anyway. The volume pots might be audio or linear taper. The pot cap might be 5nF or possibly another (smaller) value, or sometimes they had no cap there at all. Experiment and go with what works for you.

[Chuck H]

Marshall has the value for that cap on all the Marshall 1959 schematics indicated as .005 (5000pf). Not .0005 (which would be 500p). I have never seen a 500pf cap in this position in a factory built 1959. It's entirely possible that Marshall used a 4.7n cap sometimes since it would have been a common (and therefor cheaper) value late in the JMP era. .005 (or 5000pf) is the generally accepted value for this circuit used by cloners and boutiquers (new word )

The parallel caps are stock on that era of reissue for that amp. I had a '91 and can confirm this. Also, here's a photo I found to corroborate it. The highlight was already there so it seems someone else had questions about this too. As you will see, this era SLP reissue also used parallel caps to achieve the coupling cap value here.

[tedmich]

Marshall seems to use plenty of parallel caps (to get values they didn't want to pay to get made custom I guess)

in this schematic they didn't want to buy (C15/16) 2.67nF, (C17/18) 1.47nF, (C4/C5) 0.57nF caps but these specific values were required (in their minds)

Of course these caps are probably +/-10% at best...

[Chuck H]

True that! But those are odd values. In the early 90's 1959SLP the values required would have been 4.7n and 2.2n. VERY standard values. But perhaps those blue blob caps they used in there DON'T come in those values and those caps, paralleled with a ceramic disc, was CHEAPER.

I still have my 91 1959SLP. I'm told the early 90's was a good year for these amps because of the transformers used. Someday I'll rebuild mine hand wired with mustard caps, etc. and sell it. I even have a trio of good Mullard 12ax7's I got cheap that I'm saving for the project I expect the parts, with whatever is the trendy EL34's and filter caps at the time, to cost a few hundies. The labor will be about three and a half days +1 to source and acquire parts and arrange selling and shipping the thing. The profit will be about $800. So I'm not in a hurry to work for $180 a day while I have other, more interesting (to me) projects and I can make more money than that painting. But it's on my list of cool things to do. And it would add another semi genuine plexi to the rotation of gear out there, which is valid I think

[dai h.]

Marshall seems to use plenty of parallel caps (to get values they didn't want to pay to get made custom I guess)
[ATTACH=CONFIG]35179[/ATTACH]
in this schematic they didn't want to buy (C15/16) 2.67nF, (C17/18) 1.47nF, (C4/C5) 0.57nF caps but these specific values were required (in their minds)

Of course these caps are probably +/-10% at best...

that's the one with the Ritchier Fliegler involvement (IIRC there were two favorable units, one belonging to Warren DeMartini of Ratt fame with high B+ and another with lower B+. They went with the lower B+ probably so they could continue using the PTs already used in their other 100W models (1959, 2203, etc.)). (Also, I think you meant 5n7 and 590pF for C17/18 and C4/5 respectively.)

I have never seen a 500pf cap in this position in a factory built 1959

there is a 1959 from a bit earlier that used 220pF

Someday I'll rebuild mine hand wired with mustard caps, etc. and sell it.

if someone comes up with a test that proves there is some sort of "mustard magic" I'll believe it, but factually speaking, the old ones came with a number of different brands (Philips/Mullard, TCC, Hunts, Dubilier of England, Wima, Rubycon(?), Lemco, Murata, ERO, etc.)--usually film but in spots oil (2n2 position) and even sometimes ceramic (paralleled 10nFs). Mica and ceramic (non-temp. compensating and temp. compensating) for the smaller caps. From what I understand the earliest ones had parts bought from the Radiospares catalog, then later the cheapest parts they could find (standard commercial practice).

I also wonder if there were some weird layout things going on from the cap sizes and locations (the big rectangular micas, coupling caps in the tone stacks getting heated up by B+ dropping/decoupler Rs).

[Chuck H]

if someone comes up with a test that proves there is some sort of "mustard magic" I'll believe it, but factually speaking, the old ones came with a number of different brands...

I wasn't touting any mustard magic. But others do. The plan is to sell the amp Mustard, Mullard, Blah blah blah. I'm saving the Mullards because their value in the context of selling that amp is greater than my faith in hype.

[dai h.]

I would agree that installing "mustards" could improve the perceived value of the amp. Observing the way people seem to digest the info, it's sort of like stereotypes. Sometimes I've seen owners of vintage Marshalls removing non-mustards because the idea of the Philips/Mullard mustards being the "correct cap" is so powerful (which they sometimes regret when they learn otherwise).

[J M Fahey]

I'm not sure whether 4.7n was the original value. I've seen old schematics where the capacitor was listed as: 0.0005 (this is old Fender style of marking capacitors on schematic). When you have that many zeroes you have to think if the value is 5nF, or 0.5nF. And in this case it is 0.5nF = 500pF. It is were easy to make such a mistake. As you can see below, even Bob made the same mistake. He calculated that 4.7nF+1nF=57000pF. But it is only 5700pF (ten times less).

With 4.7nF the amp is too bright which makes it very difficult to use it with bright guitars like e.g. Fender Startocaster. I was fixing several amps with this problem. And the only fix was to replace 4.7nF capacitor with 470pF capacitor. In some cases I installed a switch to switch in the old capacitor (4.7nF) but later I was told that the users never use it. This is something to consider. We all know that resonable values of the bright capacitors are between 82pF and 330pF. 470pF is already too much. So you have to decide if you want to keep the amp to factory spec, or you want to make the amp sound like the owner would like it to .

Mark

Agree that the "original" "original" value, was "0.0005" or "500pF" (old 2/5/1 notation) , a.k.a. 470 pF in modern normalized scale notation (in practice both are the same) .

But at some point .005/4n7/4700pF started to be used .
Whether it was done on purpose, a happy accident, or both (which I think is the real explanation), fact is that it turned a clones Fender Tweed Bassman into a distortion machine.

Such a high (10X normal) value for a so called Bright cap turns it into not a bright cap any more (way too low turnover frequency for that) but an pot bypass combined with Bass cut , which by the way is good to clean distortion mud.

Since then, a Plexi Marshall on 2 was already distorting big way, and any higher setting only added beef/body, until on 10 the cap value is irrelevant because it's shorted.
They needed powerful pickups for that, so the de facto Rock guitar became a Les Paul (a "Jazz" guitar) or an SG, both equipped with humbuckers, instead of what was currently the Queen: the Stratocaster (think Hank Marvin, Surf guitar, etc.) .
The ones who used Strats had to add either some distortion pedal (Jimi Hendrix or Jimi Page on a few instances) or a Germanium Treble Booster (Ritchie Blackmore).

Of course, 99% users today buy completely inadequate for their reality 50 or 100W Plexis and use them in a bedroom, garage, small Club, and then complain of way too bright amps, buzzy sound, icepicks, etc. and go to the extreme of butchering (castrating?) their amps by clipping bright caps (all values, even the "standard" ones)
It's one of the most common complaints at sites such as TGP .

Real problem is, of course, that they bought this:


to do this actual job:



EDIT:
read later posts.
So the parallel caps appear on reissues?
Which were purportedly "based on particularly good sounding amps"?

Gimme a break!!!
That's 120% Marketing B.S. !!!!!

Yes, of course they pulled and measured a nominally .005uF caps and they found that that particular one was, say, .0056 uF .... that's what 10% tolerance means !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The very next one in the bin may have been perfectly acceptable .0045uF , so what?

Worst is, paralelling a ".005" cap with a "560pF" one does NOT guarantee it now has the "magic value" because *both* have 10% tolerance.

This a truckload of crap to catch Musicians who have no clue on how things work.

Fact is 10% difference on such low slope (less than 6dB/oct) circuits is absolutely impossible to catch by ear .
To boot, there are many caps in an amp, even a simple one such as a Plexi, so combined drift will be all over the place, even on "corrected" ones ... but users have no clue and wonder in awe at the "exceptional work to faithfully recreate a Piece of Art".

fact is, to make clearly audible differences, 2:1 changes are common, such as going from 220pF tom 470 pf .... or even worse ....... to 4700pF

Oh well, wonders of Marketing.

[MarkusBass]

Nice . I think that the "original" "original" "original" value was ".0005" (without the leading zero), which made it even more difficult to read the actual value. The change could have been done on purpose but I don't like it. 4.7n capacitor put across the Volume pot is just a short when the pot is set to 2, or higher. This raises a question: what is the pot for (if it does not work as a Volume pot). In my opinion, with lower value of the capacitor, the amp has more possibilities - without loosing anything regarding power.

Mark

[Chuck H]

I'm going to open with "corrections accepted"

AFAIK the "plexi" panel 1959 circuit was made from '67 to late '68 or early '69 and used a .005 (5000pf) bright cap on the volume control. I've only seen a few examples but my experience confirms this. It's the value indicated in absolutely ALL the Marshall schematics for this model.

Some later models, but not the 1959, used a .0005 (500pf) "bright" cap across one of the 470k mix resistors. This isn't the volume control "bright" cap.

There may have been other "plexi era" models that are NOT the 1959 circuit that used a different value "bright" cap on the volume control.

It's been stated twice now that .0005 (500pf) was indeed the "correct" value. Once by Mark and confirmed by Juan. But nothing in my experience supports this and neither do any official Marshall documents.

The value commonly accepted as "correct" for this circuit is .005 (5000pf). I'm certain that many values have been used other than this by someone other than Marshall. Marshall used the .005 value for the 1959 Super Lead plexi era design in question.

All the JMP Marshalls I've had open were a circuit other than 1959. Though the 1959 model was made at this time. It was common in this era for amp companies to use close "standardized values" instead of schematically correct values (.022uf instead of .02uf, .047uf instead of .05, etc.) I can imagine that Marshall may have used the 4700pf value then. Though Marshall often seemed to do things a bit different from standard.

[drewl]

Funny, in the History of Marshall amps book they recommend a 500pf.

I removed the bright cap in my 69 1959 and 73 1987.