Marshall Super Lead build, sort of

[Helmholtz]

In his book on hifi tube preamps Merlin shows cap distortion measurements over frequency for most common types.

Ceramic caps can reach 1% while the THD of KT and MKT film caps is typically lower by a factor of 100, meaning a THD of up to 0.01%.

[Chuck H]

Well I hardly think I could detect 1% distortion in a clipping tube amp. In the multiple discussions on other enthusiast forums where the tonal differences between ceramic and mica caps is discussed I've read arguments about DC bias and microphonics as they apply to ceramic caps. I know DC bias can have a pronounced effect but I think this is more of a MLCC thing and all those older ceramics were single layer. I wonder if Merlin considered DC bias or if he just tested those caps with only AC signal. As far as what I've read on THIS forum mica caps can be problematic due to microphony too so it's not just ceramics. Personally I've never detected microphonics in capacitors to have a pronounced tonal effect other than the occasional grossly bad one making ugly noise. Is there anything real that might explain the reported "smooth"ness of mica caps compared to the "grainy"ness of the single layer ceramics?

EDIT: I have never used ceramics for much in a signal chain because I always figured them to be the cheapest, and therefor less desirable option. I haven't compared the two types for tone.

[Helmholtz]

1% THD is certainly large enough to avoid them in the signal path of Hifi gear.
Merlin only shows a dependency on DCV for ecaps.

Ceramic caps (other than NPO) can show a capacitance drop up to 80% with DC bias.

Never heard of a microphonic mica cap (except if it's cracked or has a bad connection).
Cap microphony requires a ferroelectric or piezoelectric dielectric, which mica is not.
https://en.wikipedia.org/wiki/Cerami...tor#Microphony

[Helmholtz]

That video https://www.youtube.com/watch?v=rWEBE81DWQc shows a nice example ( around 51:00) of why you typically can't trust a DMM for cap measurement.
Cap is 50µ and DMM reads over 200µ.

BTW, the guy is wrong when saying that capacitance tends to increase with age. Never does with ecaps. Different with PIO caps, though.
Also a cap reading in spec on an LCR meter isn't necessarily good. Could still have excessive ESR or leakage.

[Chuck H]

1% THD is certainly large enough to avoid them in the signal path of Hifi gear.

Sure. Obviously. I'm referring to the discussions about what those "distortions" mean in clipping Marshall amps. Where 1% doesn't amount to much.

Ceramic caps (other than NPO) can show a capacitance drop up to 80% with DC bias.

I did some reading and it seems class 1 capacitors are much more thermally stable and have lower DC bias shift There are many single layer types in this class but there are some class 2 single layer types. Class 2 capacitors are much worse, but are used in circumstances where volume/capacitance economy is important and the thermal properties are less critical. Many of these capacitors are multi layers but there are some class 1 multi layers also. So I guess a good question might be, what "class" of capacitor would the technologies used to make the vintage single layers disc capacitors provide? Were these old Lemco's, RS and SRC capacitors class 1 or class 2?

[Helmholtz]

There were no multilayer ceramic caps in the old days.

Class 1 caps use a low dielectric/permittivity ceramic having low losses. Means that they are larger than higher class types and and typically below 1nF.
If you have an LCR meter the high Q value can identify a class 1 type.

Rectangular Lemco/RS caps have a very high Q above 1000@1kHz and might be mica types. Certainly no class 2 or higher ceramic.
The red "dogbone" ceramic caps have a medium Q around 150.

If you match the Q value of ceramic caps they are most certainly the same class.

Ceramics other than COG/NPO show a significant capacitance drop when heated with a hairdryer.

[Chuck H]

Well I don't have an LCR meter. And I did read about the electrical differences between the two classes. I'll look around but I have a hunch I won't find spec sheets for these vintage capacitors and even if I do I'll be surprised if they list anything like class 1 or 2 or the dielectric type.

What I'm trying to learn is whether or not there is a significant difference in tone between the types of capacitors, mica and ceramic, used in vintage Marshall (or any brand) amps. I have a hunch those old disc ceramics were closer to class 1 than class 2. Which would make the tonal difference between the old ceramic and mica caps unlikely other than a little more ESR for the ceramics. Which would be insignificant in those circuits. Yet people report there is a difference.

[Chuck H]

Ok. I looked at a bunch of old disc caps from the 60's and 70's and gathered as many type codes as were visible. Then I had to research what some of those type codes mean since they are no longer in common use. I did not find any Lemco or RS caps with type codes on them but as far as I can tell most of the ceramic disc caps in consumer electronics of the era were class 2 or 3! So DC bias would have a very large effect on tone.

EDIT: I did not find any "dog bone" caps with type codes.

[Helmholtz]

How many ceramic caps in a Marshall do actually see a DCV? Didn't check, but probably only the treble cap in the tone stack.

[Chuck H]

How many ceramic caps in a Marshall do actually see a DCV? Didn't check, but probably only the treble cap in the tone stack.

Right. The treble cap. But it sees significant DC. I had considered that regarding the PI plate coupler (low DC differential leg to leg) and the "bright" cap on the channel II volume pot. But the treble cap is a pretty big tone shaper. So it may be significant.

Looking at the dog bone (dogbone) caps I did notice that some makers marked one end. I guess this was more significant in this style cap because there's a definitive "outer foil".?. As far as I can tell the older red Lemco's have no marking. So I wonder if orientation for those caps could have been random, but made a difference no one was paying attention to.

EDIT: re: tubular ceramic polarity "but made a difference no one was paying attention to". I observe this regarding noise, not tone.

[dai h.]

re: mustard film and foil, I found a page in my files from a 1967 Radiospares catalog, and on the page it says, "CAPACITORS POLYESTER" (so I guess that's what section it is), plus:

POLYESTER CAPACITORS

All capacitors in this section employ a non-inductive metallised polyester dielectric construction. Tolerance is +-20% with a P.F. <.01 at 1 kHz for all types.

More confusion? lol (I don't see the mustard but see a boxed type which might be the pink-ish TCC used for the PI grid to feedback and Presence in some amps (100nF 250V), and possibly the Dubilier--the catalog is black and white plus all the caps seem to be house branded ("RS" logos) which makes it harder to determine the manf.

Another page lists the rectangular micas, with description as follows:

CLOSE TOLERANCE SILVERED MICAS

500 V D.C. Wkg.

S-Micas (+ Capacity)

Solid impregnant with tough cement coating, wax impregnated overall. Suitable for pulse operation. Side-entry wires. Tolerance +-0.5pF below 50pF, +-1% for 50pF and higher. All capacities are 500V d.c. working...

Dimensions:
(2.2-75 pF) W. .5" H. .32" D. .06"
(82-220 pF) W. .66" H. .47" D. .06"
(250-400 pF) W. .86" H. .66" D. .06"
(470-556 pF) W. .86" H. .66" D. .06"

Values that I know of being used in Marshalls listed on the page: 47pF, 100pF, 250pF, 270pF, 500pF, and 556pF.

re: the question of class 1 or other, Marshall used both. The red 100pF on some pot caps might be N750 (with no purple dot--I have some of these). The Murata N750 250pF 5% disk with purple mark seem to clearly be class 1 (used as treble cap in some amps). The Murata 500pF dogbone N750 (used in some amps) with purple mark also looks clearly to be a class 1. (The size is a hint because class 1 are not as volumetrically efficient.) Some of the sizes are different for same value caps (IIRC there were some amps with a relatively larger looking dogbone cap for the 250pF treble cap so the bigger one may be class 1--or, possibly a higher voltage rating). Sometimes caps have written marking of temp. coeffcient but no color dot. Sometimes a color dot but may not be class 1 (I have some relatively newer Philips 10nF with a green mark which look too small to be class 1). Class 1 and other types have existed for a long time as far as I can tell. I have a late 60s electronics catalog that lists all the different types. A lot of people seem to not know there is any difference and assumed them to all be the same, but class 1 are considered a linear (low distortion type).

[Chuck H]

Well I can't relocate the page where I saw the Philips 311/Mullard C296 caps indicated as metalized. It was a salvage and vintage overstock distributor with no affiliation to any amp genre or nerd lore. It was stated mater of factly that they were metalized. So subtle that I bought it. But I guess they're not. I don't know if this is a legit datasheet, but:

[Chuck H]

I don't know near enough about vintage capacitors to have been aware of colored dots indicating class or type. The end markers I mentioned above were in the same black ink as the value/voltage script. Some of the tubular caps I looked at had only colored dots or slashes and I assumed they were value indications.

[dai h.]

They could be value indications such as BRN-BLK-BRN = 101 = 100pF, but if you have say, a color mark on top of a disc ceramic, then that could stand for the temp. coefficient (BLK = NP0/C0G, PUR = N750, (I forget exactly), but also red, green, blue, orange (and some other ones)). I would guess that you have seen them in an amp or effect, or some piece of electronics, but didn't notice the significance. And sometimes (as I mentioned) they aren't marked with a color. Anyway (to anyone happening to read this) I'm not sure I'd get too excited. The value is probably the important thing. But maybe the tolerances are a possible factor in differences between Marshalls. Say, if the 500pF weren't really considered significantly different, but they used looser tolerance ceramics (compared to the 1 or 2% mica or 5% class 1), and maybe that made a slight difference.

(Off the top of my head) I can recall some instances where people claimed ceramics sounded better or were preferred. "Lord Valve" working on somebody's Fender (switched out mica replacements for original ceramic), John Kelley Brown (who used to post here a lot and did a lot of parts swapping experimentation in Marshall circuits) seemed to prefer a cheap ceramic over the mix R, the Roccaforte amps guy seemed to like them, also in a recent thread on a Bogner with an RF problem, the chassis seems to show a cheap ceramic over a series R (micas in other positions). However, I can also recall people recommending to change to micas, so I guess there is maybe no awesome ultimate capacitor. Not that 'sexy' talking about tolerances, lol.

[Chuck H]

Well your info about the vintage markings started me on my own venture to see just what's going on. As per your post above it seems nearly all the dog bone capacitors of the era were class 1 (black dot=NPO purple=N750). Almost none were marked with a poor temp coefficient. I also continued to look at vintage ceramic cap images on Marshall boards exclusively (and researching corresponding codes) and while most have no temp coefficient marking the ones that do are at least class 1 if not NPO. So much to their credit QC at Marshall seemed to be looking out for this.

Still no definitive on why some swear ceramics sound 'grainy' and micas sound 'smooth'.

Like your experience with those that prefer ceramics for some things, on the Marshall enthusiast forums those of a similar mind suggest "the cheap ones" as well as suggesting higher voltage ratings. No criteria for temp coefficient given but "the cheap ones" probably means a lesser rating in that regard most of the time. Which is interesting because that doesn't seem to be consistent with what Marshall selected for those capacitors.

EDIT: This is purely speculation and some of it may sound silly, but...

I think it's probable that the tonal difference between the ceramics and micas that Marshall used was probably small enough to ignore. But enthusiasts, who are all about magic parts and their nebulous properties think that ceramics sound grainy because, well, they're made of a grainy material and feel grainy in the fingers. Likewise, micas sound smooth because they're physically smooth and shiny. For non stock replacement ceramics cheap ones are suggested because it's assumed that they are more like older technologies and higher voltage rating are suggested because of the more similar size to the originals. Cheap 1kV ceramic discs look more like the originals so they must be more like the originals.