Marshall Super Lead build, sort of

[dai h.]

I remember years ago here when it was "ampage" it seemed not many people knew what caps were used (including myself of course). I realized years later after I bought some from Antique Electronic Supply that I had a couple (just a few values of 2n2, 2n7, and 4n7?). If memory serves they were sold without any pictures as part of some big sale (and pretty cheap). I think there might have been some 680nFs listed (did anybody buy these?). I remember testing those (and a bunch of other caps) in my Marshall (as coupling cap) for leakage and I think they fared well but I don't recall anything super special about them. A bit later when internet was starting to become more popular and available I remember trying to dig up info, links and brand names and such ("Iskra", "Lemco"--the micas (didn't seem clear initially what those were), the dog bones, etc. etc.) on the old Plexi Palace forum. Things became more clear over time with people sharing chassis pics, web pages with amps, and amps showing up on ebay, etc.

Years later, I suppose the mustards have the biggest presence in the popular imagination. I'm not entirely convinced of any 'magic' but guess it makes sense if you're trying to sell an amp. Interesting exercise to figure out what features will make it more sale-able. Mercury Magnetics transformers? Pink wire? Harwin turrets? Which way do you go? This Dutch place seems to have tons of cool surplus (mustards, Iskra Rs, Piher, etc.), but quite pricey for some of the more desirable stuff (100k Iskra, 22nF&680nF mustards, and so on) :

https://www.electronicparts-outlet.c...capacity-068uf

[Chuck H]

Well the amp in question goes for between two and three K (US) in good, original condition. Mine served duty in my gig days and has also been a constant mod platform. To the point where the board is damaged and repaired repeatedly. Now, at this point it hasn't been used in twenty five years and it's extensively modded. These amps in this condition go around fifteen hundred. So since I don't mind the work and I can do this rebuild for about five hundred I figure to get closer to three K for it sold when it's done. Win/win. Plus I get to hear something VERY close to an original for a brief time before it sells. That'll be fun I think.

All the coupling caps are mustards. The 680n caps will be Bianchi because those are so rare now but the rest are Mullards. The 47p on the PI is a dog bone. I went with the Valvestorm site for the turret board, F&T cans, Vishay 10uF bias and 330uF bass channel bypass cap, all the requisite beige carbon film resistors, some Welwin screen grid resistors, a set of CTS pots AND I did spring for some pink and purple lead wire Just because I had to have them for looks I sourced some Piher 2W resistors for the 56k's on the filter caps and the four 10k rail resistors on the board. I do happen to have a trio of good working Mullard 12AX7's and the power tubes that are in the amp from two and a half decades ago were recently changed and lightly used when I closeted it. A quad of black base double OO getter Teslas. We'll see how it sounds and then how it goes on the market. Maybe if I end up loving it I'll have to sell some of my blood and a kidney so I can keep it

[dai h.]

To the point where the board is damaged and repaired repeatedly.

ha. Mine are like that (actually used *two* for guinea pigs). Haven't worked on it in quite a while but I probably felt guilty and cleaned the flux off the bottom of the PCB about twice. (The treble cap is what seems to be a Japanese version of the rectangular English micas (I've seen rectangular ones here also IIRC).)

All the coupling caps are mustards. The 680n caps will be Bianchi because those are so rare now but the rest are Mullards. The 47p on the PI is a dog bone. I went with the Valvestorm site for the turret board, F&T cans, Vishay 10uF bias and 330uF bass channel bypass cap, all the requisite beige carbon film resistors, some Welwin screen grid resistors, a set of CTS pots AND I did spring for some pink and purple lead wire Just because I had to have them for looks I sourced some Piher 2W resistors for the 56k's on the filter caps and the four 10k rail resistors on the board. I do happen to have a trio of good working Mullard 12AX7's and the power tubes that are in the amp from two and a half decades ago were recently changed and lightly used when I closeted it. A quad of black base double OO getter Teslas. We'll see how it sounds and then how it goes on the market. Maybe if I end up loving it I'll have to sell some of my blood and a kidney so I can keep it

That sounds pretty cool. While I think the circuitry is more important, for some reason when I see a replica type thing that's been done really well it still has a certain appeal (maybe like when an art lover sees a Goya or whatever). But also, when I see something like that that has, say metal oxide for the V2a cathode R and 100k follower position R (I know if might be hard to find ones with leads long enough, but...). :-( (Actually on some of the old amps, they scooted the turret one position inwards, so it was easier to reach (I presume).)

[dai h.]

oh yeah, is this the "raw" sound (without the delay, reverb, and whatever else(EQ?)) on it (this is from about 2:51 in)?

[Chuck H]

That sounds pretty cool. While I think the circuitry is more important, for some reason when I see a replica type thing that's been done really well it still has a certain appeal (maybe like when an art lover sees a Goya or whatever).

Same here. I think the eye candy aspect shrinks as technical awareness grows. At any rate it's a confidence game. If I sell this thing to someone that "believes" in it for their own reasons they'll enjoy it more. At the very least I'll have some fun and I'm sure the effort will at least add the parts cost to it's value so I'm not likely to lose anything.

But also, when I see something like that that has, say metal oxide for the V2a cathode R and 100k follower position R (I know if might be hard to find ones with leads long enough, but...). :-( (Actually on some of the old amps, they scooted the turret one position inwards, so it was easier to reach (I presume).)

I've seen this too. I think Valvestorm sells some 'correct looking' resistors with for those positions with longer leads. I didn't spring for those. We'll see how it goes. I may have to move a couple of turrets or place an order for the longer resistors.

[Chuck H]

I was thinking on the matter of thermal coefficient as it relates to the treble circuit capacitor. The DC on the CF remains pretty stable. LTSpice doesn't always catch everything but it looks like a less than 10% change between min and max signal conduction levels. So if a lower thermal coefficient cap is used there DC bias shouldn't make for much dynamic change in capacitance but more of stable difference. Perhaps the guys that prefer a cheap ceramic cap for that position actually just prefer a lower capacitance since I think this would be the overall difference between, say, a mica cap and lower thermal coefficient ceramic. Maybe Helmholtz will chime in to clarify (or correct) this hypothesis.

EDIT: And maybe the description of ceramics sounding "grainy" and micas sounding "smooth" has more to do with how the amp sounds with different frequency profiles. Less upper mids with the lower thermal coefficient ceramic and more with the mica when comparable initial values are used. Rather than any actual behavioral difference or dynamic response from the capacitors.?.

[Helmholtz]

So if a lower thermal coefficient cap is used there DC bias shouldn't make for much dynamic change in capacitance but more of stable difference. Perhaps the guys that prefer a cheap ceramic cap for that position actually just prefer a lower capacitance since I think this would be the overall difference between, say, a mica cap and lower thermal coefficient ceramic.

Yes, I think a lower effective capacitance with a class 2 cap should be the main effect.
But I understand that capacitance depends on momentary voltage, so the C-value will be modulated by the signal voltage (between the ends of the cap) giving rise to THD.
BTW, the capacitance of class 2 caps also drops with frequency.

...a lower thermal coefficient..

I think you mean lower voltage coefficient here.

And maybe the description of ceramics sounding "grainy" and micas sounding "smooth" has more to do with how the amp sounds with different frequency profiles.

Maybe, but have you heard the graininess?
Why listen to opinions of others when you can try it yourself?

[Chuck H]

Maybe, but have you heard the graininess?
Why listen to opinions of others when you can try it yourself?

I've never heard graininess. It's just a word that I've heard a lot of people use to describe the tone of ceramic capacitors in guitar amps. And I absolutely will try it and see for myself but it will be awhile before I do. I don't even have a work space put together since a move. This Marshall rebuild will be the first project I've done in almost two years.

[Chuck H]

I think you mean lower voltage coefficient here.

Yes. I suppose. From what I've been reading this effect is analogous to the temp coefficient so I didn't differentiate them. But it's good to be clear about it.

But I understand that capacitance depends on momentary voltage, so the C-value will be modulated by the signal voltage (between the ends of the cap) giving rise to THD.
BTW, the capacitance of class 2 caps also drops with frequency.

This could be significant with a clipping signal. The spice shot below assumes a perfect capacitor. With frequency also affecting capacitance the differential shown would be greater I think. Modulating capacitance seems to be greatest on the upper swing in the HF which would probably create even more asymmetry in real world circumstances since lower value caps demonstrate a greater differential in the simulations. And this might well be an audible distortion (for better or worse depending on if it's a guitar amp or a radio receiver ).





EDIT / NOTE: I did insert a 10uF cap in front of the tone stack to isolate DC from the circuit and align the compared signals. Since Spice assumes a perfect capacitor DC bias affect is moot for the simulation anyway.

[dai h.]

found some interesting related threads:

Ceramic Cap Discussion from Jazz Mode Video Demo With Stratocaster and 102
https://ampgarage.com/forum/viewtopic.php?f=4&t=32126

Coupling Capacitors......An Experiment in Tone
https://www.freestompboxes.org/viewtopic.php?t=13079

Amp Capacitors, their differences and tonal affects?
https://www.tdpri.com/threads/amp-ca...ffects.166039/

[Helmholtz]

Yes. I suppose. From what I've been reading this effect is analogous to the temp coefficient so I didn't differentiate them. But it's good to be clear about it.

Temperature coefficient and voltage dependency are different things. In some cases even the sign is different.

The spice shot ...

The voltage difference between applied signal and passed signal is the voltage "drop" across the cap.
As cap impedance drops with frequency, higher frequencies cause less drop.

As a result the voltage drop is mainly fundamental frequency (+ some lower harmonics). Drop will increase at lower signal frequency.
For this reason cap THD increases at lower frequencies in a HP filter.

[Chuck H]

But I understand that capacitance depends on momentary voltage, so the C-value will be modulated by the signal voltage (between the ends of the cap)...

BTW, the capacitance of class 2 caps also drops with frequency.

I think I must have misinterpreted this.

[dai h.]

saw an interesting post re: Hi-K ceramics (post #130--also #129 is interesting as well) :

https://groupdiy.com/threads/electro...7#post-1066129


also this (preview only--you need to buy the mag to see the whole article) :

source: https://linearaudio.net/volumes/782

Audiophiles are often told that their new equipment, components or cables need to be 'broken in' to deliver their best performance. So far, technical backup of the phenomenon is (with the exception of mechanical changes in speaker drivers with use), non-existent. However, Doug Self found that there may be something to it. He found that the distortion in polyester capacitors drops significantly with use, and that the improvement is to a large extend, permanent.

Additionally, a purportedly improved version of the Duncan tone stack calculator is available (a downloadable version and browser operable version) :

https://whitecottage.org.uk/software...ck-calculator/

(browser based--click "Marshall", "Fender", etc. above to open) :

https://www.guitarscience.net/tsc/info.htm

[dai h.]

(this used to be up before but was removed, then someone changed it slightly and re-uploaded it again)

Apparently from 1985. I wonder if this is the 1984 (album) rig (Marshall through the fixed load and Bradshaw rig) but with more echo?

[dai h.]

someone gave a link to this Dutch site with old radio and tech info which apparently has some documents in English, and apparently you can search the site through Google by leaving the entry space blank and clicking "Zoeken".

https://www.nvhr.nl/frameset.htm?bib...p&ContentFrame

anyway, on a whim I searched for "Philips capacitors", and it brought up a document with a page on C296 (mustard) caps:

https://nvhrbiblio.nl/biblio/boek/Ph...dio%201960.pdf

(there's also a note to look at data sheet EP2029 for detailed information, and I googled but came up empty)