Adam Jones Marshall Superbass/Plexi

Hi Folks just wondering if someone could throw some tips for making some more advanced changes to the amp block. I'm trying to nail down Tool tones and a big part of that is a heavily modified Marshall.

Here's the info on the mods done

It's a 1959 late 70s superbass with jj el34s and chinese preamp tubes. It is super lead spec w/ 100k feedback on the 4 ohm tap it has a .68 cap for the presence and a .68 for v2 also the caps out of v1 are both .022s. The amp is biased to 25ma and he jumps the channels with a cable. It still has a superbass tonestack and a shared cathode - Dave Freedman

If anyone can make sense of this in terms of what do do in the fractals plexi models I would greatly appreciate it.

Cheers
 
I’ll start off by saying I don’t know much about vintage Marshall’s. I googled superlead schematic and found this pdf with three different versions-
https://www.thetubestore.com/lib/th...arshall-JMP-Superlead-100W-1959-Schematic.pdf

w/ 100k feedback on the 4 ohm tap

Each of the three schematics is different in this regard. I can’t really read what’s going on in the first one, looks like 27k off the 8 ohm tap? This is a lot of negative feedback.

Second is either 47k off 4 ohm tap (for 100 watt amp) or 100k off 4 ohm tap (50 watt amp), notice part value in parenthesis. If adams amp is 100 watts, then it has half as much negative feedback as this second schematic.

Third schematic is the same as the second but off the 8 ohm tap which will increase negative feedback. Also may change the sound as well, as I have read people say online that equivalent level of feedback off different taps sounded different (but I’ve never experimented with that personally).

How does this compare to the amps modeled in the axe FX? I don’t know, Cliff would have to say how his amp(s) have the negative feedback set up, and how that corresponds to the setting in the axe FX. You’ll probably want to turn it down and experiment with what sounds best.

it has a .68 cap for the presence

Presume 0.68uf. Second schematic might be the same as this, hard to read but I see ‘68’ next to the presence cap. But can’t tell what symbol is after it. Maybe a ‘K’ which I don’t think would make sense here? Or an ‘N’ which would denote nanofarad (0.068uf).

The other two appear to state 0.1uf. If we assume the axe FX model is based on 0.1uf, I believe the 0.68 would shift the cutoff frequency down so presence would boost more in the upper mids. Try reducing presence frequency (I think it’s called).

a .68 for v2

First two schematics, no cathode bypass cap on v2 gain stage.

third schematic does have 0.68uf bypass cap on 820R (or 1k) cathode resistor. This will boost gain for that gain stage and the frequency is dependent on the resistor and cap value.

see cathode bypass capacitor calculator here - https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

here is what .068 / 820R looks like. 1k resistor very little difference.

Screenshot 2023-07-15 at 09.47.56.png

If the axe FX model is like the last schematic, no change necessary. If its like the first or second schematics, you'll want to bump the gain up a bit (or input trim) though input trim will be 'full frequencied'. Maybe put an eq in front of the model with a slight tilt EQ, centered somewhere around 150hz to 200hz. gain to taste. This will be at the start of the preamp rather than the last gain stage but I dont know of any advanced controls that approximate cathode bypass cap values.

the caps out of v1 are both .022s

first schematic shows 0.022uf coupling caps for both inputs / both sides of V1.

second schematic shows .022uf for one input and .0022uf for the other input (presumably this is a 'bright' or 'treble' input). This will greatly cut bass after the first gain stage, probably turn down preamp low cut. See coupling cap calculator for how drastic of a low cut a .0022uf cap is - https://www.ampbooks.com/mobile/amplifier-calculators/coupling-capacitor/

The amp is biased to 25ma

this isn't really meaningful without knowing plate voltage. This could be a hot bias if the plate voltage is high, or a cold bias if the plate voltage is low. Someone who knows more about superleads maybe can chime in on this.

It still has a superbass tonestack

I cant find a specific 1959 superbass schematic but after looking at a few other superbass schematics, they all have a 56k slope and 250pf treble cap, which is the same as the first schematic above.

second and third appear to have 33k / 500pf which is standard marshall as far as I know. if the axe FX models are the usual 33k / 500pf (or 470pf), then switching to the JTM45 tonestack should work as that is 56k / 250p. Other caps and pots look the same. I dont know if any other parts are changed when switching tonestack in the axe FX though, or if its literally just the tonestack.

and a shared cathode - Dave Freedman

the first schematic shows both halves of V1 with a shared cathode resistor and capacitor, the cathodes are both connected together then go through 820R resistor bypassed by 250uf capacitor, to ground. like above, the capacitor provides a gain boost and this large of a cap would be full-frequencied. I believe the old bassman's have the same setup for V1, shared cathode, under 1k cathode resistor and very large bypass cap.

second and third schematics dont have a shared cathode. (what I assume is the) bass channel has the 820R / 250uf cathode, but treble channel has 2.7k / .68uf, which is a colder bias and the gain boost will not boost lows as much. If the axe FX model is like the second and third schematics, maybe turning down preamp low cut, and setting the preamp bias warmer would help.



Overall, if you're building or modding your own amp, Dave's comments tell you exactly what to change, but we dont get to do schematic-level tweaking in the axe FX for the most part. We probably have advanced controls that affect most or all of the above, however I dont know which ones, or how they correspond to specific part value changes. But hopefully this gives you a starting point for things to adjust. Maybe you'll get the vibe right after tweaking the above mentioned parameters.
 
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