Mojotone British 100w Lead/Bass

H

hankspencer

Member
I am getting ready to build a mojotone 100w British head and I have been trying to decide between building the super lead or the super bass version. Wondering if there is a reliable way to mod the design so that I can switch between the two? Has anyone done this or something similar that can offer suggestions?
 
From what I see from the schematics, there would be too many changes to do it with a single mechanical switch. You would have to build a series of relays to do the changes, and I think there would be some pretty nasty pop even with smart implementation.

Probably smarter to try out both (Mojotone are PTP so it's easy enough), write down what you like most about both, settle for one base and maybe have the most relevant change for you on a swith.
 
Just build Lead version if you’re planning to use modern speakers. I just built an amp with Bass versions preamp section and ended up changing it to resemble Lead, ’cos bass content was too superwhelming with fuzz through 15” speakers.
 
I would definitely build the lead version. The normal channel will sound very close to the bass version. It's a bit weird in the diagram above that it shows a 500pF cap in the tone stack with a 56K slope resistor. Usually the tone stack with a 56K slope resistor would have a 250pF treble cap (the values shown on a 5F6-A schematic), and a tone stack with a 500pF treble cap would use a 33K slope resistor. The 500pF/33K version was reputedly built for Jimi Hendrix to beef up his Strat. I always use that tone stack in a Marshall.

One change you can make to the lead version is the cathode cap and resistor on the normal channel. The Super Bass shared the cathodes of V1, the Lead amp split them. The Bass amp used 820R/330µF as it was copied from a Bassman. The Bassman actually specced 820r/250µF but for whatever reason Marshall used 330 - HUGE! And when they split the cathodes they should have used a 1K5 on the normal channel if they wanted to keep the gain the same. 820 is fine when the cathodes share it but as soon as you split the cathodes you need to double the value of the resistor to keep the gain the same.

The Bright channel used a 2K7 cathode resistor to up the gain. The 2k7/.68µF cap combo is a Marshall Lead channel standard. I would stick with that. But for a usable Normal channel I normally use a 1K5 cathode resistor and anywhere between 1µF and 25µF. 25µF is the standard Fender value for a preamp tube cathode in V1. Try a few and see which one you like.

Another area I mod is the volume pots. I rarely use a bright cap on the volume pots but do use the bright cap on the mixer resistors. The 470p/470K values for the bright channel are a key part of the Marshall sound.
 
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