Oddball Late 70’s Super Bass Circuit Info

A while back I picked up a Super Bass from ’79 and upon first glance I thought this thing had a ton of strange mods done to it until I did some research and found other reference photos from the same year and they all had identical parts and layout. I saw the split cathode right off the bat but things get stranger the more you look.

Here are the key differences that make it unique to any other Super Bass circuit I’ve seen:

Plate driven tone stack
Split cathode at V1
Unbypassed 820R resistor on the normal channel V1B cathode
2.7K / 47n cathode for V1A for the bright channel
22n phase inverter coupling caps
V2A is unused!!!
220K tone slope resistor
No treble peaker for bright channel
510K mixer resistors
Completely different tonestack
Only two 10K droppers for the entire B+ rail — The droppers on this one specifically were 22K/15K resistors paralleled together. None of the other references I found had them this way so I assume they were short on parts and wanted to get these out the door. They were Pihers so I'm sure it was done at the factory. I did replace them with an NOS 10K Piher from my stash.

Here's a snapshot of the oddball preamp circuit to compare with the traditional Super Bass preamp.

Stock Marshall Super Bass preamp circuit — Stock Super Bass preamp circuit.

Oddball 1979 Marshall Super Bass preamp circuit — Oddball ’79 Super Bass preamp circuit.

The photo below is before I did any work to it.

1979 Marshall Super Bass main PCB, as found, before any work — The ’79 Super Bass board as found, before any work.

Part of me wants to keep it this way since there’s not a lot of these floating around... but alas, they do not sound great... Usable, sure, but the EQ is odd, it sounds sterile when clean and doesn’t overdrive enough when pushed. I’m not sure what they were going for with this design. As much as I consider myself to be a curator of history, this one was converted to the traditional Super Bass circuit.

Since there’s barely any info on these I’ve found I have drawn up the full schematic and provided it here. This way I’ve preserved the circuit for historical curiosity and if anyone should choose to recreate this lackluster circuit, here ya go.

The full high resolution PDF schematic can be downloaded here

Marshall 1992 Super Bass schematic, 1979 oddball circuit, drawn by Purgasound (Rev 002) — Marshall 1992 Super Bass schematic — the '79 oddball circuit, drawn by Purgasound (Rev 002).

If you want to see the mid 70’s Super Bass spec of what this amp was converted to, check out the info here along with a schematic.