SpiderWars
Well-known member
Just got this '70 Marshall PA with an interesting mod...sort of. The amp is setup to be a Super Lead in channels 3/4 and a 2203 in channels 1/2. The output of each 'side' is summed right where the 470k/470k resistors are with the 470k to ground removed from the 2203 side since the other SL side is now a load. Pretty straightforward so far but the interesting part is how the Master Volume works. It is active only when plugged into the 2203 Input jack. When not plugged into the 2203 input the pot that is the Master Volume (Channel 1 Volume control) is lifted from ground by 2.5 M or something. Also note the solder joint (or not!) on the Treble wiper.
6.3vac transformer, bridge rectifier glued to chassis, and filter cap for the power supply...and the cap of death.
Left to right: Treble, Master Volume, 2203 Gain, SL Bright Volume. The Treble and Master each have what appear to be IC sockets upside down. The Treble one has a transistor tucked underneath, kind of visible in the pic below. The 15k resistor is connected to two legs of it. One leg goes to the 2203 Hi Input jack, the ground lug that gets switched when you plug in. So it's grounded when unplugged, lifted when plugged in. And that wire that connects here has what I think is a diode at the connection point (in white shrink wrap).
Here you can see the white/purple wire going to the input jack switched ground lug with something in white shrink wrap.
Hard to see but there is a transistor tucked in there.
The board and everything else is fairly intact with stock parts/values (which is a Super Bass tone stack). Both channels are lower gain than a straight up Super Lead. It seems to take pedals better tho.
And sadly this Master Volume switching does not work when variac'd to 90vac. I was wondering why the thing was so silent just sitting there idling then I realized it was in plexi mode with Master on 0. Usually plexi mode has no Master and you can hear the hiss.
6.3vac transformer, bridge rectifier glued to chassis, and filter cap for the power supply...and the cap of death.
Left to right: Treble, Master Volume, 2203 Gain, SL Bright Volume. The Treble and Master each have what appear to be IC sockets upside down. The Treble one has a transistor tucked underneath, kind of visible in the pic below. The 15k resistor is connected to two legs of it. One leg goes to the 2203 Hi Input jack, the ground lug that gets switched when you plug in. So it's grounded when unplugged, lifted when plugged in. And that wire that connects here has what I think is a diode at the connection point (in white shrink wrap).
Here you can see the white/purple wire going to the input jack switched ground lug with something in white shrink wrap.
Hard to see but there is a transistor tucked in there.
The board and everything else is fairly intact with stock parts/values (which is a Super Bass tone stack). Both channels are lower gain than a straight up Super Lead. It seems to take pedals better tho.
And sadly this Master Volume switching does not work when variac'd to 90vac. I was wondering why the thing was so silent just sitting there idling then I realized it was in plexi mode with Master on 0. Usually plexi mode has no Master and you can hear the hiss.
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