Marshall Super Lead 100 MKII

[CJ]

all done rebuilding the chassis,

testing it out is the fun part, but not for the neighbors,  ;D

that impedance switch for the speakers seems to be a sore spot on these early amps, this one had the switch hard wired for 16 ohms, which may be why the OPT may be acting weird, i bet if went open during a heavy jam session, so the guy soldered it so you can not pull out the selector part,

a new one is 17 bucks, so we just wired a few xtra spk jacks where somebody had drilled holes in the back of the chassis, they probably had a master vol i there,

if you lose connection to the speakers under full load, the OPT can pop,

there might be some issues with this OPT after all, the amp started squealing a bit after about 10 minutes of Blackmore riffs,

i sure hope we don't have to take this thing apart,  ;D

we had issues with an OPT acting up after warmup, very strange problem, probably caused by heating of some shorted turns or some other problem,

we took out a few miles of Mains wiring, since this will be using 120VAC, there was no reason to have the connections made at the jumper section of the PC board, we left the other taps there just in case,

 

[CJ]

since the diodes got cooked at one time in this amp, we put a 10 ohm surge resistor in series with the B+ line, there is resistance in the pwr xfmr sec winds, but why not add a little more, plus it might simulate a bit of rectifier sag which can add a bit of compression to the sound,

something interesting on these Marshall amps is the center tap of the sec being wired to the center of the series wired filter caps,

this is probably to keep the voltage across each cap equal, something which is done on Fender amps using a pair of 220K 1 watt resistors as a voltage divider,

here are the OPT color codes for this amp, strange that black is not ground,

those English folks like poetry, 

maybe the say "brown is ground" and "green is sixteen",  or maybe not, 

OPT secondary:
BROWN: Earth(ground)
BLACK: 4 ohms
YELLOW: 8 ohms
GREEN: 16 ohms

 

[CJ]

this amp has a problem with a sensitive grid wire on the bright channel,

since we have the chassis already soldered back together,  ripping it apart to put in a a chunk of coax would not be fun,


so we borrowed a trick from a Fender chassis which entails making a normal piece of grid wire into a shielded cable, only Fender did it to a bunch of B+ wires, probaly to keep them from radiating energy into sensitive wires,

this is done by un-soldering the green grid wire at the tube socket and wrapping it with some #26 magnet wire which has a nice coat of poly insulation,

this wire is then grounded thru the cathode of the 12AX7a,

it works great, you can stick your finger right on the grid wire and no hum is induced, the squealing at hi-volume is also gone,

here is what it looks like>