I think the biasing is to make 'em sag a bit more. stuff often sounds vastly different at stage volume. ymmv, and should, of course.īut i'm working in the real world with these things, not playing in my room or at my bench. almost any dirt pedal will sound better with a 741 or 1458 than any jfet dirt circuit. there's really a world of tonal difference. The 1458's don't have that broken glass thing, and don't waste power producing loud hiss a couple octaves higher than the guitar signal resides.įunny, many of my friends tend to agree on this. Yeah, they may mimic a tube's amplification curves somewhat, when CLEAN, when they clip? they clip super hard and ugly. i tend to find that tho i DO like mosfets in distortion, i totally despise jfets unless they're clean. The mojo-istic belief that them jfets sound better is just more internet hubris. better warmer distortion, no more broken glass or hiss. i replaced them with the proper chips, and the amp came to life. if i had to mic it outdoors, the sound guy and audience would bitch. this thing was so damn noisy tho i had to. it sounded good clean, but if cranked was so damn hissy it was horrible, brittle, and unuseable. One of my 5212's that had come in had been modified. maybe good for some things, but in this circuit, all that means is an assload of hiss. and they have just plain too much high end response. they don't clip gracefully, they tend to sound like broken glass. On a side note, i'm not getting notifications on forum posts all the sudden, really weird!! gonna probably take a while, tho, cuz i really suck at doing this stuff. I've begun drawing it up, and taking notes. if they can't help, i'll bite the bullet. I'm gonna shoot marshall an email and beg for the schematic, i'd rather at least try that than trace it on my own. most of it is VERY similar, in term of layout, but some of the part values are different. it puts all the parts numbering off a couple numbers. I think the main dif between the posted common schematic is everything is off a little bit from the way the input is set up. seriously, if ya look at the schematics, they're almost identical! for all intents, the actual amplifiers are all but virtually the same.Ĥr, 30 watts. The 50 watters do use a bigger transformer, but its the same one in the 12-20-30 watt models, believe it or not.
#TRANSISTOR SCHEMATIC PC#
Hey bro, yeah i can grok the pc thing, i just had to get a new one recently and am hating life lol
The back panel on the amp is marked 72VA but you One difference is your PCB is marked JM78-2 whereas the schematic implies the PCB is JMP27A.įWIW, the 50W models have a different transformer (larger, higher VA) and probably a higher voltage. If you take that schematic you can kind of fumble through the PCB. The other parts around IC1 match-up with the schematic. Next you have the 2圆8ks on the PCB so some small change there. R1 is 220k on the schematic and 1M on the PCB. The lower half of the board is the preamp:
You can also see C21 on PCB (100uF 25V) doesn't The designators of the 1k5's not match up with the schematic. The top half of the board is the power amp you can see the 10 ohm, 39 ohm and 1k5 ohm resistor. You can see it sort of matches this part of the circuit, So start with Solid-state, 30W, no reverb, which is this schematic, in some cases the shape of the circuit matches the schematic but the part values are different.Īt least two people have put-up revised schematics for the 12W versions.
#TRANSISTOR SCHEMATIC MODS#
when the schematic doesn't match you will find minor mods to the circuit the parts designators on the PCB don' t match-up with the schematic! Find the schematic which matches the board as close a possible.
I don't know what the hell goes on with their schematics but here's the general way forward using schematics and boards from that era. What I can tell you is the Marshall schematics you find on-line are 'sort of' correct. Quote from: Rob Strand on March 05, 2022, 10:45:19 PM I'm in the process of shifting PC's ATM so I can't do any tracing.Īs far as I can tell, your board *is* a 30W unit.
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