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http://www.thetubestore.com/lib/thetubestore/schematics/Mesa Boogie/Mesa-Boogie-Stiletto-Stage2-Schematic.pdf
and a link to Marshall's 2204 schematic / circuit diagram.
http://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JMP-Mastervol-50W-2204U-Schematic.pdf
I'll wait to find if there is indeed a cunning reason though, apart from nailing the frequency response...
(but I suspect they are fairly wide tolerance components anyway?)
And there are a couple of treble peaking cap/resistor combos just on that first page... this schematic is gold, thanks so much!
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"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Funnily enough, when I turned it on it initially fizzled a bit (I assume this is what rectifiers sound like while they're blowing) but then sounded ok and behaved almost as normal, except presumably on 25w of power so I could tell something wasn't *quite* right, walked over and smelled the smell of the screen resistor. Turned it off straight away, probably only on for 30 seconds all in, but it had melted its solder and fallen right out of the pcb, a charred cylinder.
Aside from the Valve rec (is it safe to run without one if I don't use that mode?) and said fried resistor, is there anything else that's likely to be fucked? Will the EL34 have blown? Not sure what pin corresponds to what...
I'm such an idiot.
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I think the OT should be OK because the rectifier valve doesn't have pin 3, and the bias supply should be OK because it doesn't have pin 5, so it can't have shorted those.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
And just for funsies, the bombsite;
Quite the scorch marks. According to the schematic there are two "5352" diodes connected in series between the B+ and the blown resistor. Think they're underneath the board...
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EL34:
Pin 1 - suppressor grid, pins 2 & 7 - filament, pin 3 - plate, pin 4 - screen grid, pin 5 - control grid, pin 6 - no connection, pin 8 - cathode.
Rectifier:
Pins 2 and 8 - filament/cathode, pins 4 and 6 - plates. Pin 1 is usually present on the valve but not connected, 3, 5 and 7 are not present.
So… EL34 in rectifier socket - didn't power up, so all it had to do was withstand a high voltage from the screen to the (cold) cathode, which it should have been able to.
But - rectifier in EL34 socket… because the filament is centre-tapped to ground, and the cathode is grounded in a fixed-bias amp, there's 3V between pin 2 and pin 8, which is enough to partially power up the rectifier. It would then conduct directly from pin 4 to pin 8, which blew the screen resistor. If you're lucky, the rectifier valve itself might have survived because the resistor would limit the current, but I wouldn't count on it.
The amp should still work with no rectifier valve if it's set to solid-state, but obviously you'll have to replace the screen resistor first - and it's also quite likely the Zener diodes are blown.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Is there any reason they need to be on the underside of the board? Would it matter if I soldered replacements in on the easy side?
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Does your multimeter have a beep continuity tester so you can work out where the traces go on the underside, even if you can't see them? You'll need to do that to be sure you orient the replacements correctly, if you can't see how they're fitted now.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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Whether the resistor died before the diodes depends on the current draw rather than the power rating, but I do think there's a good chance they've survived.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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And it definitely sounds better when the valves are all in the correct sockets...
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