JCM 900 SLX 5881 conversion to EL34 question

Clockworkmike said:
Honestly its had a bit of a wonky history over the past 2 years or so. It had the C8 capacitor and Bridge Rectifier 3 begin to go out and was in the early stages of trying to burn into the board. Kept dropping out the volume when it was under about 6 on the Master Volume at random. Would have to turn off the standby and turn it back on to get it to work if it was under 6. Changed those and the filter caps, held up good for a while. Changed all the pots on the preamp section cause they were really scratchy and changed the tubes out about this same time last year. Cleared that up

But it went full caput when i had it cranked with an attenuator. Not the attenuators fault but i think it stressed the already shaky PI tube and resistors. As i was showing before, lots of resistors were wrong and i think honestly it was just a time bomb waiting to blow. Rarely use it that much so judging breakdowns between recently and last year? Probably didnt even notice it honestly.
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This all sounds like a nice Rabbit Hole you got yourself into. With all the molestation going on with this amp, ya gotta ask yourself: is there any cold solder joints in there? Lot of back-tracking and retesting of components is going to be a tedious affair, but maybe worthwhile. A project like this would drive me crazy, and who knows to what extreme I would go to, but this would be a cool Marshall to get running properly... :cool:
 
syscokid said:
This all sounds like a nice Rabbit Hole you got yourself into. With all the molestation going on with this amp, ya gotta ask yourself: is there any cold solder joints in there? Lot of back-tracking and retesting of components is going to be a tedious affair, but maybe worthwhile. A project like this would drive me crazy, and who knows to what extreme I would go to, but this would be a cool Marshall to get running properly... :cool:
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Honestly, none that i can see. But hey, its been getting a full check down (literally ever resistor and capacitor) so combing over it for any bad joints is A OK in my book and i will continue to hunt any down!

What's a shame, is it wasnt sold new me ( well it was on paper anyways, but a lying ass salesman nearby ripped me off. Bought new with papers in 1996 but its clearly a 1993 by its serial numbers and someone HAD been in here messing around). Ive only ever fixed the C8, BR3 and Filter Caps but its hard to say what was done prior to me owning it: would be easier to pin point my own mistakes if something was done improperly but its rough finding theirs lol

And thank you! For not just the compliment ( it IS a wicked amplifier when it runs right) but for your invaluable help! I truly appreciate all your time and input. I know it had to be irritating listen to me fumble around but it wasnt in vain
 
Welp, i changed everything we talked about to the right values ( phase inverter capacitors, the aforementioned bad resistors), adjusted voltage to 48VDC, changed the speaker cab jack plate to a new, much better built one but the cathode ohms still sets at roughly 13 ohms even after checking the wiring and traces closely.


Im stumped. Im calling the tech now and sending it off, 2hrs north of here. Probably end up spending more than its $!#*ing worth at this point. Selling it isnt an option cause its busted. If it were up and running, poop yeah id toss it by now. Sick of this
 
Clockworkmike said:
Welp, i changed everything we talked about to the right values ( phase inverter capacitors, the aforementioned bad resistors), adjusted voltage to 48VDC, changed the speaker cab jack plate to a new, much better built one but the cathode ohms still sets at roughly 13 ohms even after checking the wiring and traces closely.


Im stumped. Im calling the tech now and sending it off, 2hrs north of here. Probably end up spending more than its $!#*ing worth at this point. Selling it isnt an option cause its busted. If it were up and running, :poo: yeah id toss it by now. Sick of this
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that's what happens when you don't listen to your advisor Ramo he knows what's best for you. from now on you only should take advice from him.
 
Clockworkmike said:
Welp, i changed everything we talked about to the right values ( phase inverter capacitors, the aforementioned bad resistors), adjusted voltage to 48VDC, changed the speaker cab jack plate to a new, much better built one but the cathode ohms still sets at roughly 13 ohms even after checking the wiring and traces closely.


Im stumped. Im calling the tech now and sending it off, 2hrs north of here. Probably end up spending more than its $!#*ing worth at this point. Selling it isnt an option cause its busted. If it were up and running, :poo: yeah id toss it by now. Sick of this
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It's only 1 more thing.
Disconnect power from the amp set meter for ohms.
Take out the power tubes.
Test the wire between pin 8 of the output socket and the circuit board with the ohm meter.
What was the resistance of that wire?

Finding the bad spot isn't that difficult. And you don't need to give up because of 1 last problem.

You have not failed, just keep working.
 
Clockworkmike said:
Welp, i changed everything we talked about to the right values ( phase inverter capacitors, the aforementioned bad resistors), adjusted voltage to 48VDC, changed the speaker cab jack plate to a new, much better built one but the cathode ohms still sets at roughly 13 ohms even after checking the wiring and traces closely.


Im stumped. Im calling the tech now and sending it off, 2hrs north of here. Probably end up spending more than its $!#*ing worth at this point. Selling it isnt an option cause its busted. If it were up and running, :poo: yeah id toss it by now. Sick of this
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You changed to a new PI tube, correct?
 
Amp Mad Scientist said:
It's only 1 more thing.
Disconnect power from the amp set meter for ohms.
Take out the power tubes.
Test the wire between pin 8 of the output socket and the circuit board with the ohm meter.
What was the resistance of that wire?

Finding the bad spot isn't that difficult. And you don't need to give up because of 1 last problem.

You have not failed, just keep working.
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Apparently, it blew the damn R30 resistor again as well. The rest of the parts are good and reading right.

Yeah i put brand new ECC83s in all the preamp tubes actually, had a new set and went ahead with that.
 
Clockworkmike said:
Apparently, it blew the damn R30 resistor again as well. The rest of the parts are good and reading right.

Yeah i put brand new ECC83s in all the preamp tubes actually, had a new set and went ahead with that.
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You powered the amp up before it was fixed?
I told you not to do that.

You test and correct everything FIRST.
You put the tubes in and power it LAST.

Don't power the amp up until EVERYTHING is confirmed.

What is the reading of the wire between pin 8 of the output tube socket and the board?
 
Amp Mad Scientist said:
You powered the amp up before it was fixed?
I told you not to do that.

You test and correct everything FIRST.
You put the tubes in and power it LAST.

Don't power the amp up until EVERYTHING is confirmed.

What is the reading of the wire between pin 8 of the output tube socket and the board?
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Im a screw up, plain and simple. What else can i say? Apparently my kid threw the other 2 22k 1w metal oxide resistors i had in the trash and thats been taken out in the bin. So i get to wait around another week for a 5pk from TN.

Ill check the resistance of the wire in a little bit. I have some 18awg 105C Hook up wire around here. But the amp will still be down.

Oh well
 
This is big F'n bummer news, sorry.

Something following the beefy R30 resistor is sucking a big poop-load of current. I'm posting the schematic that Clockworkmike posted in the beginning of this thread. The blown R30 is highlighted in the yellowish color. Like AMS said... Go tubeless and check for voltages especially around the R30 area... Follow the Anode Trail! From the grounding of the filter caps to the PI tubes anode circuit. I lost track, but is this the amp that had a questionable power tube socket?

Inspect for broken wires, or frayed wire ends shorting out?

2500SLX-60-34 Marshall - Copy.gif
 
syscokid said:
This is big F'n bummer news, sorry.

Something following the beefy R30 resistor is sucking a big poop-load of current. I'm posting the schematic that Clockworkmike posted in the beginning of this thread. The blown R30 is highlighted in the yellowish color. Like AMS said... Go tubeless and check for voltages especially around the R30 area... Follow the Anode Trail! From the grounding of the filter caps to the PI tubes anode circuit. I lost track, but is this the amp that had a questionable power tube socket?

Inspect for broken wires, or frayed wire ends shorting out?

View attachment 60083
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Yes sir, this would be the one that smoked the output tube socket. Im gonna snoop around inside and follow the path like you said. Like @Amp Mad Scientist said as well, check pin 8s wiring and might just say to hell with it and replace it entirely back from the board to socket. I have some hook up wire here for that very thing. Pretty soon, i might end up just rebuilding it from the ground up at this rate lol
 
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