Forming filter caps

[neikeel]

Oke, thanks but I think I don't get it sorry. I thought the resistor has to be inserted after de rectifier diodes and not before if I read Larry's instructions correctly....my situation is that 2 red wires from the PT go to one site of standby switch . From the other side of the standby switch I see 2 blue wires go to the rectifier diodes. After the rectifier is the situation that I mentioned before. There is also one white wire that goes to the bias section. Where do I have to put my 100 K resistor now? Across which terminals of the standby switch should I solder my resistor? Thanks

If your amp is a later Marshall the standby is NOT the place for the resistor (the two ac feeds go to the standby and the ac then goes from the other two poles to the diodes). Early amps (and Georges clones switch rectified DC).

Insert the 100k after the diodes between red wire and the fuse holder tip, ideally solder and then follow instructions over probably only 3 or 4 hours with new F&Ts.

DON'T forget to bleed the caps before deoldering and reconnecting!!!

[bahady30]

Thanks man, that's what I want to hear. I appreciate this! Yes I will drain them . It's the first thing I always do when I'm gonna mess with my amp. Is it more safer to drain them through a resistor to ground to avoid sparks?

[danman]

Thanks for catching that Neikeel! I should have checked his schematic before assuming the power supply was the same used in the older amps.

[bahady30]

Thanks guys for clearing this up. Last question: Do I have to attach a speaker or dummy load when forming these caps? Thanks

[neikeel]

No - you have no tubes in, so no need (make sure the yellow wire is down stream of the resistor or your mains cans will not reform

[bahady30]

Thanks for the help! I soldered the resistor in and fired the amp up. After 45 min I measured 3,3 VDC across the resistor so it's doing well I think. I let it sit for a while maybe the voltage will drop another 1V or so. Then drain the new caps, remove the resistor and put the new tubes back in. Maybe I hear some difference but that's not that important because the old Daly's were still good. I changed them because I want a reliable amp when gigging. I let you know how it sounds.

[neikeel]

Thanks for the help! I soldered the resistor in and fired the amp up. After 45 min I measured 3,3 VDC across the resistor so it's doing well I think. I let it sit for a while maybe the voltage will drop another 1V or so. Then drain the new caps, remove the resistor and put the new tubes back in. Maybe I hear some difference but that's not that important because the old Daly's were still good. I changed them because I want a reliable amp when gigging. I let you know how it sounds.

[bahady30]

Yesterday I had a gig in a large pub and I wanted to test the JMP at decent volume. After the cap job, new pre and output tubes, adding screen grid resistors , cleaning all of the electronics, I noticed the real potential of this amp. There is no hum or his or popping anymore, she is punchier, and has one of the fattest overdrive I heard from a combo. She's freakin loud, I couldn't get the master above 3 with our punk rock band and we are playing loud. Maybe she's a bit to bright because I couldn't turn up the treble above 1...I'm gonna experiment with the bright cap or clip it. Thank you all for your advice!!

[Amp Healer]

Yesterday I had a gig in a large pub and I wanted to test the JMP at decent volume. After the cap job, new pre and output tubes, adding screen grid resistors , cleaning all of the electronics, I noticed the real potential of this amp. There is no hum or his or popping anymore, she is punchier, and has one of the fattest overdrive I heard from a combo. She's freakin loud, I couldn't get the master above 3 with our punk rock band and we are playing loud. Maybe she's a bit to bright because I couldn't turn up the treble above 1...I'm gonna experiment with the bright cap or clip it. Thank you all for your advice!!

Well don't know that amp u have ?
Saying JMP is saying I have a Volvo and not the model
But I guess u have a 2203 as u mention a master .
Install a cut kontroll an promise u will like it

[bahady30]

It's a Marshall JMP 2104 from 1980 .

[danman]

Try a couple of RFT ECC83's in it first. If they don't knock enough treble off then you can experiment with bright cap and treble peaker circuit.

[Zentner]

Hi all, I'm trying this method to reform some old Erie caps I got. The amp works fine with the f&ts that came in it, but old caps are cool yada yada. Put a set of eries in and installed the 100k across the standby and the lowest it would go is about 37v. Tried another set and the same thing happened. I then put the f&ts back in and they did the same thing, so I think I have to change the circuit slightly to reform. I've outlined in the last two pictures where the red wire comes off the can to the turret board. I also tried this with a 50k resistor and the voltage did drop lower to about 20v, I expect it would continue with a lower value resistor. Any ideas as to what I can do?

[coldengray]

I have scored a huge stash of old 50-50 and 8uf cap cans, among many other values. How can I reform them out of an amp? I do have a spare 50W PT if that will be useful. Also have a variac.

[novosibir]

You don't need a variac.
Take four diodes 1N4007 or UF4007 and build a rectifier of it and hook up your electrolytics all in parallel, check for their rated voltages, so that they won't be exceeded. Swap in a resistor between rectifier positive and electrolytics positive. A 100K is ok for a total capacitance of 150...250uF
If your total capacitance is bigger, then go smaller with the resistor's value accordingly.

And yes - don't touch anything during the forming process, close the room and keep kids & cats outside the room

Larry

[coldengray]

Thanks Larry

I found this elsewhere on the forum, does this match what you're saying? I'm doing this outside of an amp so what's the best way to apply AC voltage?