Snubber Caps Question

[denrrou]

I have a 50w 1971 jmp lead that i am replacing filter caps and bias caps and i also noted it has these 2 caps of the attached pic. I was curious if would be better to replace them as well and did some research.

Have read that it would be better to simple get rid of them as if it was a marshall mistake that caused lots of PT to blow. I have indeed seen many circuits that lacks them and now i am in doubt...

In the past this amp had indeed a know mistake corrected, it had to do with the standby voltage going to the power tubes, my tech explained me marshall wired it incorrectly and he fixed it by just repositioning one red wire, and i have read it is a well know error. But he said nothing about these 2 caps.

They seems to be original since one leg has the red soldering paint, but i dont understand why someone in the past cutted one end and jointed them together as shown in the pic, except for this rest of the board seems completely stock an untouched.


Now i wonder: as a recap job should i replace them with the filter caps and bias caps, leave them alone or get rid of them?

 

[Pete Farrington]

They’ve had a lot of voltage on them for many decades, if keeping the snubber cap thing it may be a good idea to replace them.
I wonder what the voltage rating of that FWB package is?

 

[StingRay85]

This amp needs some serious cleaning and TLC. Looks like something already burned out in there, or is this all corrosion?

 

[denrrou]

You may be getting a purported snubber cap issue mixed up with that stock awful standby arrangement in many late 60s / 70s 50 watters? In which the bias supply is deenergised in standby, and needs a wire moving to fix it.
The amp in the photo looks to still be stock. The output valves would be very grateful for it to be modded. Until that’s done it would be best not to use standby.

The amp is ok, i will do the recap because the amp tech said the Plate Voltage is lowering along the time, what is an indicative of the filter caps going bad according to him. He keeps track of the value of each retube / bias and the last time he said me it would probably be better to replace the daly caps with the bias caps. Thats why i will do the recap now.

The standby wirring error was correct many years ago.

I was just wondering if i shold replace the Snubber Caps or even it they could also be a mistake as i read.

 

[denrrou]

This amp needs some serious cleaning and TLC. Looks like something already burned out in there, or is this all corrosion?

Its corrossion, i have seem it on some other marshalls as well. at some extend it looks like the red painting marshall uses on the soldering, but dont really know whats the cause. All transformers are stock. The amp souds great.

 

[denrrou]

They’ve had a lot of voltage on them for many decades, if keeping the snubber cap thing it may be a good idea to replace them.
I wonder what the voltage rating of that FWB package is?

Any ideas on why would the last owner had cutted one and and resoldered them like that? thats the only thing modified in the board.
How can i check the rectifier voltage? the Snubber Caps are .01uf 1000vdc

 

[Pete Farrington]

They perhaps are replacements, stock value looks to be 0.22uF https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_lead_50w_1987.pdf

 

[denrrou]

They perhaps are replacements, stock value looks to be 0.22uF https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_lead_50w_1987.pdf

perhaps, but they dont look like replacement, since only one end is unsoldered, the other end keeps the red paint just like the rest of the board.
Probably marshall used more than one value along the year, as with other components, thats what i found searching for the values:

"If each diode has its own cap, a .01 to .022 is enough. If altogether two caps are running from the AC sides of the bridge rectifier to ground, then the min. value should be .047 to .1 - or better, as Marshall did it in the past - a value of .22"

 

[neikeel]

Those caps are not the originals, they are 600v rated (by size at least).
The originals are usually also Dubilier but 1000v rated (0.22uF). Some of that era 100w have 1500v rated 0.25uF.
The reason that PTs blow was because of the bias supply not typically those caps. The diode block is slightly borderline for PIV on these as only two diodes of the 4 are in circuit (unlike the full bridge of the 100w).
Someone has already rewired your bias feed with that chunky red wire that goes between one half of the ac HT feed and the bias feed dropping resistor. Normally it is a standard gauge white wire under the board and the turret is fed from below (ie the rewire was a bit slapdash).

 

[denrrou]

Those caps are not the originals, they are 600v rated (by size at least).
The originals are usually also Dubilier but 1000v rated (0.22uF). Some of that era 100w have 1500v rated 0.25uF.
The reason that PTs blow was because of the bias supply not typically those caps. The diode block is slightly borderline for PIV on these as only two diodes of the 4 are in circuit (unlike the full bridge of the 100w).
Someone has already rewired your bias feed with that chunky red wire that goes between one half of the ac HT feed and the bias feed dropping resistor. Normally it is a standard gauge white wire under the board and the turret is fed from below (ie the rewire was a bit slapdash).

they are not 600v, they both are ".01uf 1000VDC WKG" both Dubilier. Original red paints on one side, just the opposite side was unsoldered for some reason.

here is where i read about them as a possible cause of blowing PT http://forum.metropoulos.net/viewtopic.php?t=38191

The tech that that rewired that chunky red wire said it was the solution for the bias standby error, that red wire is the original one, he did not added or removed anything, he just removed it from the point it was originally soldered and resoldered it just like its now. I think he thought it would be the more non invasive way of doing.

Is there any problem the way its currently setted? do you have any suggestion?
thanks for the reply

 

[neikeel]

Ok.
But the snubbers are not the originals (as per prev post) you can see the ghost of the originals. Early amps (pre 70 used 0.05uF 1000v so 0.02 is not too off the mark.
I remember the Metro thread.
You don’t have to believe me but that red wire is not the original (FWIW).
Suggest you do the bias caps too.
Good luck

 

[denrrou]

Ok.
But the snubbers are not the originals (as per prev post) you can see the ghost of the originals. Early amps (pre 70 used 0.05uF 1000v so 0.02 is not too off the mark.
I remember the Metro thread.
You don’t have to believe me but that red wire is not the original (FWIW).
Suggest you do the bias caps too.
Good luck

Regarding the red wire indeed i dont remember exactly if it was just a reposition of one original existing wire or some extra wire he putted, he did it in front of me, but it was years ago. I just remember it was something very simple. But i think its indeed some extra wire he added, but if its correctly done that does not matter at all to me, nor if the caps are original or not, i am actually willing to replace it with the bias caps and filter caps if that is also recommended, based on some readings i just thought they might be an error as well since lots of same circuits dont have them and i was wondering if i should replace or get rid of them.

Even though ist not my concern, I still think the caps might still be the original ones because its clear that for some reason someone just cutted one side leaving the opposite untouched still with the red paints just link the rest of all the original components on the board. The ghosts you might be referring is probably due to the fact that before being cutted the legs on both sides were not crooked down and therefore the body of the caps were and upper in the board with its legs straight. The rest of the cutted legs are still hanging on the other side. Does not make sense someone trying to fake the marshall red painting while leaving the soldering like that clearly screaming someone messed with it. This amp also uses an original 1k iskra resistor in the place where the stock diagram asks for a 820 ohm, and there are other small
deviations as well. Seems more probably .01uf instead of .22 Dubilier is what they had at hand.

 

[denrrou]

or maybe thew blow and and all this dirty around is the guts of the blown ones

 

[Matthews Guitars]

The corrosion is likely due to a vented capacitor that may or may not still be in the amp. Or it could be an indicator of a burned up transformer. Either can vent corrosive chemicals into the chassis. The corrosion pattern is going to be centered around whatever blew up.

 

[denrrou]

The corrosion is likely due to a vented capacitor that may or may not still be in the amp. Or it could be an indicator of a burned up transformer. Either can vent corrosive chemicals into the chassis. The corrosion pattern is going to be centered around whatever blew up.

i was thinking about it and looking more closely... starting to think the snubbers caps are really replaced ones and the original ones made all that mess.Theres a little bit in the opposite side of the chassis right in front of them and the way the board is dirty right in front of them indicates the source probably started from there since its above the chassis, i think it would be difficult for it to climb above the board from the chassis. The snubbers caps indeed seem newer than the rest of the amp Filter caps are the old daly ones and the PT is drake. I have no idea if these Dubilier caps are capable of this mess when they go bad...

 

[john l]

Ok.
But the snubbers are not the originals (as per prev post) you can see the ghost of the originals. Early amps (pre 70 used 0.05uF 1000v so 0.02 is not too off the mark.
I remember the Metro thread.
You don’t have to believe me but that red wire is not the original (FWIW).
Suggest you do the bias caps too.
Good luck

So Neil Im curious. What do believe happens if one of those caps shorts? Im in no way shape or form being snarky. I believe you to know vastly more than I do and listen when you weigh in. To me the claim on metro computes.

 

[denrrou]

I was searching mouser for .22 1000v caps, but there are different types. Not sure what to choose.

What would be the best current production replacement caps for the Snubber?

 

[neikeel]

They do not have to be polarised (RS sell some big yellow jobs). You do not have to use them for your amp to work properly (as per other threads) but you can use if you find the correct ones. If you do not you may get some pops from the amp when you switch off.
The main point I was making is that the PT problem was predominantly on 50w (not 100w) which use the same snubbers. Marshall (to my knowledge) never acknowledged the bias feed issue just changed the arrangement mid 70s.

 

[ELS]

if you can, I suggest replacing them with ceramic disc caps, ceramic caps practically never fail, and in the power supply it doesnt really matter if they're microphonic, wont change the tone of the hum
they're also more effective at surpressing high frequency transients

the voltage rating should be at least 500V, if you decide to change it to a single cap, then double that voltage rating.

and if the caps are connected to the B+ rail instead of ground, you need 1000V rated caps anyway.

 

[neikeel]

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