Marshall class 5 crackling issue

[Iron Broadsword]

And it ain't the tubes.. It is modded with the 'orpheus' attenuator, just a single low ohms pot in series with the headphone out that when engaged (true-bypassed otherwise) uses the headphone amp to drive the speaker. This can be used in full power mode as well. Here's a link: http://www.marshallforum.com/marshall-a ... 5-mod.html
I also did the 'hardwired headphone mod' to this amp, which is connecting a single wire between two points on the output pcb, which enables the orpheus mod to work in full power as well.

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Anyways here's my problem, the amp when in full power without the pot engaged sounds beautiful. No issues. When in low power mode, and/or whenever the pot is engaged, I hear crackling going on while notes ring out, but only during this time. The crunch also becomes very fizzy and undesirable.

So.. because the amp works fine in full power without the pot engaged, that leads me to believe that it's not the pot, tubes (tried changing them anyways..), transformer and more likely something within the headphone amp. I'm looking at C50 here on the schem:

http://forum.metroamp.com/viewtopic.php?f=25&t=33575

They used a 10uF 50v, and I've got a 10uF 25v on hand. Anybody know if 25v for this part would be still within a safe range? I'd like to pop it in and see if this cures the problem.

[Iron Broadsword]

I do have a 200v 15uF. Anybody know what c50 does in the circuit? If it wouldn't damage it I could at least find out if it solves the problem.

[dobbhill]

Could you post a schematic for the "mod", please? It would help us to help you.
C50 appears to me to be a high-frequency attenuator, probably to roll off the highs for headphone listening.
Let us see the circuit and maybe we can figgure this out.
D

[Iron Broadsword]

Thank you! I just drew this up myself to show what I did as the original post didn't have a schem to go with it.

I also just ordered a new 15ohm/25watt resistor, and replacements for c50, r51, and r50 as well just in case one of these parts are bad.

[Iron Broadsword]

I should note.. I've completely undone these changes while troubleshooting and the problem persists, so to me it sounds like something on the output board, since the only time it occurs when the mod is removed is when in low power mode.