Marshall Origin 20 head and fizzy breakup/noise

[Loomer]

So I have a Marshall Origin 20 head  I do like the sound and feel of - very headroomy with the Master down and takes pedals extremely well, and sounds good wheen overdriven, too. Basic and cheap it might be, but does its job well!

However, on the edge of breakup it gives a weird, fizzy and cardboard-y (???) sound in the background which I'd love to eliminate if at all possible - sound clip here.

Anyone got any idea of what might be the culprit and what I might be able to try? Or is it simply the character of the distortion? I have a couple spare 12AX7's lying around and will start by replacing those in the pre to see if that changes anything in the meanwhile.

Thanks in advance!

[tubeswell]

Inject a 1kHz 10s-of-mV sine wave at input and scope the plates throughout.

[Loomer]

Inject a 1kHz 10s-of-mV sine wave at input and scope the plates throughout.

Sadly dont have access to a scope so it's going to have to be more agricultural than that!

[stratomaster]

See my reply #4 here: https://el34world.com/Forum/index.php?topic=31122.0

The fixed filters in these amps are voiced very harshly.  Clipping out the 220pF cap eliminates much of the hash, and turning the Tilt up still has plenty of high end.

I ended up adding a Cut control to the output stage to further cut the highs as I found the lower power options to be a bit grainy.

[Loomer]

See my reply #4 here: https://el34world.com/Forum/index.php?topic=31122.0

The fixed filters in these amps are voiced very harshly.  Clipping out the 220pF cap eliminates much of the hash, and turning the Tilt up still has plenty of high end.

I ended up adding a Cut control to the output stage to further cut the highs as I found the lower power options to be a bit grainy.

Thanks for the link, I'll have a proper read when I'm fresher and not just online because a party next door is keeping me up... 

Did you find the extra treble that the 220pF introduced (or technically preserved I suppose) did cause similar fizzyness in the OD? Did the character of the breakup change as a result?

Thanks!

[stratomaster]

Yes. The tone changed in both clean and distorted tones. Both were smoothed out in a pleasant way.  Removing and replacing this cap is very easy, so it's worth trying. Very low risk/high reward.