What causes excess white noise in Vol 1 of vintage Plexi?

[page361]

Just curious - what are some of the known causes for extreme "white noise" in a vintage JMP bright channel ? Obviously some level is acceptable...wouldn't be a Marshall without some.
Could it be the bright cap? volume pot? original filter caps? resistors? ...all of the above? I exclude tubes as the assumption is all tubes would already be replaced as the first step in troubleshooting.

[demonufo]

Usually the bright cap. An actual plexi though? Or a later model?

[page361]

Yes '69 Plexi...all original.

[neikeel]

Yes '69 Plexi...all original.

1) Bright cap - 5nF
2) cc plate resistors/dropping resistors
3) contacts on the input jacks
er.....what else?

[page361]

Well removing the Lemco bright cap absolutely did the trick - but geez...sure does take ALOT of top end away.

So my next question is....could that cap just have seen better days? I just don't recall this amp being as noisy as it was prior to me removing the cap. But I would really like to keep the amp in spec and original.

[maxwedge572]

Save the old cap but try a 100-150pf but the more you crank this channel the less that bright cap will have any effect.