AusTex64
Well-known member
- Joined
- Jun 3, 2013
- Messages
- 525
I'm chasing a "ghost note" problem in a Marshall Super Lead Model 1959 100W amp. "Ghost note" referring to 120hz getting into the audio path somewhere and intermodulating with the fundamental note. This is real interesting topic to read up about. Some people say "all 100W Marshalls do it to some extent". Yet my buddy has a 100W Model 1959 that doesn't do it at all, that I can hear. So the obvious questions is "if they all have the ghosting problem, why doesn't my buddy's amp have it too?"
The first advice across the board is to change out all the big filter caps. Another suggestion is to increase the first stage filtering from 100uf to 200uf. So we changed out the first two to 200uf. It did help the problem, but it's still there, and it did change the feel of the amp a bit. I just keep coming back to "why does my buddy's amp not have this problem, with the stock filtering?" Guessing I'm gonna have to try to talk him into letting me study his amp.
Back to my question. I checked the two LCR 50uf + 50uf 500VDC caps commonly used by Marshall that were replaced. Here's an interesting quote about LCR caps - "Certain capacitor brands like F+T and LCR have high leakage current." I used our LCR meter, who's power supply consists of a 9V battery. It showed all sections were within 10% of 50uf. But it was pointed out that this isn't at the 400VDC that these Marshall run at, and it only takes one little spot between foils to conduct at that voltage and render the cap no-go.
SO, is there any method for testing high voltage caps that is reliable other than just substituting them in the amp and hoping for the best?
Sorry for the long winded post.
The first advice across the board is to change out all the big filter caps. Another suggestion is to increase the first stage filtering from 100uf to 200uf. So we changed out the first two to 200uf. It did help the problem, but it's still there, and it did change the feel of the amp a bit. I just keep coming back to "why does my buddy's amp not have this problem, with the stock filtering?" Guessing I'm gonna have to try to talk him into letting me study his amp.
Back to my question. I checked the two LCR 50uf + 50uf 500VDC caps commonly used by Marshall that were replaced. Here's an interesting quote about LCR caps - "Certain capacitor brands like F+T and LCR have high leakage current." I used our LCR meter, who's power supply consists of a 9V battery. It showed all sections were within 10% of 50uf. But it was pointed out that this isn't at the 400VDC that these Marshall run at, and it only takes one little spot between foils to conduct at that voltage and render the cap no-go.
SO, is there any method for testing high voltage caps that is reliable other than just substituting them in the amp and hoping for the best?
Sorry for the long winded post.