Marshall 6100 30th Anniversary trouble

[mtlbasslad]

A pencil is unsuitable for use inside an amp, as the 'lead' (graphite) is conductive.
In a live amp, using one as a probe could result in electrocution, and even in a isolated, de-energised amp, conductive deposits might be inadvertently made in there.

Note that schematic notes to set bias to a specific voltage only apply to tubes selected by that manufacturer at that time.
Tube characteristics have always varied over a wide range from their bogie type standard.
See http://www.mif.pg.gda.pl/homepages/f...086/k/KT66.pdf 'bias must be variable over a +/- 25% range for each tube'.
Hence it's good practice, even when using tubes that conform to that manufacturer's selection criteria, to verify that the plate / cathode current resulting from the bias voltage is reasonable.


A bias voltage of -60 is 'colder' than -50, ie it will result in a lower plate current / dissipation at idle.

You are right of course about the pencil, using a dowel now

[mtlbasslad]

Whichever connector that was crackling, double check it's solder connections. If they are fine you may have solved it by cleaning the connection.

Unable to get to solder connections without removing the board...

[mtlbasslad]

I think we got it

With all connections cleaned up, I ran the amp (with old tubes) for 8 hours while doing other shop chores, while applying special diagnostic tool (rubber hammer) every 1/2 hour - no noises, no redplating

Guitarist showed up with new JJ 5881's, so installed them & he played for a hour while I played with bias - he seems to like it at -48V so that's where it will stay.

How that bad connection ribbon cable (from the 'master output' board to 'power amp' board could cause redplating I don't understand, but living with women has taught me that I will never understand everything...

Thanks all for your advice, let's hope the thing holds up...

Lorne