Producer, Engineer, Live Sound Man
Q:I recently bought a vintage Marshall JMP in real Clean Shape. After carefully replacing all of the tubes and
setting the bias, using a bias probe, it squeals badly and is hard to control at full volume. I know they're
supposed to run wide open. But I'm sure they're not supposed to squeal either. I've looked in the amp and like I
said before It's clean nothing burnt as far as I can see.
Got any suggestions?
A: First of all Congrats on your Vintage Marshall Purchase. The Reality is there aren't that many around any more
that haven't been "moded" to death or just out right trashed.
Second, good for you retubing it completely and biasing it your self. Marshall's, like all point to
point tube amps, require regular maintenance. Takes guts to tear into an amp. But It's Important to know
your amps' needs. You find that out by working on them.
As to your problem, You have a leaking or dried out filter cap. In fact, since I assume these are
the same ones that came out of the factory. More than likely they are all fried. The life expectancy
of an electrolitic Capacitor is between 7 and 10 years. No matter what anybody says
thats the most your going to get before they start to give out. You'll need to replace every Electrolitic Cap in the head. Like over hauling an engine, this
will restore it to its full capabilities. The caps run around 100$, labor may cost you a little more.
A small price to pay considering the value of the amp as both a collectible and a tone.
If you haven't done this before I'd take it to someone who has, as you don't want to learn on a vintage amp, especially if it's clean.
Remember, love your amp and it will love you back
Peace out, C.E.
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