From: tom@cpac.washington.edu (Tom May)
Newsgroups: rec.music.makers.guitar
Subject: Re: Damage to a tube Marshall?
Date: 19 Nov 92 08:36:19 GMT
	<1992Nov16.222849.25501@news.mentorg.com>
	<1992Nov16.211048.6967@nntpd2.cxo.dec.com>
Organization: Center for Process Analytical Chemistry, UW

In article <1992Nov16.222849.25501@news.mentorg.com> andyb@andyb.mentorg.com (Andy Bishop) writes:

>I've owned my JCM900 2x12 50W combo now for about 2 years and I love
>it.  I've owned other Marshalls and this is by far the best sounding
>Marshall amp I've ever owned.  I've had guitarists come up and ask
>what the hell I'm playing through, and who mod'd my amp.  Everyone
>that's heard it loves it.  I did replace the 75W speaks with 2 25W
>Celestions greenbacks, as well as replace the power tubes and have
>the amp re-biased. (The tech had to add some resistance into the
>bias circuits to make things happen)  I've also replaced the three
>12AX7s, but since they're class A, there's no need to re-bias.

>As for you...it would be hard to say what might have been damaged if
>anything at all.  I would suggest that you take you amp directly to
>a competent technician and have him check out everything from the
>pre-amp (12AX7s) to the outputs.  

And then, in article <1992Nov16.211048.6967@nntpd2.cxo.dec.com> cooper@kdx200.enet.dec.com (Uh-huh...) writes:

>I know people will piss in my sneakers for saying this, but I'll 
>comment ANYWAY.  :)  :)

>I've played guitar for quite a while, and I've owned SEVERAL Marshalls.
>I always like them at first, because of the fat tone and pure ballz.
>But they NEVER last with me.  Why??

>Because they have lame-gain instead of insane-gain.    I like to have a 
>REAL metal sound ala Marty Friendman of MegaDeth.  Ya know, a really fizzle-y
>top end, lots of squeelies etc...   Marshalls don't do that - not for me
>anyway.  

I just took my Marshall to Mr. Science at SoftScience here in Seattle
to get an effects loop installed.  When he opened it up, he said it had
a common Marshall wiring bogosity: one of the 12AX7s was wired so the
signal went through the two sections backwards (i.e., right to left
instead of left to right when viewed from the back).  Apparently there
are problems with wires crossing each other in this configuration which
causes Marshall to leave out or change the value of some components which
*drastically reduces the gain* (if they gain were higher, they'd get crosstalk
in the wires or something).  The whole amp was also riddled with ground loops.

In addition to adding the effects loop, Mr. Science also redid the grounds
so nearly everything grounds to a common point, and he also rewired the bogus
12AX7 and stuck in a resistor and capacitor that Marshall had omitted, so the
gain is now up to what it should be and it will "sound like a real Marshall!"
i.e., insane gain instead of lame gain.  (If the gain is too hot for me, I can
cut it back down again by simply snipping out the resistor and capacitor.)

He also changed some resistor value which is supposed to make the tubes react
faster on attacks.

Sadly, I just picked it up last night and haven't had a chance to test it yet.
I'll let you know next week how it works.
--
Tom.