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Voland 23rd December 2008 03:28 PM

Marshall AVT 50 preamp gain/distortion increase
Hi guys!

I'm new to this forum and I've got a question regarding
modification of my Marshall amp. I'm not really satisfied with the distortion channel and
would like to change that(if that's possible of course).
How can I possibly increase the gain (and therefore the distortion?)
of the 12ax7 preamp tube? I don't really have much theoretical knowledge about
amplification, but I am familiar to electronics and soldering and I also have
a schematic of the amp. It'd be really nice if someone could help me with this.

Thanks, Ivan

Merlinb 23rd December 2008 03:38 PM

Assuming you have the same schematic as this:

I would suggest you startby putting a cap in parallel with the first cathode resistor, R29. Try between 470n and 1uF. Larger values up to 10uF may give you too much bass ('woofy'), but by all means try them. (They need only be low-voltage types, say 10V).

If that doesn't give you enough then I would try increasing R32, to between 100k and 470k, but don't get too carried away or you may get some ugly distortion.

Voland 4th January 2009 04:41 PM

Thanks a lot for the useful tip - I've tried it with 2 2.2uF caps in parallel and it certainly did improve the gain. What I want to try now are a couple of 33uF electrolytic caps - is it possible to use these? I've seen people do it, but isn't there an AC current passing through?

mc1951uk 27th November 2010 01:05 PM


Sorry can,t help with your query but I was wondering if you could send me a copy of your schematic as I am having a problem with my amp.


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