Master Volume Question

Marshall Amp Discussion

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lpd
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Master Volume Question

Post by lpd »

Hey guys I'm trying to resurrect a build of David Roots that was finished but had a couple issues. It is a Plexi/JCM 800 with a footswitch for each channel. One input standard volume tone stack except bright and normal can be blended together and two bright switches. What perplexes me is two master volumes pre and post. Any thoughts why this would be used? I can dial it in really well at low and high volumes keeping the post at 12:00 and using the pre post as a volume pot. Seems complicated and David liked to experiment.

Can post pics if helpful?
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martin manning
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Re: Master Volume Question

Post by martin manning »

You have an extra degree of freedom there. The pre-PI MV can be used to set the level going into the PI and thus the PI distortion that results from overdriving it. Then, the post-PI MV can be used to set the overall volume. The common add-on MV setup for plexi's and JTM45's (post-PI) allows setting overall volume while preserving the preamp and PI distortion as determined by the volume pot in the original circuit. Marshall's MV on the JCM 800's was pre-PI only, but some people prefer that. You have both.
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M Fowler
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Re: Master Volume Question

Post by M Fowler »

I have worked on a Lee Jackson Metaltronics M-1000 head and it is JCM flavor with Master Vol. I & II.
m1000preamp_schem.pdf
m1000psu_schem.pdf
m1000 Rev1.jpg
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lpd
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Re: Master Volume Question

Post by lpd »

Thanks guys. Here's a pic for reference. First master is definitely a pre PI second connects to the output of tubes and bias pots?

Image
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romberg
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Re: Master Volume Question

Post by romberg »

lpd wrote: Mon Aug 14, 2017 2:48 pm Thanks guys. Here's a pic for reference. First master is definitely a pre PI second connects to the output of tubes and bias pots?
This style of post PIVMV replaces the grid leak (bias feed) resistors with a dual ganged pot. The pot is "grounded" to the bias supply and servers as both a master volume and the grid leak resistors. I like this style of MV as it does not require adding additional coupling caps.

Also, as Martin pointed out, having these two masters in an amp gives alot of control over how the amp sounds when it is overdriven at lower volumes. If you poke around with an osciliscope in an over-driven amp you will find that an over-driven PI looks similar to an overdriven set of power tubes. With just a pre-PI master you will never have the power tubes or the PI overdrive. So, all of the distortion comes from the preamp. But, with two masters you can crank the pre-PI master, overdrive the PI and then use the post-PIMV to control the overall volume.
lpd
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Re: Master Volume Question

Post by lpd »

Thanks !
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