Hi Tom,
Have you tried looking here?
http://www.drtube.com/marshall.htm
Just a little way down there is a link to the Majors and you can see a pic of your Pighead there. Unfortunately it doesn't seem to be mentioned directly in the schematics but one of the listings may help.
If the only difference is the tone control section then it should be simpler that what is shown and most of Marshall's tone control circuitry is in a simple passive branch of only a handful of components. Can you trace what you currently have in that area in your Pig?
Have you tried looking here?
http://www.drtube.com/marshall.htm
Just a little way down there is a link to the Majors and you can see a pic of your Pighead there. Unfortunately it doesn't seem to be mentioned directly in the schematics but one of the listings may help.
If the only difference is the tone control section then it should be simpler that what is shown and most of Marshall's tone control circuitry is in a simple passive branch of only a handful of components. Can you trace what you currently have in that area in your Pig?
There is also a link here:
http://www.musicparts.com/products.asp?Company=Marshall
Which will take you to a site where you can definitely BUY the schematic for $15!!!! (Shock, Horror, I know, but if you are really in a bind......)
The Marshall presence control is usually just a cap and pot across the lower leg of the feedback network from the output Tx. That would probably just be removed.
The other Bass, Treble, Middle controls are also a pretty standard layout. The middle control is a pot at the bottom of the tree fed to its wiper through a cap. Again this may simply be removed in your setup.
If it is a problem in any other area of the Pig, eg power supply, phase splitter, power amp or input amp, then the original schematics may be enough to give you your answer.
http://www.musicparts.com/products.asp?Company=Marshall
Which will take you to a site where you can definitely BUY the schematic for $15!!!! (Shock, Horror, I know, but if you are really in a bind......)
The Marshall presence control is usually just a cap and pot across the lower leg of the feedback network from the output Tx. That would probably just be removed.
The other Bass, Treble, Middle controls are also a pretty standard layout. The middle control is a pot at the bottom of the tree fed to its wiper through a cap. Again this may simply be removed in your setup.
If it is a problem in any other area of the Pig, eg power supply, phase splitter, power amp or input amp, then the original schematics may be enough to give you your answer.
Albertb- Thanks for the reply. The Marshall Major and the 200 are two different amps. The Major has the usual tone circuit as most amps of the time whereas the 200 has what is claimed as an "active tone control circuit" to give a "cut and boost" to the bass and treble areas. The schematics/links you refered to appear to be the Major circuit which I already have. I don't have a 200/Pighead but I would like a copy of the schemo so that I use or incorporate it into some of my other projects. I'm guessing that it's like the Baxandall tone circuits for most audio systems, but I may be wrong. Thx again, Thom.
I see what you are looking for, but as I am sure you know, it is generally considered that Baxendall tone circuitry is not best suited for guitar amps so I am surprised to hear Marshall used it in your Pig's case.
Have a look at the PA20s on that site and you will see a much simpler tone layout. Looks like 2 channels with a volume and 1 tone each. That gets a bit closer to your own.
But better than that!!! Have a look at the "JMP Lead Series Standard and Master Volume, 50W/100W, Standard pre amp circuits with 2x ECC83 (Marshall, 1978?)". In the bottom left you will find the Bass preamp variant which uses - guess what - Baxendall tone circuitry. This has an additional pot for the Mid control but it is only a variable value of the standard Baxendall circuitry. If it were a fixed value the circuitry would be absolutely bog standard. Component values are given in the table on the right. That may help.
As an aside, if you haven't already seen this, you should download the little utility here:-
http://www.duncanamps.com/zips/cigna.zip
It is a tone control design application which shows curves dynamically as you change component values and pot settings for a number of the standard configurations. It is really useful when you are playing around here. It's free and small!
Have a look at the PA20s on that site and you will see a much simpler tone layout. Looks like 2 channels with a volume and 1 tone each. That gets a bit closer to your own.
But better than that!!! Have a look at the "JMP Lead Series Standard and Master Volume, 50W/100W, Standard pre amp circuits with 2x ECC83 (Marshall, 1978?)". In the bottom left you will find the Bass preamp variant which uses - guess what - Baxendall tone circuitry. This has an additional pot for the Mid control but it is only a variable value of the standard Baxendall circuitry. If it were a fixed value the circuitry would be absolutely bog standard. Component values are given in the table on the right. That may help.
As an aside, if you haven't already seen this, you should download the little utility here:-
http://www.duncanamps.com/zips/cigna.zip
It is a tone control design application which shows curves dynamically as you change component values and pot settings for a number of the standard configurations. It is really useful when you are playing around here. It's free and small!
http://www.kbapps.com/audio/schematics/tubeamps/marshall/JCM800bass1996-50W.html
Here's a link to a 50W Marshall with a Braxenhall B/T tone. Has an extra passive mid/sweep, but otherwise I'm 'told' it's a good little 'pig'.
Here's a link to a 50W Marshall with a Braxenhall B/T tone. Has an extra passive mid/sweep, but otherwise I'm 'told' it's a good little 'pig'.
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