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Old 1st January 2010, 01:11 AM   #1
jmilch is offline jmilch  United States
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Default Need help adding a master volume, and some troubleshooting ...

I have a custom built guitar amplifier, with the following tubes:

2 x 12AX7
2 x 7189 (EL84 equiv.)
1 x EZ81 recitifer

It was originally an old Boulevard ST-17 amplifier, meant to be a PA or used with instruments. Someone before me already customized it to the point that the original schematic was useless.

I did manage to add the following:

Power Switch (yes it did not have one!)
Standby Switch
1/4" Input for instrument
1/4" output for ext speakers
4/8/16 ohm 1p3t selector switch

Everything is working. My issue is, I use the control on my guitar to control the max volume, but of course when I change guitars, the amp might be much louder or softer. I would like to add a master switch.

There is no tone stack on this. From what I have gathered, I need to add a 500K audio pot between the first preamp tube (12AX7) and the second preamp stage. I have identified the pin with the signal, and verified with a multimeter it was the correct pin, but I wanted to double check before I screwed soemthing up...

Any clues? I can post a picture if needed...

thanks for any and all help!
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Old 1st January 2010, 02:47 AM   #2
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I think the popular "Rich" master mod adds the master after the Phase spliter. This is PP right?

I just reread your post. Are you looking to add an "addtional" master volume? Does this amp have any volume control at all? If not, I would think you just need a standard volume right at the input?
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Old 1st January 2010, 02:51 AM   #3
jmilch is offline jmilch  United States
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Quote:
Originally Posted by TubeMack View Post
I think the popular "Rich" master mod adds the master after the Phase spliter. This is PP right?

I just reread your post. Are you looking to add an "addtional" master volume? Does this amp have any volume control at all? If not, I would think you just need a standard volume right at the input?
It has absolutely no volume or tone controls. It was all removed, and just pretty much wired up as a 18 Watt marshall Lite clone (or close to it from what I can tell)

I tried a 500k audio pot right from the guitar input, and it was not very effective.

It was suggested that I add it before the second preamp stage. It is wired PP correct. Sorry for the noob question, what exactly is the phase splitter?

**edit** I think I understand what you are saying. Here is the schematic I am working off of. I know it does not match exactly, but it is near what I have. Same tubes, and same PP setup.

http://yfrog.com/j718wattliteg

With that in mind, should I follow it and try to place the volume where it is in the schem above?

Last edited by jmilch; 1st January 2010 at 03:01 AM.
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Old 1st January 2010, 02:58 AM   #4
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Ok, that really wouldn't be referred to as a master, as a master is a "second" volume after the gain stage. This allows for a overdriven preamp with low output volume. Do you want that ability?

Google "rich master mod" Here's a start

[http://www.scribd.com/doc/6705426/Ma...-by-Step-Guide

as for where to put the "first volume", I always thought it was at the input. I'd need to see a schematic.

Experts, please step in!

Last edited by TubeMack; 1st January 2010 at 03:04 AM.
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Old 1st January 2010, 03:11 AM   #5
jmilch is offline jmilch  United States
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Quote:
Originally Posted by TubeMack View Post
Ok, that really wouldn't be referred to as a master, as a master is a "second" volume after the gain stage. This allows for a overdriven preamp with low output volume. Do you want that ability?

Google "rich master mod" Here's a start

[Marshall Post Phase Inverter Master Volume Step by Step Guide

as for where to put the "first volume", I always thought it was at the input. I'd need to see a schematic.

Experts, please step in!
I get what you are saying. I thought I was mis using the word master My other amps including a SS amps have of course a master volume and a separate volume for each channel, allowing low volume overdriven sound.

I am going to put it right on the guitar input again and see if I get some different results. I am going to try another pot as well.

Any other suggestions?
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Old 1st January 2010, 03:16 AM   #6
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Yeah it does seem to go after the first stage, same with the plexi. I never noticed that! Not sure how an addtional master after the splitter would work or if you even want to bother with that.

Time for an expert....
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Old 1st January 2010, 03:18 AM   #7
jmilch is offline jmilch  United States
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Quote:
Originally Posted by TubeMack View Post
Yeah it does seem to go after the first stage, same with the plexi. I never noticed that! Not sure how an addtional master after the splitter would work or if you even want to bother with that.

Time for an expert....
For some reason directly off the input doesnt do much. I am going to attempt to put it after the first stage right now and see. I wouldnt mind having a master either...

Where abouts in the schematic would I go about putting it after the splitter?
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Old 1st January 2010, 03:25 AM   #8
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http://blueguitar.org//new/schem/misc_mod/pi-m-vol.pdf
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Old 1st January 2010, 03:35 AM   #9
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Typically, the first stage is wide open... then volume/gain... then additional gain stages.. then master volume, then output stage. This format reduces noise to a minimum in a maximum gain environment.

In the case of the attached previous schematic, there are 2 gain stages (1 low 1 high) then the volume control. To add a master to this would be simple -- wire in a stereo pot into the control grid circuits of the el-84s....

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Old 1st January 2010, 03:42 AM   #10
jmilch is offline jmilch  United States
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Quote:
Originally Posted by Cycline3 View Post
Typically, the first stage is wide open... then volume/gain... then additional gain stages.. then master volume, then output stage. This format reduces noise to a minimum in a maximum gain environment.

In the case of the attached previous schematic, there are 2 gain stages (1 low 1 high) then the volume control. To add a master to this would be simple -- wire in a stereo pot into the control grid circuits of the el-84s....

-sean
Could you point me in the right direction here... Should I stick with a standard 500K stereo pot? Ok so I would just wire the control grid from each EL84 into the stereo pot? Makes perfect sense, if that makes sense

Do I have the right idea?

And as far as the volume goes, I should put it in after the second stage? I am just about to solder it in here, thanks for the help

thanks so much for the help guys.
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