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!
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!
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?
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?
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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/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!
Google "rich master mod" Here's a start
[http://www.scribd.com/doc/6705426/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!
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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?
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?
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
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.
I tried the A500K pot where it said in the schematic, and it did not adjust anything. When it was turned down low, the pot sparked.
So I dont have a clue... I tried using a A100K inline right after the guitar input, and it worked alright, except I noted alot of popping sounds when the amp began to distort.
I would be happy with just a volume knob, what is the simplest way I can add one?
So I dont have a clue... I tried using a A100K inline right after the guitar input, and it worked alright, except I noted alot of popping sounds when the amp began to distort.
I would be happy with just a volume knob, what is the simplest way I can add one?
That's a little strange.
I just breadboarded my first guitar amp with 3 x 12aU7 gain stages with a CF to finish.
I had a pot after the first gain stage to adjust overdrive and a master volume pot just after the second gain stage.
Worked pretty well. But the diagram shows 500K, I was using 100K as thats all I had.
I just breadboarded my first guitar amp with 3 x 12aU7 gain stages with a CF to finish.
I had a pot after the first gain stage to adjust overdrive and a master volume pot just after the second gain stage.
Worked pretty well. But the diagram shows 500K, I was using 100K as thats all I had.
Attachments
Volume control on a guitar amp goes between the output of the 1st stage and the grid of the 2nd stage. It also acts as the grid resistor for the 2nd stage so you should remove that assuming it's currently there.
Here's the simplest common example I know of:
http://www.schematicheaven.com/fenderamps/champ_5f1_schem.pdf
Here's the simplest common example I know of:
http://www.schematicheaven.com/fenderamps/champ_5f1_schem.pdf
Whaddaya mean 'it was not very effective'? I mean, you got a tube input with 500k across it, which should make virtually no difference, or you got a tube input shorted to ground at the other extreme, which should be silent apart from noise at the output. Or you got it miswired. There's just nothing to not be effective. It might not be very controllable although it should be with an audio taper.
Post diagrams of exactly what you are doing, preferably embedded jpegs.
w
Sorry perhaps I should have clarified more. When it was placed inline according to the schematic, the volume would increase as normal until the pot was turned about half way up, and the amp would cutout.
I can post some images, but I do not have a schematic or diagram since someone previous to me changed much of the original design.
I have the original schematic for the amp, I will upload that also. I appreciate any and all help guys
This is a very knowledgable forum!I have identified the output of the first stage, and the gid of the second stage. In between them, the line runs to a filter capacitor. (large can one) I did not know the 500k pot also acted as the grid resistor also, (although now it makes sense). So maybe that is the issue. I should remove the grid resistor on the second stage, and see what happens. I will try it when I get home. Thanks for the help
What do you mean 'inline'? Code is is 'inline'.
OK, so here's your input (or input to a stage):-
...and here's your input with a 500k pot added as volume control...
There are no hard-and-fast rules about where a volume control should go in an amplifier.
Forget the grid resistor in the second stage. A 500k pot should not be loading this significantly but the exact topology might be important. You can't do this stuff by rambling on. Draw a diagram on a piece of paper and photograph it at lo res with a digital camera so there's no confusion about what's going on. Remember pencils?
w
OK, so here's your input (or input to a stage):-
An externally hosted image should be here but it was not working when we last tested it.
...and here's your input with a 500k pot added as volume control...
An externally hosted image should be here but it was not working when we last tested it.
There are no hard-and-fast rules about where a volume control should go in an amplifier.
Forget the grid resistor in the second stage. A 500k pot should not be loading this significantly but the exact topology might be important. You can't do this stuff by rambling on. Draw a diagram on a piece of paper and photograph it at lo res with a digital camera so there's no confusion about what's going on. Remember pencils?
w
I will put together a diagram. I am not familiar with the correct symbology for a schematic, and since I was not the original designer/builder of this amp, it isnt the easiest thing to just follow the signal path.
I did notice on each and every schem I looked at involving my particular tube set, the volume always went between the first and second pre amp stage. Maybe this is just a guitar amp common proactice, I really do not know.
I have it just like the second diagram. I am going to look again and see if I can make a diagram ....
I will try it
I would guess that over 99% of guitar amps (not preamps nor power amps) have the volume control between the 1st and 2nd stage. There are some very old designs with the volume 1st but I think those were for guitars/instruments that don't have a volume knob themselves. Nowadays there's always a volume control on the guitar which is very close to having one before the 1st stage inside the amp.
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