Need help with presence mod on this Purplexi

G

gtrwun

Well-known member
I know a bit about amps, but im unable to figure out what someone was trying to do on this amp? Id really like to get it back to stock. The dual ganged pot is really a head scratcher to me. Any help would be greatly appreciated! Steve
 
It's hard to tell what is connected to that pot from the photos.
- It doesn't look like anything is connected to one gang of the pot (the one farthest from the faceplate); is that correct?
- Does the blue/wht striped wire connect to a lug of the front-most gang?
- What else is connected to that pot?
- What is that WIMA .68uF cap connected to?
- Did the stock amp have a presence control?
 
Can't you just get a layout of the stock version and copy that? What year is that one supposed to be? '68?
 
Hard to see in those pics, but the presence pot appears to be wired up normally. Only one half that pot is used. Maybe a different value pot? There's some other stuff going on there, but the presence pot looks normal.
 
V2a":j6s3uybu said:
It's hard to tell what is connected to that pot from the photos.
- It doesn't look like anything is connected to one gang of the pot (the one farthest from the faceplate); is that correct?
- Does the blue/wht striped wire connect to a lug of the front-most gang?
- What else is connected to that pot?
- What is that WIMA .68uF cap connected to?
- Did the stock amp have a presence control?
Click to expand...

The pics arent great, I know. I was doing this whole thing from my phone and it was next to impossible to get pics to upload. And my photo skills are challenged at best.
1. There is nothing connected to the outermost gang of the pot. Maybe thats all someone had?
2. That blue/white wire is connected to the inner gang of the presence pot
3. Half that WIMA is connected to ground and the other half goes to a lug of the inner pot.
4. I wish I knew what the stock amp looked like , but ive been unable to find a picture or get in contact with the guy who built these amps. I asked someone I know who mods amps if he would work on it for me, and he said these amps are a bitch to work on because there is so much going on in its stock form.

It's pretty obvious looking at the amp, that someone has been inside and ive heard these amps sound good in stock form. Probably too much for me to deal with on my own. I do appreciate the replies though.
 
Is that 430k total NFB resistance? Looks like a 330k was added in series with the board-mounted 100k (the resistor-to-ground part of the NFB divider is the standard Marshall 4k7, dangling off the opposite side of board as the 330k). If so, that's not very much NFB. Does the Presence even work very well?

My guess is that the person did not have the right value pot (usually 25k-L) and just used what they had.

EDIT: and FWIW and IMO, just based on the photos you posted, there is not a lot going on in that amp. Pretty much any tech (even a hack like me) should be able to hook you up.
 
SpiderWars":3s8zjgx2 said:
Is that 430k total NFB resistance? Looks like a 330k was added in series with the board-mounted 100k (the resistor-to-ground part of the NFB divider is the standard Marshall 4k7, dangling off the opposite side of board as the 330k). If so, that's not very much NFB. Does the Presence even work very well?

My guess is that the person did not have the right value pot (usually 25k-L) and just used what they had.

EDIT: and FWIW and IMO, just based on the photos you posted, there is not a lot going on in that amp. Pretty much any tech (even a hack like me) should be able to hook you up.
Click to expand...

I appreciate the response. I honestly haven't played the amp in quite a while and took the chassis out a long time ago because the amp sounded like it had something wrong. Very dark sounding and the gain was not clear. Thought it might just need tubes and bias, but the tubes actually tested good. So, yeah maybe I just need to find someone willing to work on her. Thank you again, for taking the time.
 
JerryP":qvk6izgk said:
Hard to see in those pics, but the presence pot appears to be wired up normally. Only one half that pot is used. Maybe a different value pot? There's some other stuff going on there, but the presence pot looks normal.
Click to expand...

Wish you were closer Jerry!
 
gtrwun":3swg1vdi said:
Rick Lee":3swg1vdi said:
Can't you just get a layout of the stock version and copy that? What year is that one supposed to be? '68?
Click to expand...

http://www.guitaramplifierblueprinting.com/jw1.html

If you know where I can find a schem or layout, PLEASE , point me in the right direction. I've tried to contact Jim and he has never replied. :confused:
Click to expand...

This is the section from that link you need:

100 watt master volume configuration. The master is a pull pot, and the master when defeated, is totally out of the circuit, and also will put into play other aspects. With the master out of the circuit, the amp is an original pure plexi circuit. It will also when defeated, change the amps input configuration from series (when using the master - VERY high gain), to parallel (plexi) using the original plexi circuitry. You can take this to the bank - this is the amp that does it all.


It looks like they used the master volume pull pot to disconnect the MV feature and make it run one way all the time. I can't tell if they made any other changes, perhaps a one-wire or cascade mod. Below is where you can probably figure out how to return it to stock. Ceriatone has some good layouts online too. The board-mounted can caps in your photo make me think it's based on a '68 12000, but the '69 Plexi in the link below has a better layout and instructions to follow.

http://valvestorm.com/1969%20Kit
 
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