50w Plexi mods

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Greg_L
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50w Plexi mods

Post by Greg_L »

I'm gonna try two mods to this 50w Plexi reissue.

1) PPIMV - Post Phase Inverter Master Volume. These amps have no volume control. They're just loud as fuck all the time. I've been happily using an attenuator, but I want the PPIMV as well. The good thing about the PPIMV mod is that when you crank it, it becomes invisible in the circuit. It's like it's not there.

2) One-wire cascaded gain mod. These amps have two channels and they don't share gain stages. This mod will send one stage into the other and boost the gain to like hair-metal shred territory. That's not a sound I typically use, but it'd be neat to have and supposedly the gain mod is very simple. As far as I can tell it nukes the "normal" channel, but that's okay as I only use the bright channel anyway. I'm not sure yet how it affects the bright channel. If I can make it switchable, I will.

First up....taking this thing apart...
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The resistors circled in yellow gotta go away. The dual 250k volume pot will now will serve their purpose. The orange and green wires circled in red will have to be removed from the power tube sockets.
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Gotta get under the board...
Image

I gotta stop for now....this is as far as I've gotten. Hopefully tomorrow this thing will be alive again.
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Re: 50w Plexi mods

Post by Bubba »

Wow. Brave man! I wouldn't dare do something like this!

Will watch with great interest. :smiles:
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Re: 50w Plexi mods

Post by Greg_L »

Bubba wrote: Thu May 04, 2017 5:24 am Wow. Brave man! I wouldn't dare do something like this!

Will watch with great interest. :smiles:
Ha, well let me put your mind at ease as I tell you that I have no idea what I'm doing. :lollers2:

What I can do though is search every corner of the internet and follow instructions. The PPIMV mod has been done for decades and it's been pretty well figured out and successfully implemented by people way smarter than me at this stuff. I don't know the tech and theory behind what I'm doing or why it works, but if I do it right, it will work. My 100w Plexi has the same PPIMV (not done by me) and it works flawlessly. What I have not done yet, and I should do it right away, is crack open the 100 watter to see how it's wired up compared to what I'm doing to it's 50w little brother. They're basically the same amp.

Now the cascaded gain mod to come afterwards? I'm kind of still in the dark on that one. I'd say I'm about 75% sure what to do on that one. :crazy: :cuckoo:

The amp is very simple though. If I nuke it I know just the guy and place to take it to to get it properly fixed.
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Re: 50w Plexi mods

Post by rayc »

Then you can come over to Oz & mod my Marshall.
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Re: 50w Plexi mods

Post by Greg_L »

rayc wrote: Thu May 04, 2017 7:35 am Then you can come over to Oz & mod my Marshall.
Lol no just send it to me. I'll get it back to you eventually. :nyuk:
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Re: 50w Plexi mods

Post by Bubba »

Greg_L wrote: Thu May 04, 2017 7:19 am
Bubba wrote: Thu May 04, 2017 5:24 am Wow. Brave man! I wouldn't dare do something like this!

Will watch with great interest. :smiles:
Ha, well let me put your mind at ease as I tell you that I have no idea what I'm doing. :lollers2:

What I can do though is search every corner of the internet and follow instructions. The PPIMV mod has been done for decades and it's been pretty well figured out and successfully implemented by people way smarter than me at this stuff. I don't know the tech and theory behind what I'm doing or why it works, but if I do it right, it will work. My 100w Plexi has the same PPIMV (not done by me) and it works flawlessly. What I have not done yet, and I should do it right away, is crack open the 100 watter to see how it's wired up compared to what I'm doing to it's 50w little brother. They're basically the same amp.

Now the cascaded gain mod to come afterwards? I'm kind of still in the dark on that one. I'd say I'm about 75% sure what to do on that one. :crazy: :cuckoo:

The amp is very simple though. If I nuke it I know just the guy and place to take it to to get it properly fixed.
Just don't get zapped by one of those big caps! :eek: I assume there are protocols for discharging them safely before you start any work?
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Re: 50w Plexi mods

Post by Greg_L »

Bubba wrote: Thu May 04, 2017 7:40 am

Just don't get zapped by one of those big caps! :eek: I assume there are protocols for discharging them safely before you start any work?
Yeah, I just short them to ground. It's not very elegant, but it works. A jumper wire and a quick spark does the trick. Then I just leave the jumper connected so they don't recharge. Some amps have a tiny resistor in the filtering circuit to bleed them dry when the amp turns off. I don't know. The potential for shock is there if you don't bleed the caps, but I think it might be overstated just to keep everyone safe.

In my experience, which not much, those big filter caps don't hold a charge for long. If the tubes are installed, the caps trickle down to near empty just by turning the amp off and leaving it off for a bit. This amp was off for nearly a week before I opened it up and there was nothing in the caps when I checked them with a multimeter.
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Re: 50w Plexi mods

Post by Bubba »

Greg_L wrote: Thu May 04, 2017 7:53 am Yeah, I just short them to ground. It's not very elegant, but it works. A jumper wire and a quick spark does the trick. Then I just leave the jumper connected so they don't recharge. Some amps have a tiny resistor in the filtering circuit to bleed them dry when the amp turns off. I don't know. The potential for shock is there if you don't bleed the caps, but I think it might be overstated just to keep everyone safe.

In my experience, which not much, those big filter caps don't hold a charge for long. If the tubes are installed, the caps trickle down to near empty just by turning the amp off and leaving it off for a bit. This amp was off for nearly a week before I opened it up and there was nothing in the caps when I checked them with a multimeter.
It's that fear of the unknown that makes me wary. I'm sure the risk is overstated as you say. I've had a few "belts" as we call them over here, mainly from working in the building trade. I got a bad one from a first-fix light switch cable curled up in its back box. I was fitting a door right next to it but was told the lighting circuit was off. Of course, it wasn't - the bare wires touched the back of my hand and gave me such a jolt I had to sit straight down on the stairs. I could taste my fillings.
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Re: 50w Plexi mods

Post by Greg_L »

Bubba wrote: Thu May 04, 2017 8:02 am

It's that fear of the unknown that makes me wary. I'm sure the risk is overstated as you say. I've had a few "belts" as we call them over here, mainly from working in the building trade. I got a bad one from a first-fix light switch cable curled up in its back box. I was fitting a door right next to it but was told the lighting circuit was off. Of course, it wasn't - the bare wires touched the back of my hand and gave me such a jolt I had to sit straight down on the stairs. I could taste my fillings.
Lol. Damn. I've luckily never had a serious A/C current shock. I've had some really powerful zaps though from D/C race car ignitions. They kick out like 60,000-70,000 volts. It hurts real bad. It definitely gets your attention. But it's so quick that it just stuns you and you pull away, collect yourself, and cautiously carry on.
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Re: 50w Plexi mods

Post by Tadpui »

Awesome, Greg! I'll be following along! I still wouldn't dare try this myself, but I like to see the process.
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Re: 50w Plexi mods

Post by WhiskeyJack »

Tadpui wrote: Thu May 04, 2017 3:35 pm Awesome, Greg! I'll be following along! I still wouldn't dare try this myself, but I like to see the process.
Ditto. :lollers2:

I looked at building a Hiwatt clone a year or two ago and as quickly as i started researching it, i very quickly closed that chapter of my personal exploration for now. That's a big "maybe someday" project for me.

I am interested to see what greg, comes out with though for sure. :like:
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Re: 50w Plexi mods

Post by Greg_L »

Well fellas, here we go.....

So I stopped last night with the board loose and ready to flip. But I decided to go after the easy shit first. This amp has two speaker jacks. I only need one since I link my cabs together instead of having a bunch of cables plugged in behind he head. So the secondary speaker jack got yanked out and in it's place goes the master volume pot. The volume in the back is cool because the front still looks stock. And it doesn't require another hole to be drilled. That black piece with the long bent wires hanging off is the primary speaker jack. I left those wires long and just bent them out of the way so I could easily put the other jack back in if I want.
Image


That was all for the easy shit so it was time to get down and dirty. I tilted up the board and gently de-soldered the two resistors that had to go. R25 and R26 now empty and ready to have some wires penetrate their delicate holes. GN5 and OR are where the Green and Orange wires ran to pin 5 of the power tubes. Those got removed as well.
Image


With the resistors out cleanly, in goes what I think is the negative bias voltage wire. Without this wire the power tubes immediately go kaboom. I chose a blue wire because I had it laying around. This also goes up to two lugs of the volume pot. On the board side I looped it under to make contact to both negative voltage spots just like the resistors did. That little titty sticking up through R26 is the end of the blue wire. The resistors on the pot are part of the bias circuit and they're just mocked in there for this pic.
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Next was to solder the the swamp resistors to the leads going to the pot and solder the assembly to Pin 5 of power tubes V4 and V5. This part was pretty harrowing because the connections have to be very good and very clean and very sturdy and very insulated and there aint much room down there to fuck around with a boiling hot soldering iron. These pin 5 spots are where the green and orange wires went..
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I added the other bias supply wires to the board and this is the underside. Pretty clean! I didn't melt anything or lift any traces.
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Now it's time to connect everything at the pot. This was kind of a bitch because everything is pretty cramped and the shielding needed to be grounded. I trimmed everything to just fit nice and tidy and stuck it all together. Good clean solders and nothing is touching.
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The finished PPIMV mod.
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I used a chicken head knob because it's easy to grab and tell where it is when reaching behind an amp. So now this amp has a master volume!
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And the verdict...........SUCCESS! :crazy: :coolstorybro: :coolstorybro: :coolstorybro: :coolstorybro:

Bias is perfect. No redplating. No death. No noises. No explosions. No tube meltdowns. Perfectly functional master volume control on an amp that would normally blow your face off. I've taken it from cranked to whisper quiet and it works perfectly. Sounds great, works great. :not worthy: :nyuk: :punk:

So now with that done, it's time to get into the cascaded gain mod. Stay tuned.............
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Re: 50w Plexi mods

Post by Roman »

Excellent! Wonderful pics as well. :cool: :like:
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Re: 50w Plexi mods

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Roman wrote: Thu May 04, 2017 5:44 pm Excellent! Wonderful pics as well. :cool: :like:
Thanks man! It feels awesome to tackle something like this when you have no idea what you're doing and have it work. :coolstorybro: :lollers:
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Re: 50w Plexi mods

Post by Bubba »

I would have to put some shrink-tube over those redundant output wires. :eek: :biggrin:

But that is as cool as fuck and a professional-looking result. :cool:
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Re: 50w Plexi mods

Post by Roman »

My first amp I built was a Heathkit, solid state "fender twin copy". Other than it being solid state, it was similar in look, power, controls etc.

Drove 3 hours to Chicago picked it up, drove back to my place, and stayed up till the sun came up building it. It worked!!!!


A good amp to build if the bug strikes you are the Marshall 18 watt clones! Fun, relatively easy to build and have the classic Marshall sound.
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Re: 50w Plexi mods

Post by Greg_L »

Bubba wrote: Thu May 04, 2017 5:57 pm I would have to put some shrink-tube over those redundant output wires. :eek: :biggrin:

But that is as cool as fuck and a professional-looking result. :cool:
Thanks. Yeah I'm not done with it yet. I'm gonna tidy some shit up still. And now I'm gonna do this goofy "one wire" mod. Thankfully it's basic and simple and easily reversible.
Roman wrote: Thu May 04, 2017 5:58 pm My first amp I built was a Heathkit, solid state "fender twin copy". Other than it being solid state, it was similar in look, power, controls etc.

Drove 3 hours to Chicago picked it up, drove back to my place, and stayed up till the sun came up building it. It worked!!!!


A good amp to build if the bug strikes you are the Marshall 18 watt clones! Fun, relatively easy to build and have the classic Marshall sound.
I've got two amps in mind to build - a JTM45, or some kind of Fender reverb/tremolo clone. Either of those would be great and don't seem to complicated.
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Re: 50w Plexi mods

Post by Roman »

Excellent choices as well. One thing I like about the JTM 45 is it's smaller chassis. It lends itself to may different cabinet designs.
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Re: 50w Plexi mods

Post by Greg_L »

Okay so I did the one-wire mod the way I thought it was supposed to be done. Nothing. Very clean sound, the exact opposite of what it's supposed to do. With mine being a PCB instead of a turret board, I'm just guessing here. I straddled two resistors with the "one wire" like I thought I was supposed to and nothing. So I undid that and ran a jumper wire to just one of the resistors that I think it's supposed to be. And this is what I got....
one wire 1987.mp3
Lol. I don't know. Sounds pretty wild and hairy but it also kills the normal functionality of the amp. I'm gonna put it back to stock and do more research.
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Re: 50w Plexi mods

Post by Greg_L »

Ok so I found a version of the cascade gain mod using a push/pull pot for the normal channel volume pot. It makes the amp work as stock and as a hairy shred monster. But I don't think I'm gonna mess with it anymore.
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