50w Plexi mods

Swapping out speakers in cab -OR- putting a hammer though your Line6 Spider? Tell us about it.
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rayc
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Re: 50w Plexi mods

Post by rayc »

I'd often wondered about -feedback but was too lazy to look it up. Now I know a bit more. there was a bit of chatter about it either in a Vox doco or a Marshall doco - can't recall.
I'm a lot closer to modding the Superbass - unfortunately the bloke who does the best amp work in my region has moved just recently.
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Greg_L
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Re: 50w Plexi mods

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Re: 50w Plexi mods

Post by cakewalkKaKed »

aha! great thread thanks,all links still work,listening to tone thread clipsies as i type :D ...

defo braver than me ... /hides at thought of hot solder in back of my amp
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Re: 50w Plexi mods

Post by Greg_L »

cakewalkKaKed wrote: Wed Oct 04, 2017 12:24 pm aha! great thread thanks,all links still work,listening to tone thread clipsies as i type :D ...

defo braver than me ... /hides at thought of hot solder in back of my amp
There are still some tweaks I want to do that I just haven't gotten to. :facepalm2:
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Re: 50w Plexi mods

Post by cakewalkKaKed »

time yet ... and will be fun to see how/what you get up to :)
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Re: 50w Plexi mods

Post by Greg_L »

cakewalkKaKed wrote: Wed Oct 04, 2017 2:21 pm time yet ... and will be fun to see how/what you get up to :)
There is a resistor on the push/pull pot that shelves off some signal. The value of that resistor can be toyed with, or removed completely. It's 100k right now. I'm gonna try some different values and see what happens. Or maybe remove it completely.
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Re: 50w Plexi mods

Post by cakewalkKaKed »

kewl! braver man than me ! ... lower voltage i have no problem with .. but poking around in an amp .. looking forward to these tweaks :)
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Re: 50w Plexi mods

Post by Greg_L »

cakewalkKaKed wrote: Wed Oct 04, 2017 3:38 pm kewl! braver man than me ! ... lower voltage i have no problem with .. but poking around in an amp .. looking forward to these tweaks :)
It's really not that bad. Yes, you can get shocked, but not if you go through the proper precautions, which themselves are very simple and rudimentary. If you can work a jumper wire with alligator clips, you can discharge filter caps and be safe.
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Re: 50w Plexi mods

Post by Greg_L »

Update - I'm about to tear back into this thing. I'm gonna turn it into a Plexi and hot-rod JCM 800 all-in-one. Stay tuned for the destruction of a currently perfectly fine functioning amp. :lollers2:
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Re: 50w Plexi mods

Post by Bubba »

Good news! Not something I'd attempt, but go for it. :)
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Re: 50w Plexi mods

Post by WhiskeyJack »

Greg_L wrote: Mon Jan 29, 2018 9:20 am Plexi and hot-rod JCM 800 all-in-one.
Then when you are done you can drop in the mail to Canada ATTN: Shan.

:like:
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Re: 50w Plexi mods

Post by Greg_L »

WhiskeyJack wrote: Tue Jan 30, 2018 6:12 pm

Then when you are done you can drop in the mail to Canada ATTN: Shan.

:like:
It took a month for pickups to get to you. It'll take 6 years for an entire amp.
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Re: 50w Plexi mods

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Greg_L wrote: Tue Jan 30, 2018 6:57 pm
WhiskeyJack wrote: Tue Jan 30, 2018 6:12 pm

Then when you are done you can drop in the mail to Canada ATTN: Shan.

:like:
It took a month for pickups to get to you. It'll take 6 years for an entire amp.
I'm not going anywhere. :chillin
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Re: 50w Plexi mods

Post by Greg_L »

WhiskeyJack wrote: Tue Jan 30, 2018 9:14 pm

I'm not going anywhere. :chillin
:lollers:
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Re: 50w Plexi mods

Post by Greg_L »

Okay gentlefucks, round 2....or 3? I don't know, but I'm tearing into this 50w Plexi again. Why? Because I can. If you're squeamish about tearing up a perfectly good amp and soldering on tiny bits, look away. I don't know the end result yet, but the process has begun.

This time I'm doing a variation on the cascade gain mod. I did the cascade mod before and it worked great, but I never used it. I still just used the amp as stock. And when I did kick it into high gain gear, I didn't really like it that much anyway. So what had happened was, in cascade mode before, the darker normal channel feeds into the brighter channel and it was just too much muck. So I'm reversing the flow...which *should* basically turn this four-input Plexi amp into one half 50W JCM 800, and one half 50W Plexi! How cool is that? Very. If it works. A JCM 800's gain stages are cascaded, and that's what this will achieve, and in the right order of gain staging. The downsides 1) it just might not work, and 2) it kills the "normal" channel of the Plexi. The latter doesn't matter to me as I don't like or use that channel anyway, so win/win! The signal will pass through the bright Plexi channel like normal and on to the rest of the amp, like normal, or I can redirect the signal back through the now defunct "normal" channel and use it's now unused gain stage. With some resistor and cap changes the darkness of that channel will be gone and it will be just another gain stage. And then it will pass back into the rest of the amp like normal. The JCM800 portion's gain control and engage switch will be on a 1 meg push/pull pot, and the old normal channel's volume pot will now be a pre phase inverter master volume - just like any JCM 800. The tone stack, phase inverter, and power section will be unchanged stock Plexi style. All of this will happen before the tone stack. It's basically all in the preamp. So....

This is where we start.
Image


This is the current push/pull pot for the current cascade mod. It will be removed, cleaned up, and re-purposed for the new version.
Image
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These are the four jacks for any typical Plexi. One of these gotta go, and it will be the normal channel high input. The push/pull will go in it's place and the normal channel low input will remain to function as the low (clean) input on a typical JCM 800.
Image
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This one will go back in later. All cleaned up.
Image


This high input will go bye-bye. Note the resistor. That's the 1 megaohm grid leak and it sets the input impedance for the high input. Its the big difference between the high and low inputs...but we don't need it for the JCM 800 portion of the preamp because the signal will have already passed through the Plexi inputs which also have this resistor.
Image


This is how and where the push/pull will now live. It's gonna be a bitch soldering to that thing. I'm gonna do it loose and then install it with the wires already hooked up.

Image



And now we gotta go under the board....
Image


First thing is to remove the old normal channel's cathode resistor and bypass cap. We don't need them anymore. We need that cathode part of the circuit to resemble the second stage of a JCM 800, and that means all that goes there is one 10k resistor.
Image


Shan, if you're watching, this is how you do it.....but cut the excess off. :razz:
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And here's a pile of shit that has been removed so far...some of this obviously needs to go back. Some of it is reversing what I had to do for the previous cascade mod.
Image



And that's where I'm stopping for now. My head hurts from hyper-focusing on this tiny stuff. Lots more to do.
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Tadpui
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Re: 50w Plexi mods

Post by Tadpui »

You're a brave man! Looks like its coming along nicely.
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Re: 50w Plexi mods

Post by Greg_L »

Tadpui wrote: Tue Feb 20, 2018 7:48 pm You're a brave man! Looks like its coming along nicely.
Thanks. I don't know if it's bravery or stupidity, maybe both, but you just gotta jump in and do it. I feel like I've armed myself with enough understanding to do it right. Hopefully. The payoff could be two of my all time favorite amps in one. :coolstorybro:
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muttley
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Re: 50w Plexi mods

Post by muttley »

Good work boy. Keep these people real..;)
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rayc
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Re: 50w Plexi mods

Post by rayc »

Impressive both in terms of dealing with soldering tiny bits and having a clue about what you do.
I'd rather teach reading to a reluctant kid - I understand and can do that. This stuff requires a different mind set and different everything really.
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Re: 50w Plexi mods

Post by Greg_L »

muttley wrote: Tue Feb 20, 2018 8:27 pm Good work boy. Keep these people real..;)
rayc wrote: Tue Feb 20, 2018 10:26 pm Impressive both in terms of dealing with soldering tiny bits and having a clue about what you do.
I'd rather teach reading to a reluctant kid - I understand and can do that. This stuff requires a different mind set and different everything really.
Thanks fellas, but we still got a long way to go. More updates tomorrow.
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