Plexi “Spoiler Mod” (All info in post #1)

[johnnyeggz]

I’m an all heavy all the time guy, and when I use a Plexi, I dime the High Treble channel and blend in some Normal. Well that got me thinking, being I’d never use the Normal channel for anything on its own, why not tailor it specifically for blending purpose? As it stands now, ya really can’t blend it more than 3 1/2 to 4 on the knob or it gets too woofy, but if I were to drastically lower v1a 330pf bypass cap, that might get me even more drive to blend in. Maybe raise that 820 a bit? Maybe add a small Bright cap on the Vol pot?

Or maybe it’ll be a complete waste of time.

These are the things I think of during a 12 hour round trip drive from yesterdays gig.

What says yous?

UPDATE:

To save you the hassle of reading through, here’s the keeper version (v3)



Channels jumped at v1

PPIMV

v1a 1.5k/1uf
v1b 1.5k/.68
.0022 Couplers

Normal Vol 500pf Bright cap
210k wiper to ground

Bring PI voltage down to 320 if it’s way higher.

The rest is 68 Plexi specs.

 

[jagrenet]

I’m an all heavy all the time guy, and when I use a Plexi, I dime the High Treble channel and blend in some Normal. Well that got me thinking, being I’d never use the Normal channel for anything on its own, why not tailor it specifically for blending purpose? As it stands now, ya really can’t blend it more than 3 1/2 to 4 on the knob or it gets too woofy, but if I were to drastically lower v1a 330pf bypass cap, that might get me even more drive to blend in. Maybe raise that 820 a bit? Maybe add a small Bright cap on the Vol pot?

Or maybe it’ll be a complete waste of time.

These are the things I think of during a 12 hour round trip drive from yesterdays gig.

What says yous?

I too have pondered these very ideas. Not 2 hours ago I put the chassis of my Plexi back in the head shell after it rested on my bench, during a few mods. Sounds like maybe I'll have to pull it back out - LOL

 

[johnnyeggz]

I’m gonna start with lowering that 330uf bypass. I got an 8 & 10 electrolytic laying around and a 1uf Mallory. I’ll start playing around tomorrow.

 

[StingRay85]

Highly doubt you'll notice a difference. Maybe you get more noticeable effect from lowering the coupling cap size

 

[johnnyeggz]

Highly doubt you'll notice a difference. Maybe you get more noticeable effect from lowering the coupling cap size

Maybe so.

Id still want the Normal channel to be darker though, in theory.
Maybe a .0047 instead of a .0022?

Or maybe I’m better off converting the Normal channel complete to High Treble and work backwards from there?

Well see in a lil while, I’m not 100% awake yet.

 

[Marcomel79]

After getting the suggestion from esteemed member @AndyD, i fitted a 500pF cap on the mixer resistor of channel 2, and a 500pF bright cap on vol2 pot. Huge improvement! I didnt change the bypass cap on V1A. No need....

 

[johnnyeggz]

After getting the suggestion from esteemed member @AndyD, i fitted a 500pF cap on the mixer resistor of channel 2, and a 500pF bright cap on vol2 pot. Huge improvement! I didnt change the bypass cap on V1A. No need....

So now it’s more “blendable”?

 

[Purgasound]

The 330uF cathode bypass cap on the normal channel is insanely too large. You can drop to a 2.2uF and it will still be flubby. 1uF is not too bad but most amps that use a 1uF cathode cap will also use a smaller coupling cap for that stage. For example on some newer Orange amps they might use a 10uF cathode cap with a .001uF coupling cap.
I wouldn't be nervous about dropping the cathode value to something usable. Even 0.68uF will still have a darker feel due to not having the 470K/470pF treble peaker in it.
Some Traynor BassMaster amps had two identical channels aside from one channel having a bright cap. It's common to see those modded to have a real Marshall style bright channel.
On my super lead circuits I usually leave the bright channel alone but I will tailor the normal channel to something more usable. I like being able to Cascade the normal channel into the bright channel and add a master volume. This way I can conjure up the sounds of the bright channel cranked without having to go stupid loud. It takes a little finesse to get it to sound right. It's not as simple it's just dumping one channel into the other. The old school one wire mod has a lot of drawbacks and there is definitely room for improvement there. Of course there are two ways to go with it. You can tailor it to be very high gain or be more creative and get it to recreate that plexi crunch at low volume. That takes some experimentation with the right voltage divider and some other values between stages.

 

[johnnyeggz]

The 330uF cathode bypass cap on the normal channel is insanely too large. You can drop to a 2.2uF and it will still be flubby. 1uF is not too bad but most amps that use a 1uF cathode cap will also use a smaller coupling cap for that stage. For example on some newer Orange amps they might use a 10uF cathode cap with a .001uF coupling cap.
I wouldn't be nervous about dropping the cathode value to something usable. Even 0.68uF will still have a darker feel due to not having the 470K/470pF treble peaker in it.
Some Traynor BassMaster amps had two identical channels aside from one channel having a bright cap. It's common to see those modded to have a real Marshall style bright channel.
On my super lead circuits I usually leave the bright channel alone but I will tailor the normal channel to something more usable. I like being able to Cascade the normal channel into the bright channel and add a master volume. This way I can conjure up the sounds of the bright channel cranked without having to go stupid loud. It takes a little finesse to get it to sound right. It's not as simple it's just dumping one channel into the other. The old school one wire mod has a lot of drawbacks and there is definitely room for improvement there. Of course there are two ways to go with it. You can tailor it to be very high gain or be more creative and get it to recreate that plexi crunch at low volume. That takes some experimentation with the right voltage divider and some other values between stages.

I’m not going for the cascade, but I’m gonna internally jumper at v1, gonna start up right now and see whats what.

 

[Purgasound]

Cool. I'm sure you might also be aware of that how you jumper it can also affect the tone. Depending on how you plug in can affect the gain level quite a bit. Experiment there as well

 

[johnnyeggz]

Cool. I'm sure you might also be aware of that how you jumper it can also affect the tone. Depending on how you plug in can affect the gain level quite a bit. Experiment there as well

I’ve only seen it jumpered internally at the tube.

 

[FleshOnGear]

If your goal is to be able to turn up the normal volume higher, I’d remove the cathode bypass cap altogether, and maybe even increase the cathode resistor. However, if you want to be able to get more gain than the stock sound, Purgasound seems to have the right approach. It kind of depends on whether you want to use the normal channel to double your gain, or if you want it just to add depth to your tone.

 

[Marcomel79]

So now it’s more “blendable”?

Not only its very blendable, you can actually use it on its own!

 

[johnnyeggz]

If your goal is to be able to turn up the normal volume higher, I’d remove the cathode bypass cap altogether, and maybe even increase the cathode resistor. However, if you want to be able to get more gain than the stock sound, Purgasound seems to have the right approach. It kind of depends on whether you want to use the normal channel to double your gain, or if you want it just to add depth to your tone.

I dunno about double the gain, but being able to add more gain is always welcome around this household.

Honestly after two very late nights in a row, I just didn’t have it in me to work on the amp today, I haven’t even gotten dressed yet.

Tomorrows another day….

 

[johnnyeggz]

Definitely on the right track.


Still tweaking……

 

[johnnyeggz]

Here it is….

 

[jagrenet]

Here it is….

Sounds KILLER !!! - Now you gotta tell us what you did to get there

 

[johnnyeggz]

Sounds KILLER !!! - Now you gotta tell us what you did to get there

Normal Channel
1.5k/1uf
.0022 Coupler
250pf bypassing the 470k mixer

Bright Channel
1.5k/.68

Jumpered at the preamp tube. ( it’s the only way I know how)

PPIMV

 

[jagrenet]

Normal Channel
1.5k/1uf
.0022 Coupler
250pf bypassing the 470k mixer

Bright Channel
1.5k/.68

Jumpered at the preamp tube. ( it’s the only way I know how)

PPIMV

I appreciate the feedback. I didn't know how to jumper V1 so instead, I installed a DPDT kit from Granger Amplification. The DPDT replaces the Normal pot which is cool because, with the pot "pushed in" it's basically still a stock Plexi. Pull the pot out and V1 gets jumper-ed or cascaded. It really is pretty cool to have both worlds available with it and is a fairly simple mod to do. However, .... I am on the fence about 'will I ever really use this amp ..... truly as a Plexi" ........ OR ..... do I go ahead and go full blown JMP style or JCM800 style with it and take it up a notch. Now you've got me wanting to tweak V1A with the values you listed above. - I also installed a Lar-Mar style PPIMV as well.

 

[johnnyeggz]

I appreciate the feedback. I didn't know how to jumper V1 so instead, I installed a DPDT kit from Granger Amplification. The DPDT replaces the Normal pot which is cool because, with the pot "pushed in" it's basically still a stock Plexi. Pull the pot out and V1 gets jumper-ed or cascaded. It really is pretty cool to have both worlds available with it and is a fairly simple mod to do. However, .... I am on the fence about 'will I ever really use this amp ..... truly as a Plexi" ........ OR ..... do I go ahead and go full blown JMP style or JCM800 style with it and take it up a notch. Now you've got me wanting to tweak V1A with the values you listed above. - I also installed a Lar-Mar style PPIMV as well.

I wanted to keep it “Plexi” so I didn’t do the cascade thing.

I jumped them by disconnecting the wire that goes to the 68k/68k of the Normal channel (pin 2), then ran a wire from pin2 to pin 7(where the 68k/68k of the Bright channels hooked to)