Plexi volume boost for leads?

ledvedder

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I use a variac. I run my amp on full blast, even the eq knobs (presence to taste though). Hendrix/early EVH tone for days, gets the power tubes cooking. But the problem is it’s too loud! That’s where the variac comes in. I adjust the voltage to the amp down—same tone but less than half the volume. Very manageable. So then I rigged a very long bolt on the knob of the variac so I can adjust the volume with my foot while playing. I’m in a 2 guitar band. I need a volume boost for my solo? Kick the voltage up. I have a whole lot of volume at my disposal now and I don’t have to sacrifice tone in any way.
How to you compensate for the bias change, if you're changing voltage on the fly? When I built my Plexi, I added a switch that adjusts the bias appropriately for 120v or 90v.
 

circusinthesky

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Yeah, after that point (roughly 7 on the treble volume), you don't really get much more volume. You can expect more gain, and that might mean a mushy tone depending on how strong a signal hits the input of the amp. (Meaning, if the amp has reached the end of headroom, boosting the signal going into the amp (and/or turning up the amp vol) might just overdrive it into an inarticulate mess (woolly, mushy, etc).

The comment about getting your lead tone, then backing off your guitar volume is a classic method of getting rhythm/lead tones.

I try to remind myself:
-Often, getting the lead tone means focusing the tone by removing other frequencies. An EQ pedal, or using an OD's tone controls (without much gain) can do a lot to make a lead tone stand out. (A cocked Wah is great for this. Pickup height can also help here.)
-A cleaner signal will seem louder, because there aren't as much competing frequencies, and the amp/speakers aren't working hard to reproduce unwanted freq.
-Jumper into the low bass input, to add a little less bass.
-Forget the usual EQ'ing settings. A Plexi might benefit from having the bass turned down more than usual, especially in a two guitar situation.

YMMV.

I already have it pretty gained up. Around 7 on the treble channel and 2-3 on the normal channel. Everything I've tried into the front of the amp only seems to add more gain, not volume.
 

Verminator25

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How to you compensate for the bias change, if you're changing voltage on the fly? When I built my Plexi, I added a switch that adjusts the bias appropriately for 120v or 90v.
It’s not a plexi but it is from the same school of thought. It’s a Jet City. Basically Soldano. But I run the preamp gain extremely low because of the added power tube distortion. Definitely in plexi territory as far as tone goes. I never bothered with the bias and the amp has been fine for 3+ years. Not to say it wouldn’t be an issue for another amp, but this one has been fine. May or may not reduce tube life…but I digress; a Marshall may not be as resilient under these conditions…

IDK EVH did it right? Lol
 

Antti Heikkinen

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I played my Plexi for the first time last night at band rehearsal. Overall, it sounded ok. I'm still trying to get it dialed in to where I have a good overdrive tone with a good amount of cleanup when I roll back the guitar volume. It also seemed like a constant fight between too bassy and not enough bass. But my biggest issue is that leads just seemed to disappear in the mix. I've always boosted the lead volume through an effects loop, but the Plexi obviously doesn't have one. I had both a Boss SD1 with level up, gain down, and an EQ pedal set to frown with level up. I also tried using the guitar volume, but since I already have the amp dialed in with overdrive, raising the guitar volume just seemed to increase gain, not volume. Nothing seemed to really cut my leads up in the mix. Looking for some suggestions.
The very reason I sold mine and went for a JVM. I tried them both head to head and found out that I could get pretty much am exact copy of the sound with the JVM and the same cab. You can't tell them apart really. But the JVM is so much more usable.

Indeed when the plexi sounds just right, there's like zero headroom left. Very hard to get a clearly louder solo sound. The only way is to just use an EQ pedal or such to tweak the lead to really cutting vs rhythm. Some sort of switchable attenuator would also work but couldn't find any except maybe tge Boss one that's super expensive.

Another option is to just dial in the plexi for clean and plenty of headroom and then use OD pedals for dirt. That's after all how Hendrix used his Marshalls as well. Fuzz in front.
 

ledvedder

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It’s not a plexi but it is from the same school of thought. It’s a Jet City. Basically Soldano. But I run the preamp gain extremely low because of the added power tube distortion. Definitely in plexi territory as far as tone goes. I never bothered with the bias and the amp has been fine for 3+ years. Not to say it wouldn’t be an issue for another amp, but this one has been fine. May or may not reduce tube life…but I digress; a Marshall may not be as resilient under these conditions…

IDK EVH did it right? Lol
He sure did! And I love his tones. But it's been confirmed that he biased his plexi to 50mV due to the voltage drop from his variac.

Which JCA do you have? I have a few modded 22H's.
 

Raimo

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I played my Plexi for the first time last night at band rehearsal. Overall, it sounded ok. I'm still trying to get it dialed in to where I have a good overdrive tone with a good amount of cleanup when I roll back the guitar volume. It also seemed like a constant fight between too bassy and not enough bass. But my biggest issue is that leads just seemed to disappear in the mix. I've always boosted the lead volume through an effects loop, but the Plexi obviously doesn't have one. I had both a Boss SD1 with level up, gain down, and an EQ pedal set to frown with level up. I also tried using the guitar volume, but since I already have the amp dialed in with overdrive, raising the guitar volume just seemed to increase gain, not volume. Nothing seemed to really cut my leads up in the mix. Looking for some suggestions.
The trick with plexi is to back down the volume on the SD-1 so its lower than the guitar straight in to amp. That way you get headroom and can use the eq pedal last in chain as a vokume boost for solos. The SD-1 is ok for plexis but please try the Boss blues driver and stack it with the SD-1, using no gain on SD-1, only bring up the volume. Killer Sound. Place them like guitar->SD1->BD1-EQ. Doesnt get better than that.
 

GaryJ

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I played my Plexi for the first time last night at band rehearsal. Overall, it sounded ok. I'm still trying to get it dialed in to where I have a good overdrive tone with a good amount of cleanup when I roll back the guitar volume. It also seemed like a constant fight between too bassy and not enough bass. But my biggest issue is that leads just seemed to disappear in the mix. I've always boosted the lead volume through an effects loop, but the Plexi obviously doesn't have one. I had both a Boss SD1 with level up, gain down, and an EQ pedal set to frown with level up. I also tried using the guitar volume, but since I already have the amp dialed in with overdrive, raising the guitar volume just seemed to increase gain, not volume. Nothing seemed to really cut my leads up in the mix. Looking for some suggestions.
Set your lead tone up for your bridge(lead) pickup…. Crank up the treble so your leads cut through the mix. If you get too bassy flipping to neck pickup incorporate a bass cut either with the guitar (depending on what kind of guitar you have) or via a boost pedal with tone controls or an eq pedal just set to suck out some mud when activated.
 

Giblespaul2001

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Yeah.. he is turning up right as soon as you do.
The other guitarist does that shit to me it’s okay that I can’t hear my leads but he thinks his leads needs to be WAY over everyone else. He don’t know how to use his volume knob when I take a lead. At some point I usually just bury him to remind him who has the louder amp.
 

playloud

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They didn’t use the wah wah cocked only when they used it as a regular wah wah. Also they usually turned down the volume on the guitar a bit for rythm and turned it up for solos their soudman turned up the volume for solos.

Are you sure? I'm fairly confident I recall Tony Visconti mentioning the cocked wah thing. There are also live clips where you can see Brian Robertson rocking the wah at one section of the song and then continuing with the wah still engaged - whether intentional or not, it worked!

I already have it pretty gained up. Around 7 on the treble channel and 2-3 on the normal channel. Everything I've tried into the front of the amp only seems to add more gain, not volume.

This is precisely why treble booster/EQ is being suggested.

If you assume constant output power, then attenuating low end means you get more treble "for your" 50/100W. And 50W of treble is a lot more annoying than 50W of bass! In a dense mix, it's precisely this "annoying" quality that will allow you to cut through.
 

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I played my Plexi for the first time last night at band rehearsal. Overall, it sounded ok. I'm still trying to get it dialed in to where I have a good overdrive tone with a good amount of cleanup when I roll back the guitar volume. It also seemed like a constant fight between too bassy and not enough bass. But my biggest issue is that leads just seemed to disappear in the mix. I've always boosted the lead volume through an effects loop, but the Plexi obviously doesn't have one. I had both a Boss SD1 with level up, gain down, and an EQ pedal set to frown with level up. I also tried using the guitar volume, but since I already have the amp dialed in with overdrive, raising the guitar volume just seemed to increase gain, not volume. Nothing seemed to really cut my leads up in the mix. Looking for some suggestions.
Hey Led, in my opinion, the 4 input Plexi type pre-amp is the best Marshall tone going for live levels. I'm a lucky owner of a limited run JCM800 1987 50 watt, which is basically a JCM800 class A/B power amp with a 4 input Plexi pre-amp. I also have a JCM900 SLX which is just a great amp to have around. I mention that since the SLX was the amp where Marshall got it right as far as lead boost is concerned. Two Master Volume knobs which are foot switchable between the two. Simple and unbeatable. Switching to the second Master Volume gives you a simultaneous boost in everything - gain, level, sensitivity, bite and the EQ. So getting that into perspective, the SLX is the last of the super leads, but with the addition of a permanent on valve driven line booster in front of the standard super lead pre-amp circuit. Still, as great as that amp is, it's just not quite as "musical" as the freakish 4 input Plexi super lead. Nothing else has the same balance between bite and tone. In saying that, I'm not a fan of "dulling down" a kick ass rhythm tone for the sake of having a lead tone that cuts through the mix, let's face it, there might be typically 20 or 30 seconds of lead solo in a 4 minute song. Like yourself, I wanted a way around that limitation. My solution was a totally reversible mod. based on the dual master volume set up of the SLX. To describe it in words, I've disconnected the wiring to the DI socket (which I never use) and stored the wire ends safely. The DI jack was replaced with a multi-contact switchable jack, the original jack stored in safe keeping, the idea being that I'm using that DI socket entry point rather than drilling holes in the chasis like so many others have done. The Master Volume knob of the amp is de-soldered from the board with a link from the zero volume pin of the potentiometer re-established to its original circuit point of the board. The centre pin and hot pin of the Master pot. have been wired to the switchable DI jack with return wiring to the board. When nothing is plugged into that socket, everything operates as normal, like nothing has been done at all. It gets tricky describing in words from here and this is just a rough description, but in a small ABS instrument box which plugs straight into the DI socket opening the switchable contacts, I've set up a stereo plug, from memory maybe a 2Mohm log potentiometer and a 12 volt DPDT mini relay (which easily operates from standard effect pedal power supplies or even a small 9volt battery). The mini relay is operated from a remote footswitch and basically parallels the outside potentiometer across the existing internal Master potentiometer, effectively giving you a foot switchable lead Master Volume which you can set to lead level. The outside pot. is wired in reverse across the existing Master centre pin and hot pin circuit board points, effectively reducing that resistance when clicked in parallel via the DPDT relay. It can be done and has been done. Nothing will give you a better lead tone out of a Plexi other than cranking the Master Volume up to where it's required when it's required, all the right things happen, gain increases, level increases, bite increases, sensitivity increases. I've taken this to the next level and co - wired a mini DPDT to an original DOD fet 100 line booster which also clicks on simultaneously when I kick the lead footswitch. That has a small slider switch in line so that I can opt. to have the line boost co - switched or not. All the switching is dead quiet, no pops or buzzes / hums. That's been my bullet proof live set up for years running as a two guitar band and later as a single guitar band, where you really need it to avoid the arse dropping out of the live act every time a solo comes about if you can't rely on a mixer operator.
 

ledvedder

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Hey Led, in my opinion, the 4 input Plexi type pre-amp is the best Marshall tone going for live levels. I'm a lucky owner of a limited run JCM800 1987 50 watt, which is basically a JCM800 class A/B power amp with a 4 input Plexi pre-amp. I also have a JCM900 SLX which is just a great amp to have around. I mention that since the SLX was the amp where Marshall got it right as far as lead boost is concerned. Two Master Volume knobs which are foot switchable between the two. Simple and unbeatable. Switching to the second Master Volume gives you a simultaneous boost in everything - gain, level, sensitivity, bite and the EQ. So getting that into perspective, the SLX is the last of the super leads, but with the addition of a permanent on valve driven line booster in front of the standard super lead pre-amp circuit. Still, as great as that amp is, it's just not quite as "musical" as the freakish 4 input Plexi super lead. Nothing else has the same balance between bite and tone. In saying that, I'm not a fan of "dulling down" a kick ass rhythm tone for the sake of having a lead tone that cuts through the mix, let's face it, there might be typically 20 or 30 seconds of lead solo in a 4 minute song. Like yourself, I wanted a way around that limitation. My solution was a totally reversible mod. based on the dual master volume set up of the SLX. To describe it in words, I've disconnected the wiring to the DI socket (which I never use) and stored the wire ends safely. The DI jack was replaced with a multi-contact switchable jack, the original jack stored in safe keeping, the idea being that I'm using that DI socket entry point rather than drilling holes in the chasis like so many others have done. The Master Volume knob of the amp is de-soldered from the board with a link from the zero volume pin of the potentiometer re-established to its original circuit point of the board. The centre pin and hot pin of the Master pot. have been wired to the switchable DI jack with return wiring to the board. When nothing is plugged into that socket, everything operates as normal, like nothing has been done at all. It gets tricky describing in words from here and this is just a rough description, but in a small ABS instrument box which plugs straight into the DI socket opening the switchable contacts, I've set up a stereo plug, from memory maybe a 2Mohm log potentiometer and a 12 volt DPDT mini relay (which easily operates from standard effect pedal power supplies or even a small 9volt battery). The mini relay is operated from a remote footswitch and basically parallels the outside potentiometer across the existing internal Master potentiometer, effectively giving you a foot switchable lead Master Volume which you can set to lead level. The outside pot. is wired in reverse across the existing Master centre pin and hot pin circuit board points, effectively reducing that resistance when clicked in parallel via the DPDT relay. It can be done and has been done. Nothing will give you a better lead tone out of a Plexi other than cranking the Master Volume up to where it's required when it's required, all the right things happen, gain increases, level increases, bite increases, sensitivity increases. I've taken this to the next level and co - wired a mini DPDT to an original DOD fet 100 line booster which also clicks on simultaneously when I kick the lead footswitch. That has a small slider switch in line so that I can opt. to have the line boost co - switched or not. All the switching is dead quiet, no pops or buzzes / hums. That's been my bullet proof live set up for years running as a two guitar band and later as a single guitar band, where you really need it to avoid the arse dropping out of the live act every time a solo comes about if you can't rely on a mixer operator.
Wow, I'd love to hear more about this, or if you have a schematic or layout. I've been considering the dual PPIMV option, which I posted earlier in the thread.
 

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Wow, I'd love to hear more about this, or if you have a schematic or layout. I've been considering the dual PPIMV option, which I posted earlier in the thread.
Photo286.jpg
Wow, I'd love to hear more about this, or if you have a schematic or layout. I've been considering the dual PPIMV option, which I posted earlier in the thread.
 

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Wow, I'd love to hear more about this, or if you have a schematic or layout. I've been considering the dual PPIMV option, which I posted earlier in the thread.
This is actually an experimental prototype that worked so well, I just ran with it and never fully completed the project. So I've used the the stereo plug to totally replicate the 3 pins of the Master potentiometer in the amp. As it it turned out, my memory was a bit off since I did this about 7 or 8 years ago. The internal master pot is totally disconnected from the board, wired to the new DI jack which has 3 make/break contacts leaving everything totally normal when not in use. When the external box is plugged in, the large knob effectively becomes your new master volume knob, the small knob becomes your lead level master volume connected across the new external master knob in parallel via one side of only of a mini DPDT when energised. I've added an LED to confirm the lead boost operated. Being experimental, I never got around to making up any volume markings, just used the pen marks to set to gig level at different venues I played. Best part about all this, no alteration of the pure Plexi tone, no intrusive circuit mods, just straight up like someone cranks up your master volume for you when a lead solo kicks in, but it happens from a footswitch.
 

ledvedder

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Thanks MP! I'm assuming this is similar to the dual PPIMV that I posted earlier? I'm trying to find more info on it, but there doesn't seem to be much out there.
 

Steven van der Vegt

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I had the same issue when I got my 1987x. We also play with two guitars and I couldn't cut through for solos and playing with the volume all the time is difficult since I also sing.

There isn't a lot of headroom for clean boosting, even though you've got more than me if you have a 100 watt head. I've tried so many pedals. An Echoplex adds too much low mids to cut through, same with a Klon (although better) and something like a Tubescreamer has to be a sound you like.

What worked the best (and I've tried a lot!) is either a Treble Booster, but once again you have to like that sound, or a TC Spark Booster. I stuck with the latter, put the toggle switch on mids with the bass at 10 o'clock and the treble on 2 o'clock. I keep the gain at 8 o'clock and set the level to taste anywhere above 12 o'clock and that works very well.
 

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Thanks MP! I'm assuming this is similar to the dual PPIMV that I posted earlier? I'm trying to find more info on it, but there doesn't seem to be much out there.
There's most likely dozens of ways to get the same outcome. This approach appealed to me as it was the "best fit" for my otherwise stock vintage amp. Driving the input harder or softer has limited results and generally just results in more or less gain, but no genuine increase in headroom when you need it. It makes total sense to use the master volume as the volume boost device. Even my 50 watt is more than enough for small to medium venues so why not use the extra volume that's just sitting there waiting to be used? In saying that, my 50 watt is a blazer, it's right up there with the 100 watt SLX 900. So recapping - I've disconnected the master volume pot from the board (I use V1 from the top left bright input) and left it rotated at about 90 degrees to stay clear of the board. I've repositioned the face knob on the knurled pot shaft so that it still reads normally from 0 -10. Soldered 3 wires from the master pot pins which run directly to a 3 contact cliff connector placed in the DI port. Three wires run back from the normally closed contacts and solder onto the board where the master pot was. At this point, the whole upgrade is invisible in every sense, operationally and sound wise, which is just what you want for home practice or recording. From there, I just plug in the external master module when rehearsing or gigging. Plugging that in opens the normally closed cliff contacts and the pot in the module becomes the operational master volume with the face panel master disconnected. The second pot in the module is wired in reverse across the centre pin and hot pin so that when you turn it up, it effectively reduces the resistance between that centre pin and hot pin, resulting in two volume knobs across the same circuit point. The setting on the second pot is applied via the mini DPDT sitting inside the case. I run a two core lead which plugs into the module DC socket from a footswitch. The DC operates the DPDT which parallels the smaller resistance of the second pot across the main pot resulting in more volume boost than you can use for your lead solo. It's a must and the only system that works in small venues. I would never have my amp run through the house mixer, it is way loud enough as it is and would spill through the on stage vocal mics. anyway, so relying on the mix operator was out, but not having your lead solos heard is your worst on stage nightmare, it just looks like a lot of effort resulting in what looks and sounds like the guts dropping out of the whole band. As mentioned, I have the same footswitch wired to a line booster which triggers simultaneously, but that's more for when you're working with distorted tones, not required if you're playing clean or mild bluesy breakup.
 

ledvedder

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There's most likely dozens of ways to get the same outcome. This approach appealed to me as it was the "best fit" for my otherwise stock vintage amp. Driving the input harder or softer has limited results and generally just results in more or less gain, but no genuine increase in headroom when you need it. It makes total sense to use the master volume as the volume boost device. Even my 50 watt is more than enough for small to medium venues so why not use the extra volume that's just sitting there waiting to be used? In saying that, my 50 watt is a blazer, it's right up there with the 100 watt SLX 900. So recapping - I've disconnected the master volume pot from the board (I use V1 from the top left bright input) and left it rotated at about 90 degrees to stay clear of the board. I've repositioned the face knob on the knurled pot shaft so that it still reads normally from 0 -10. Soldered 3 wires from the master pot pins which run directly to a 3 contact cliff connector placed in the DI port. Three wires run back from the normally closed contacts and solder onto the board where the master pot was. At this point, the whole upgrade is invisible in every sense, operationally and sound wise, which is just what you want for home practice or recording. From there, I just plug in the external master module when rehearsing or gigging. Plugging that in opens the normally closed cliff contacts and the pot in the module becomes the operational master volume with the face panel master disconnected. The second pot in the module is wired in reverse across the centre pin and hot pin so that when you turn it up, it effectively reduces the resistance between that centre pin and hot pin, resulting in two volume knobs across the same circuit point. The setting on the second pot is applied via the mini DPDT sitting inside the case. I run a two core lead which plugs into the module DC socket from a footswitch. The DC operates the DPDT which parallels the smaller resistance of the second pot across the main pot resulting in more volume boost than you can use for your lead solo. It's a must and the only system that works in small venues. I would never have my amp run through the house mixer, it is way loud enough as it is and would spill through the on stage vocal mics. anyway, so relying on the mix operator was out, but not having your lead solos heard is your worst on stage nightmare, it just looks like a lot of effort resulting in what looks and sounds like the guts dropping out of the whole band. As mentioned, I have the same footswitch wired to a line booster which triggers simultaneously, but that's more for when you're working with distorted tones, not required if you're playing clean or mild bluesy breakup.
When you mention the master volume pot, are you talking about a PPIMV? Or the high treble channel pot?
 

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