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gretsch58
12-19-2006, 03:40 PM
I'm sure this post is on here somewhere. However I could not find one so I will ask. I just bought a Ceriatone Plexi 50 1987 Lead and I want to install a Master Volume. Is it pretty easy to do? does anyone have a wiring diagram? here's a link to the amp layout.





http://www.ceriatone.com/images/layoutPic/marshallLayout/Plexi50Leadceriatone.jpgOk, I'm sure this post is on here somewhere. However I could not find one so I will ask. I just bought a Ceriatone Plexi 50 1987 Lead and I want to install a Master Volume. Is it pretty easy to do? does anyone have a wiring diagram? here's a link to the amp layout.

http://www.ceriatone.com/images/layoutPic/marshallLayout/Plexi50Leadceriatone.jpg

gag halfrunt
12-20-2006, 02:06 AM
Man, why can't Marshall do them like this? That is one clear, easy-to-read schematic.

hasserl
12-20-2006, 12:42 PM
Man, why can't Marshall do them like this? That is one clear, easy-to-read schematic.

It's not a schematic, it's a layout drawing, or a diagram. Schematics are very different, and serve a diferent purpose. FWIW, I'd much rather have and work from a schematic than a diagram.

ChickenLover
12-20-2006, 12:55 PM
Go to either the Metro amp site, or the Vintage amps BB, or the Trainwreck discussion board and look for posts by 'rockstah'. He's made several posts recently and put a layout of a good post-PI MV in many of them. It's basically the Ken Fischer MV (the one that replaces the bias splitters with a dual 100K pot) except you use a 250K dual-pot and add 2M2 resistors connecting the 'ungrounded' lugs of each pot. I think he got it from a builder in Germany (novosibir aka Larry). I had used this in one of my amps (I came up with it myself...only because I was very limited with what parts I had on hand...I guess necessity really is the mother of invention).

gag halfrunt
02-20-2007, 11:33 PM
It's not a schematic, it's a layout drawing, or a diagram. Schematics are very different, and serve a diferent purpose. FWIW, I'd much rather have and work from a schematic than a diagram.
I guess we now know why I can't read a schematic :o

Affectronix
02-21-2007, 06:17 AM
Schematics are much easier to read and follow then layouts when you understand them.

Heres how to do it:
1. Disconnect the Treble pot's middle lug (lug 2).
2. Attach lug 3 of a 1M Log pot to the middle lug of the Treble pot.
3. Attach lug 1 of the 1M Log pot to ground.
4. Attach lug two to the capacitor which you had removed the Treble pot's lug 2 from.

BTW- That 1M Log pot will be Master Volume. Very easy mod.

Good luck,
Kerry

vibroverbus
02-21-2007, 08:37 AM
Go to either the Metro amp site, or the Vintage amps BB, or the Trainwreck discussion board and look for posts by 'rockstah'. He's made several posts recently and put a layout of a good post-PI MV in many of them. It's basically the Ken Fischer MV (the one that replaces the bias splitters with a dual 100K pot) except you use a 250K dual-pot and add 2M2 resistors connecting the 'ungrounded' lugs of each pot. I think he got it from a builder in Germany (novosibir aka Larry). I had used this in one of my amps (I came up with it myself...only because I was very limited with what parts I had on hand...I guess necessity really is the mother of invention).

Unless you put something in place for 'protection' that is a really bad MV idea since if the pot wiper lifts it will drop bias and send your output tubes to the next world. (or is that the purpose of the 2M2 resistors?)

I just did my first 'cross-line' post-PI master volume on the other hand, and that is super easy, low part count, and sounds excellent. Because it doesn't interact with the previous gain stage like a simple attenuation MV, it doesn't have the tone impact that a lot of people complain about. Really is the best MV I've found so far.

wilder
02-21-2007, 08:47 AM
Vibroverbus,

Could you eloborate on the cross-line PPI MV? I'm not familiar with the term.

Thanks,

Chris

ChickenLover
02-21-2007, 08:54 AM
vibroverbus,
Yes, you are correct if the pot wiper 'lifts' then you lose bias on that power tube. But I've never heard of that happening nor have it happen to me. The 2M2 resistors would still provide a a bias but you'd be way, way over the max grid-to-ground resistance and it would probably be thermally unstable.

However, this MV sounds better than any I've tried and I've tried cross-line MVs (in several different amps) and I hate them...I think they sound terrible when compared to a dual-pot post-PI MV (just my stinkin' opinion :D).

Also, the MV call1800 is describing is a pre-PI MV and IMO is only useful if you're trying to get cleaner tones. If you're trying to get great Marshall crunch at lower volumes that MV is the worst choice. Again, just my stinkin' opinion.:Spank

ChickenLover
02-21-2007, 09:17 AM
Oh yea...two amps ago I put a post-PI MV in a homebrew and I only had a 1M-A dual pot handy. So I just increased the existing 220K bias splitters to 270K (this puts the net resistance to ground @ 213K). So now if the wiper lifts I only have a slightly increased ground reference of 270K. The taper of this MV is very good...very even.

However I did another similar MV but I did use two extra caps to isolate from the bias supply because it has Power Scaling and the bias voltage is extremely low when scaled back and I didn't want the MV interacting with it at all. I then took my "I prefer dual-pot post-PI MVs over cross-line MVs" a step further and installed a dual-pot post PI tone control instead of a Vox-style 'Cut' control (which is just a cross-line MV with a cap in series). I didn't contrast/compare but I really like this post-PI tone control for removing some of high-end hash from the PI distorting. It's just another dual 1M-A pot with two .0022uF caps. Below is a pic of that one; the little yellow caps are the .0022uFs and the bigger blue ones are .68uF caps (they're so big so they don't affect the bass too much if at all).

http://i108.photobucket.com/albums/n4/mtbucket_2006/PSXgutshot1.jpg

vibroverbus
02-21-2007, 09:29 AM
A cross-line master volume basically takes advantage of the fact that in theory the PI signals are perfect mirror images. You put a pot across those two signals, and as you dial the resistance down, phase-cancellation reduces the signal level.

There are designs for this both on the front of the PI (at the grids) and post-PI. Pre-PI the grid signals could have slightly more difference in magnitude so in theory you'd have a bit of distortion from imperfect phase cancellation, although you might like how that sounds too...

My latest hack looks like this - I wouldn't put it up as a perfect example (I probably didn't need .1uf blocking caps for the MV but as I replaced the old coupling caps at the same time, I then used the old ones for this purpose, and the 512k pot is about twice what it needs to be but again was what was in there & handy - I need to replace that with a 250k-ish value, it has a ton of unusable range).

http://j2c3.com/misc/YRM-Mod-MV.JPG

Here's another description of cross-line (as well as other MV techniques)

http://people.cornell.edu/pages/mt24/Amp/mm/mvols.html

vibroverbus
02-21-2007, 09:38 AM
Hey whatever works for you... in my build the cross-line is very very transparent, but my guess is that this is highly dependent on how accurate the PI is in the amp - amps with more asymmetry will be increasing that distortion as you turn the cross-line down.

I like the PI-stage tone-control idea, there's actually a lot of places you could take that... you say you have both .022 and .68 caps in there though - how do you have that set up?

hasserl
02-21-2007, 09:58 AM
I've used the crossline MV in a couple of amps and it has worked very good. I've tried it in a couple and it didn't work very good at all. When they don't work with a particular amp they sound terrible at low volume; brittle, harsh and thin and squashed, just awful. But when they work with an amp they work great and are extremely easy to install.

Affectronix
02-21-2007, 10:37 AM
My way was for Preamp overdrive. It works in the way that a JCM800 works.

ChickenLover
02-21-2007, 01:02 PM
My way was for Preamp overdrive. It works in the way that a JCM800 works.
Yes but the preamp isn't a JCM800 (220X) preamp and the Super Lead/Plexi type preamp circuits don't have enough gain to overdrive that gain stage/cathode follower pair enough. In the 220X amps there is enough gain to overdrive them pretty hard so that's why it sounds OK. IIRC, Marshall did use this on the very first 'master volume' amps. It's was a pretty common mod back in the day and people learned...in droves...that it just doesn't sound that good for that cranked non-MV Marshall distortion.

you say you have both .022 and .68 caps in there though - how do you have that set up?
The .68uF caps are for the MV. Basically it's the signal from the coupling cap to the right lug, left lug to ground, wiper to a .68uF cap which then goes to the junction of the bias splitter and grid stopper for that tube (then repeat for other tube/pot). The .0022uF caps and 1M-A dual-pot tone control are just simple 'treble bleeds' to ground (you can see that I just tapped off of the dual-pot MV).

Affectronix
02-21-2007, 01:11 PM
Ahh. Well it works for me. I can get some nice crunch there and its also great for turning up and adding an overdrive pedal. But yes, cranked it sounds much better.

VanR
02-21-2007, 01:42 PM
Schematics are much easier to read and follow then layouts when you understand them.

Heres how to do it:
1. Disconnect the Treble pot's middle lug (lug 2).
2. Attach lug 3 of a 1M Log pot to the middle lug of the Treble pot.
3. Attach lug 1 of the 1M Log pot to ground.
4. Attach lug two to the capacitor which you had removed the Treble pot's lug 2 from.

BTW- That 1M Log pot will be Master Volume. Very easy mod.

Good luck,
Kerry

I'm no expert, but this is the way I did on my 2 amp builds. Too easy.

samwheat
02-21-2007, 06:33 PM
i use this ppimv ....... http://blueguitar.org/ ...... under schematics and links ..... by bruce collins ..... about half way down the page

ChickenLover
02-22-2007, 06:35 AM
There was a time when I used a smallbox set clean and used pedals. With non-MV Marshalls the PI gets hit pretty hard by the preamp and is one of the first things to distort. Also, when there is a bright cap across the volume control the volume comes on really fast and sometimes you can't find the sweet spot on the volume control because it just gets too loud...too early. With that simple pre-PI MV you can turn down the signal hitting the PI and then turn up the preamp volume to find that sweet spot (i.e. perfect mix regarding the bright cap) and have a great clean tone that takes pedals really well. That's where I think the pre-PI MV shines. But IME a post-PI MV is best for getting crunch at lower volumes.

It appears the cross-line MV is very dependant on the balance of the PI and it seems that some amps (with a particular balance) do well with. It just never worked that well for me in the 3 or 4 amps I tried it in. Very 'grainy' (for lack of a better word) and not as fat.

vibroverbus
02-22-2007, 09:12 AM
The .68uF caps are for the MV. Basically it's the signal from the coupling cap to the right lug, left lug to ground, wiper to a .68uF cap which then goes to the junction of the bias splitter and grid stopper for that tube (then repeat for other tube/pot). The .0022uF caps and 1M-A dual-pot tone control are just simple 'treble bleeds' to ground (you can see that I just tapped off of the dual-pot MV).

ahh i dig it - you have BOTH the MV with coupling caps and a MV-tone with low-pass RC... cool. I'd like to try that MV tone just running out of chassis holes for pots!