Plexi 50 ! Work in progress !

[uki]

 Hey guys!

What would be the circuit board dimension for the Plexi 50?
I've searched several other places but I didn't find some solid numbers, there are several that goes around 3"x11" something like that.
With this tool is possible to do a very thin cut with around .03mm.

related topics:
http://el34world.com/Forum/index.php?topic=20556.msg216668#msg216668
https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697

Thanks in advance !

[HotBluePlates]

3.125" x 11.25"

The actual length is in the Hoffman store page where the boards are sold.

[pompeiisneaks]

But is uki asking for the size of the original plexi 50 or the hoffman version?  (Or am I lost and the plexi 50 is a hoffman only invention).

I know the plexi was the older marshalls, but don't know if there was a 50watter.  I know a little about vox and fender but next to nothing yet about marshalls ;)

~Phil

[uki]

@HBP Thanks sir !

But is uki asking for the size of the original plexi 50 or the hoffman version?  (Or am I lost and the plexi 50 is a hoffman only invention).

I know the plexi was the older marshalls, but don't know if there was a 50watter.  I know a little about vox and fender but next to nothing yet about marshalls ;)

~Phil

Thanks Phil !

Well an approximate size would do, not really looking for an exact size, since I'm custom building it, and I'll have to do with what I got, which is a celeron sheet with 22"x10" gotta make it be enough for at least 2 boards. I think the board will have around 14" x 4" , I got some bulk components from the old amp where I salvaged the parts, so not sure if will be possible to reduce it some inches.

Got a pic for an approximate dimension of the board.

[Ed_Chambley]

My original has a perfboard that measures Length 11.75" x Width 3.125" x Thickness .125"

• Hoffman's board are longer as he adds components, but if you want the original look there is where to buy.  Prices are good as well.
  
  http://www.kurtsequipment.com/reproboards/indexwithbin.htm
  

[HotBluePlates]

But is uki asking for the size of the original plexi 50 or the hoffman version?  (Or am I lost and the plexi 50 is a hoffman only invention)

The term "Plexi 50" is a modern invention; Marshall never called their amps by such a name in the 60's when they made amps with plexiglass faceplates. So that was the tip-off Uki wanted to know about the Hoffman board by that name.

[Paul1453]

But is uki asking for the size of the original plexi 50 or the hoffman version?  (Or am I lost and the plexi 50 is a hoffman only invention).

I know the plexi was the older marshalls, but don't know if there was a 50watter.  I know a little about vox and fender but next to nothing yet about marshalls ;)

~Phil

http://www.premierguitar.com/articles/1968_Marshall_50_Watt_JMP_and_4x12

[pompeiisneaks]

Thanks that was great info!   I liked listening to the sounds in the video first, pretty amazing what the old plexi could do.  I may have to try that build soon :D

~Phil

[uki]

Thanks guys !!

I got the board cut with 3"x14¹/2" , working on the templet now to make the holes for the turrets.

As you can see in the pic I have shaped and sharped(teeth) a special tool to cut the celeron sheet.

Here some pics:

[pompeiisneaks]

Very cool uki

[PRR]

> I have shaped and sharped(teeth) a special tool to cut the celeron sheet.

FWIW--- my home-store sells a tool, I own one, I use it, it scratches pretty well.
http://www.homedepot.com/p/Plastic-Sheet-Cutting-Tool-GE-41/202038073

I do not like the 5-buck price for essentially a hacksaw blade with a sharp nick ground on it.

[uki]

> I have shaped and sharped(teeth) a special tool to cut the celeron sheet.

I do not like the 5-buck price for essentially a hacksaw blade with a sharp nick ground on it.

I did make the tool myself with a piece of old hacksaw blade, turned it into something usable! That is 2 tools for the price of one !
The handle is missing, gotta make one, for now it is a piece of cloth rolled in the blade !!   

Very cool uki

  

 I'll keep posting the progress, looks much easier now as it is my second build, the 1st one was questions and search for answers all they way.

[uki]

Some progress with the board, got it drilled ! More updates soon !

[uki]

Which Marshall model is the Plexi 50 based on ?!

Cutting the "turrets" pic !

[sluckey]

JMP 1987

[uki]

JMP 1987

Thanks Sluckey !  Is the JMP 1987 based on the old JTM 45 ?

[sluckey]

I believe all the big tube Marshalls have roots back to the JTM 45, which is just an euro style Fender 5F6A.

[uki]

Board ready for components !!

Check it out:

[sluckey]

Did you really cut all those turrets with that tubing cutter? How are they attached to the board?

[uki]

Did you really cut all those turrets with that tubing cutter? How are they attached to the board?

Yes I did    lots of work, I got a tool(look post #13) made where I put the turret inside and then I punch it into the hole in the board, the copper pipe is 1/8 and the holes as well, it goes tight in, then in the other side I do punch the bottom of the turrets to expand like a rivet to stuck the turrets.

Total cost of the board less than 10 bucks or 2,5 US$. 

[uki]

The fun begins !!!

Some components already in place !!

[Willabe]

Looking real good Uki. 

[pompeiisneaks]

Yup that looks great!

~Phil

[uki]

Hey thanks guys, there is lots to do yet!

Here another pic, figuring out the outer foil before soldering the caps.

Found this post on another thread, didn't want to hijack:

Have you tried jumping the two channels together?

I have two plexi style amps, my November and a 6V6 plexi. Both have preamps that are an exact copy of the JMP 1987 plexi circuit. The bright channel is very bright, almost brittle, ice pick kinda bright. The normal channel is too dark and muffled. Neither channel sounds good to me until I jump them together. Then I can blend the bright with the dark to get a really full sound, plenty of tight bottom and plenty of top end highs too. Both of these amps now only have one input jack and the channels are permanently jumped together.

I'm planning to use only one input jack does it change much the circuit , how does this jumping the channels work ?

[pompeiisneaks]

If you're using one input jack, then you can't jumper the 'two channels' you're building a single channel amp no?  Or if you are using a dual channel setup with one input and a switch, then you'd likely need to have a three position switch, something like
1. channel 1
2. channel 2
3. channel 1 & 2

That would feed the instrument input into both preamp grids for two channels.   

I seem to recall, though, that this was a one channel design. 

Basically you need two channels to jumper them.

Edit: above is all wrong, sorry... Basically i realized that the circuit is two inputs and you meant you want just one input jack per channel.  In that case you can provide yourself a switch that jumpers them together somewhere in the design, so that it takes input from say input 1 and passes it over to input 2 grid as well.  I'd think your best bet would be to connect the tip of the one to a switch straight to the tip of the other, simple on/off type, and if you 'jumper' them, it sends to both tip inputs.  But some here may have a nice picture of the best way to do something like that.  It's easiest to just provide 4 inputs to 2 channels and jumper that way normally.

~Phil


~Phil

[sluckey]

Using one input jack and internally jumping it to both channels is easy. Look here...

     http://sluckeyamps.com/6v6plexi/6v6plexi.pdf

[uki]

Using one input jack and internally jumping it to both channels is easy. Look here...

     http://sluckeyamps.com/6v6plexi/6v6plexi.pdf

Thanks Sluckey !

Looks easy, here a picture, just replace those resistors(33k to 68k) and send the jack input to both sides of the first tube.

[uki]

Hey guys could you give a little help with outer foil side of the caps orientation?!, I've marked most of them in the picture with a green dot but some of them I can't figure it out, I did follow the info on this page.

Where to connect the outside foil?

    The proper way to connect the outside foil is to the low impedance side of the circuit, which, in the case of coupling caps, will normally be the plate of the previous stage. If it is a bypass cap to ground, connect the outside foil to the grounded side. If it is a bypass cap from a signal to B+, connect the outside foil to B+. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.

Thanks

[sluckey]

I like to put my caps so the writing is all pointing in the same direction.

[uki]

I like to put my caps so the writing is all pointing in the same direction.

Yes I could do that but I want to understand the ones I'm missing, I still can't figure it out. The caps for treble, middle and presence, how and why the caps outer foil would be placed ?

[PRR]

I'll go further than Sluckey. Most caps should be positioned to "read right side up" to the repair technician.

The M and B caps are often 0.050uFd, 50,000pFd. The stray capacitance to the rest of the world is surely less than 5pFd. If you piss in the river, it makes no difference which bank you piss on; 0.05uFd is a "good short" relative to any stray capacitance.

The T cap is often in the pFd range. In my experience we like mica or ceramic here; these typically don't have "marked foil". If you do use film, outer foil toward the tone-driver tube plate; this is clearly lower impedance than a half-up treble pot.

[uki]

Thanks PRR!

Not bothering with it, if it doesn't make any difference!!

Different question, my PT have CT for the heater wires, but in the schematic it does use those two 100ohms resistors to ground, which one should be used?

[mresistor]

Uki - folks here recommend using the 100 ohm resistors, which offers some protection for the PT in the event of an internal tube short. Just trim the center tap or fold it up and tuck it away.

[mresistor]

Most caps should be positioned to "read right side up" to the repair technician.

PRR - I've come across work by some technicians or "tube gurus" where they have purposefully put the data side down with their caps, to inhibit people from seeing the values of those components. For instance if someone were to take a pic of the circuit and post it on the internet. I also had one of these amps to work on here at the house, and the caps name info etc was all face down. I had to de-solder to ascertain their valaues. I wonder why some people are so protective, bordering on paranoia, but I suppose they have their reasons to protect their work. Oh and the caps were also glued to the board .


As for foil direction, there is this thread Uki - http://el34world.com/Forum/index.php?topic=11427.0

[uki]

Uki - folks here recommend using the 100 ohm resistors, which offers some protection for the PT in the event of an internal tube short. Just trim the center tap or fold it up and tuck it away.

Oh I see why now to use the artificial tap , thanks for clarifying !!!

About the labels, I have seen some circuits with IC labels sanded away ! Human knowledge belongs to the human specie, not to one individual alone !

[uki]

Hey guys !

Finally I've got some aluminum plate at fair price, got couple pics about how the layout could be, my concern is to not put the cap cans bellow the circuit board, what do you guys think?

After looking at this pic/link, I've added two more pics(layout_02, layout_03) with different layout options. Does this make sense ?

[sluckey]

After looking at this pic/link

That pic/link mostly follows the original plexi layout. I highly recommend you make your chassis layout look just like that. Better yet, make it look exactly like a real plexi. There are plenty of pics on the net.

I definitely don't like your pics that have the filter caps near the input jacks. A couple of those filter caps have some high level noise and hum on them.

[pompeiisneaks]

agreed, put the filter caps near the power input section so they are at the source of the noise they're filtering.   Looking good though.

[uki]

Thanks guys !! I have found this pic of the same type of amp, I'll follow it, the cap can near to the choke is for the preamp section yes?

[pompeiisneaks]

Although I don't know the amp, yes that looks right.  Often there would be 2 cap cans each and they'd use the two on the right for primary power filtering and the one on the left for a few stages of preamp filtering, just due to location, Someone that knows the plexi 50 layout a bit better could confirm.  Here's someone's plexi layout that has the same kind of "two cap cans for power filtering, one for preamp filtering" and they do a similar layout except the cap for the preamp is on the turret board.  http://s77.photobucket.com/user/patlinux/media/MetroAmpPlexi.jpg.html

that way the 'noisy' power entry area is cleaned up before even getting close to the power stage of the tubes, and doesn't even remotely approach the input side or preamp section.

~Phil

[uki]

Got some work done with the chassis today here a few pics !

[Joel]

Uki, using 1/8 copper tube as turrets is the most brilliant idea I've seen in ages!  I don't know if it's your original idea or not, but it's bloody brilliant.  And I'm now frustrated that I didn't see this thread two weeks ago when I made my eyelet board for my AB763 build.  Those copper turrets look great.

[uki]

Uki, using 1/8 copper tube as turrets is the most brilliant idea I've seen in ages!  I don't know if it's your original idea or not, but it's bloody brilliant.  And I'm now frustrated that I didn't see this thread two weeks ago when I made my eyelet board for my AB763 build.  Those copper turrets look great.

Hey man thanks, yeah it is my idea to use 1/8 copper pipes as turrets, I'm quiet pleased with it, and the result is very good, I thought about use 1/4 but looks to be too much, there are no 3/16 which would be perfect, sometimes 1/8 is too small if 3 components goes in the same turret. I have done some more about a year ago when I built a Fender 6G8!
You gonna need few things to build the board this way, pipe cutter, thin pliers, and the special tool (post #13) to punch the turret in the board , it have a hole about 7 or 8 millimeters deep and the hole must be a little larger than 1/8 or the turret will get stuck inside when puching the other and to rivet it, I cut the turrets with about 10 millimeters, the celeron sheet I'm using is 1 millimeter thick. It is a lot work to do, cut all the turrets, and punch it and rivet each one of them to the board but it is very cheap this way, at least for me here where the regular ones are not available and are expensive buying online from far way.

[uki]

Things are starting to looks like an amp !!!!

[pompeiisneaks]

outstanding, making progress on my own, it's a blast :)

~Phil

[uki]

This one is been a slow progress, but things are going well so far, here some update, chassis almost done, a few more holes to do, I'm waiting the weather to warm up a little more, it is better to work with painting stuff,  so I can do the front and back panels.

Also I got a nice idea for the B+ fuse, check it out!!

[Willabe]

Also I got a nice idea for the B+ fuse, check it out!!

Nice! 

Build is coming along very nicely and looking good. 

[sluckey]

How are those turrets attached to the board?

[kagliostro]

Ciao Steve

Reply #19

Franco

[PRR]

Jul 12> turrets ... How are they attached to the board?
Nov 4> How are those turrets attached to the board?

3-month echo in here... here.... here...............