Custom 50W plexi chassis_Please critique layout

[RobBozic]

Hi,

I'm putting together a 50W plexi, based on Doug's "hotswitch" schematic. I've already got the transformers, the PT is a Hammond 290GX and 159S Choke, with a custom wound OT. I'm getting a custom made chassis from Ceriatone with my own layout for the hardware.

Can someone have a look at my placement of the transformers, filters caps, and tubes. Any advice is greatly appreciated.

Thanks in advance
Rob

[DummyLoad]

the pot between the input jacks... that's not gonna work out too well.


--pete

[sluckey]

I would swap places with the choke and OT (just like the original). Why do you have 5 cap cans?

[RobBozic]

Thanks,

the pot between the input jacks... that's not gonna work out too well.


--pete

Pete, it's there for flexibility. So it's either 4 inputs, OR, 2 inputs and the hotswitch adjacent to the two vertical inputs.

Regarding the cap cans, it's one for each stage, thinking of having them externally mounted as opposed to internal. Not a big deal, will probably put them inside.

Thanks
Rob

[Willabe]

If it's a pot it won't fit between the 2 input jacks.

And depending on what would it be wired to it will cause problems. The input signal can easily be infected by larger signals because the signal coming from the guitars PUP's is soooo small.   

[EKDENTON]

I was looking at hoffmans plexi 50 and I only see 6 pots, your layout has 9 holes for pots, what are the three extra holes for? Maybe you could delete the one hole between the inputs and use one of the other holes for the switch?

[DummyLoad]

If it's a pot it won't fit between the 2 input jacks.

And depending on what would it be wired to it will cause problems. The input signal can easily be infected by larger signals because the signal coming from the guitars PUP's is soooo small.

brad, i think he means that if the pot is used, the two input jacks are covered by a faceplate. if the pot is not used, you can make it a 4 input amp and the faceplate covers the pot hole.


--pete

[Willabe]

Hey Pete, ya think you could lend me that monkey of yours for a while so he can teach me how to understand what I'm reading?         

[DummyLoad]

Hey Pete, ya think you could lend me that monkey of yours for a while so he can teach me how to understand what I'm reading?         

   monkey sez okey dokey but bring bananas...


--pete

[Willabe]

Deal! 

[kagliostro]

Here they are


       


 


Franco

[Willabe]

     Dancing Italian bananas. 

[PRR]

> bring bananas...

You can't ride a bike with a banana in your pocket.

[jojokeo]

    Dancing Italian bananas.

Need hot fudge & vanilla ice cream.
 
Rob: only my opinions - don't drill those position holes for the pots, they're a waste of time and effort. 4 input jacks are outdated, a waste of space, parts, & money. I would move the PT towards the back of the chassis and cap cans in front nearer to the switches where they will be run to. Don't turn the PT - you don't want the laminations close and pointing towards the EL34s. I'd space the preamp tubes a bit closer to each other then move them in & towards the power tubes a bit so that the OT could be placed in that far back corner area. This provides a number of advantages; better weight balance, isolation from the PT, don't have to worry about it's lamination's positioning, and it's far enough away from your input jacks and eyelet/turret board to not cause issues there either. The circuit board will be positioned more directly in front of your control pots and near & between your tubes. Overall your lead dress should be shorter and more direct and less possibilities for negative issues.

[RobBozic]

So, have the OT on the opposite end of the chassis from the PT, and move the preamp tubes closer to the front panel like a Soldano? If so, I would have thought the OT would be too close to the input jack

[sluckey]

So, have the OT on the opposite end of the chassis from the PT, and move the preamp tubes closer to the front panel like a Soldano? If so, I would have thought the OT would be too close to the input jack

No. Put the OT next to the PT. Look at a pic of a real plexi. Or just follow the Ceriatone layout, which closely follows the original layout.

[jojokeo]

So, have the OT on the opposite end of the chassis from the PT, and move the preamp tubes closer to the front panel like a Soldano? If so, I would have thought the OT would be too close to the input jack

Yes on the first part and no, since it's towards the back of the chassis it's not close to the input jacks at all.

[RobBozic]

So, have the OT on the opposite end of the chassis from the PT, and move the preamp tubes closer to the front panel like a Soldano? If so, I would have thought the OT would be too close to the input jack

No. Put the OT next to the PT. Look at a pic of a real plexi. Or just follow the Ceriatone layout, which closely follows the original layout.

I always thought it would be better to have the choke between the PT & OT. I know the originals weren't done this way, however most of the aftermarket Marshall clones are done this way because they want the layout / aesthetic of the originals.

Thanks
Rob

[jojokeo]

Do what you like or do what works for you but there's no single way to do something. I find that just because somebody else does something a certain way doesn't mean it's the best working or best looking or best tasting. By that I mean if everyone copied each others' BBQ technique and recipes does that mean there's only one way to BBQ a brisket or tri tip or even a hamburger? Of course not. In the end what matters most is what you like best for your own taste and reasons.

Look at the photo and the transformers. When this amp came to me it made all kinds of noise so badly that the ppimv could not even be turned up half way. The OT's laminations were both mounted in the same direction. Turning it 90º helped solve the issue but it didn't go away completely until both proper grounding AND effective shielded wiring was implemented. Using my "listening device" set-up I was picking up noise from the PT just a tad even that far away and properly oriented. But I had no choice as to leave it there since the amp was already built that way along with all the other stuff. However, the end resulting floor noise is extremely quiet w/ two gain stages and with all four going it's perfectly normal in relation to power applied. All controls can now be dimed without the crazy oscillations screeching too. This is only an example of one guy's recent experience. The higher the amp's power the more important are the things described to you.

[RobBozic]

Thanks jojokeo, I agree with what your saying.


Thanks for your help.


Rob

[sluckey]

The reason I suggested following the original layout is because it's a proven layout that works very well. Of course there are plenty of other ways to build an amp. My reasoning behind swapping places with the OT and choke is to get the OT closer to the output tubes and farther away from sensitive preamp board circuitry.

[RobBozic]

Thanks Sluckey,


My chassis is also 3 inches longer than the standard plexi /800 chassis, so the OT is further away from the preamp.


Thanks again
Rob

[jojokeo]

My reasoning behind swapping places with the OT and choke is to get the OT closer to the output tubes and farther away from sensitive preamp board circuitry.

Steve I preface this with ultimate respect for you and your huge knowledge base. You are an icon on this site.
 
Please look at my photo above - where is the OT positioned? That's right, it is directly above the circuit board and right over the pots. It couldn't be any better wrongly positioned unless it was installed directly over the input jacks. True it may "work" and be "proven" but that doesn't make it correct or right or even preferred...I see it as a recipe for problems especially for lesser experienced individuals with limited knowledge of lead dress and trouble shooting skills.
 
Rob appears to have a choice here since his chassis and circuit isn't already pre-fabricated and laid out so no need to take an unnecessary risk and at the same time he (and others) can learn something on how to better his build and future builds.

[sluckey]

He asked for a critique and I gave it. My preference to swap positions with the OT and choke is a proven design. And would be just a very slight change from his proposed layout. My suggestion is in no way meant to be "THE ONLY WAY TO DO IT".

If you gonna build a Plexi, you can't go wrong by following Jim Marshall's plan. If you want to BBQ a brisket, you'll have a yum yum moment if you follow the Steven Raichlen plan.

[mresistor]

regarding BBQ - oh yeah, Steven is the wizard when it comes to heat and smoke and meat.  Lordy!  But hey, how about Sterling Ball?

[sluckey]

There are plenty of other great BBQers out there. Steven probably ain't even close to the top. But he has a great writing style. And a great presentation. How can you go wrong with 40 acres of smoking' Webers? 

[Ritchie200]

I don't think either one of you guys know what the heck you are talking about.  I guess I will have to give a tutorial on proper transformer orientation and mounting techniques.

Jim (sigh....)

[mresistor]

I'd be interested Ritchie.                                      https://www.bigpoppasmokers.com/

[sluckey]

You kinda late Jim. We talking about grillin'!

[Ritchie200]

I'm talkin about grillin' Jojo!

Jim

[Ritchie200]

I'd be interested Ritchie.                                      https://www.bigpoppasmokers.com/

Imagine if you will...  A mere lad of 10, building a head to enclose the hi-fi amp his dad converted for him for guitar.  Measured once, cut once (with a hand saw even), nailed together, added faceplate to enclosure (yes, I know, it works better on the chassis...), covered with black vinyl, and added black grill cloth above the faceplate so I could see the tubes glow! In a dark room.... Ok in a very dark room...., All trimmed out with a handle and rubber feet.  So, I slid the chassis into my head for the very first time in triumphant fashion and....what the heck!  How did this power tranny get so tall!  It wasn't like that when I first measured it?!?!?!  Soooo I got to thinkin.......  I aint gonna build another head, this thing took me a week!!!!  Hmmm, ya know, that tranny isn't as wide as it is tall...  If I take the screws out, maybe I have room to lay it on its side?  A few yards of that cool radio shack red vinyl tape to properly secure it, and walah.  Haha!  Never changed it and it's almost 50 years old now.  Went on three Midwest tours with a church group I was in back then so it is proven road worthy!!!  Quiet as a church mouse and it still cracks me up when I look at it.

I can provide specific drawings showing orientation and taping points for those interested.  I accept PayPal.

Jim

[jojokeo]

Wow that's some seriously good Radio Shack tape. I wonder if it could pass for ding repair tape? I used to think I was BBQ'ing when in fact I was only grillin' and chillin' like a villain. I'm a grilling master - but this past x-mas I got my first smoker...I'm a total rookie with that thing. It's fun and kinda like using my rotisserie, once you get the prep work out of the way it's pretty much set it and forget it.

[Paul1453]

That looks like one of my builds, Ritchie!   

I'm usually very happy if my duct taped together amp plays and sounds good.

After seeing all your high quality builds, I've become a little self conscientious of mine.

I'm hoping to someday make something as pretty as your guys builds.