Getting a head of myself - Marshall 2204 JCM800 inspired build

[kuDo]

Hello everyone,

As the title of this thread already explains, there are two things going on.
I am planning to start a build of a JCM inspired head and by starting this topic, I am getting ahead of myself (Yes, sad pun intended)

I joined a couple of weeks ago and I started my first post here:
Merlin's 12AU7 effects loop https://ampgarage.com/forum/viewtopic.php?f=6&t=33865
The big idea was to learn a thin or two, and I did.
The topic was a about a effects loop that I found on the internet and which I would like to incorporate in my upcoming build.

I want to use this thread as a log of my proceedings building this amp.
Hopefully it will be fun to read for someone out there and maybe even useful.
In the end I hope to end up with a working head that I have build myself. But the main objective is that I wand to learn about valve amps. By attempting to build one, I will have to face a lot of different aspects of valve amps so it is as much about the journey as the destination...

[kuDo]

As I am already working on this a while, I thought it would be nice to share my first drawing of the amp and how I am planning to build it.

I started out with a schematic I found on the super nice website of Rob Robinette.
There, I already red a couple of things about the amp. After that, I did a lot of calculations and loadline drawing to see how the amp is working.
I had the feeling I was starting the get a rough picture of this schematic, so I decided to convert the schematic into a layout drawing of the amp.

Trying to create a conversion of the schematic made me realize that there is a border between theory and reality. A border that took me some time to cross.
But eventually I finished, and I am really curious of what you guys think of it. It probably has already been done a million times, but I want to have my own shot at it.
It's the only way I will learn. So thanks for your patience

[pompeiisneaks]

Very cool indeed, do you have the schematic as well? Or just a layout? Often times people don't realize things they've missed because they don't have a schematic as well. The layout is more just a builders cookie cutter recipe, but the schematic shows exactly how the amp should be done correctly to function.

~Phil

[xtian]

I didn't examine your layout carefully, but one thing jumps out at me. Shove the main turret board to the left an inch, in order to shorten the yellow wires running to the PI.

[brewdude]

Looks like all the wires end up going to and from that secondary eyelet board... it seems excessive to me (perhaps there is some advantage I haven’t yet noticed)... I think a few tag strips to anchor grid resistors would be more economical and not fill up as much of the interior volume.

[kuDo]

Very cool indeed, do you have the schematic as well? Or just a layout? Often times people don't realize things they've missed because they don't have a schematic as well. The layout is more just a builders cookie cutter recipe, but the schematic shows exactly how the amp should be done correctly to function.

~Phil

Hi Phil,

I do have the schematic, it’s been retrieved from Rob Robinette’s site.
I translated it into a cookie cutter recipe myself.
I did however add a effects loop, switchable Cathode resistor for the second stage and a elevated heater supplie.

This is it

[kuDo]

I didn't examine your layout carefully, but one thing jumps out at me. Shove the main turret board to the left an inch, in order to shorten the yellow wires running to the PI.

The length of the yellow leeds are caused by the fact that I added a effects loop in between the stages. In an earlier version, I had a separate board, but I decided to insert it in the supply chain I had going.
But I agree, the length of those leeds are a real eyesore.

[kuDo]

Looks like all the wires end up going to and from that secondary eyelet board... it seems excessive to me (perhaps there is some advantage I haven’t yet noticed)... I think a few tag strips to anchor grid resistors would be more economical and not fill up as much of the interior volume.

I was thinking of using tag strips at first, but then I though I could aswel use a piece of turretboard.
The thinking behind it was to get the wirring to the valves neat and organised. Furthermore I was not a fan of the wires flopping arround when they are as long as the seem.

[romberg]

Nice looking layout! I agree with xtian in that the yellow (and to some extent blue) leads kinda jump out at you. If it were me I might try moving around components on the main board to move them closer to the tubes they are connected to. This is not fun to think about after you have finished a layout. But it is also much easier to do than after you have actually built the amp.

I've done a few amps with custom layouts I've drawn up myself. And it has always taken me a few revisions (or more) to get things to a state I can live with. This is the time to do it. You will be much happier with the end result.

Mike

[kuDo]

Nice looking layout! I agree with xtian in that the yellow (and to some extent blue) leads kinda jump out at you. If it were me I might try moving around components on the main board to move them closer to the tubes they are connected to. This is not fun to think about after you have finished a layout. But it is also much easier to do than after you have actually built the amp.

I've done a few amps with custom layouts I've drawn up myself. And it has always taken me a few revisions (or more) to get things to a state I can live with. This is the time to do it. You will be much happier with the end result.

Mike

Hello Mike,

Thank you for your reply! You are right. I think I will try and redo it.

The current layout is guided by a rule that I set for myself. I could have been a lot more compact, but I have been sticking to the plan of having the positive supply “lane” going from right to left on the top of the main board and the ground on the bottom side.

Durring the lay out I have been very conscience about where each part of the stages are connected to these power supply. “lanes”. So all the noisy parts are starting on the right and the most sensitive parts are on the left.
The grounding scheme has in a way been the driving force.
Nowhere in this layout has there been a ground connection that hasn’t been thought of. All very deliberate.

I went this route after reading the grounding chapters of Merlin’s amp design book.

Today I will be having some spare time, so I know what I will be up to

[pdf64]

How is it that the 47uF HT caps on the main board are half the diameter of those used for the g2 node and reservoir caps?
With such a surfeit of turrets in there, it’s weird that some components are still left with a leg dangling, eg cold clipper cathode switch, coupling cap to master vol.
With only 2 more common cathode gain stages, a 39k unbypassed cold clipper cathode resistor may reduce gain too much.

[kuDo]

How is it that the 47uF HT caps on the main board are half the diameter of those used for the g2 node and reservoir caps?
With such a surfeit of turrets in there, it’s weird that some components are still left with a leg dangling, eg cold clipper cathode switch, coupling cap to master vol.
With only 2 more common cathode gain stages, a 39k unbypassed cold clipper cathode resistor may reduce gain too much.

The 47uF HT cap are meant to be as big as the reservoir caps, but for the sake of visibility I, for now, have reduced the diameter. So when it is time to build I know what resistors are place next to it.
I figured it should just be possible to fit it all.

Well, no shortage of scrutiny so far. Which is really nice. The resistors for the cold clipper, were added last minute and to be fair. This might not be the best solution. The thing is that the turret board is already a little to wide, so this was my way cheating a little.

Thanks for the eye openers!

[kuDo]

After a lot of back and forth, I now came up with a layout that is a lot more satisfying.

There still seem to be a couple of long leads, but I thing that will be unavoidable.

Thanks for pointing me in the right direction.

[xtian]

Looking good! (Again, not looking hard at details, sorry, just overview).

[kuDo]

Looking good! (Again, not looking hard at details, sorry, just overview).

Thank you. No need for the apologies, I am really glad with your advice.

If there are no more issues that I can find, the next step will be making the turret board