Marshall JTM45 Combo design

[TuBo]

Hello all,

Relatively new to tube amp building and design.  Hoping some of you could take a look at it and let me know if you see any major problems or improvements on the layout.  Thanks in advance!

[sluckey]

IMO .040" aluminum is too thin for the weight of the iron in that amp. .080 or .090 would be much more sturdy.

EDIT... I just looked at your electronics layout. You're using a standard board layout but you have scrambled the tubes and the controls which will make your wiring look like a rat's nest, inviting all kinds of gremlins. Your layout is simply not logical. I advise you to stick to the tried and true layout for best results. It's a very good layout and you're not likely to improve on it. Especially with such extreme changes.

I'm sure you've seen a standard layout, but just in case, here's one that works very well. Study the way the signal flows in a logical way from right to left.

http://www.ceriatone.com/images/layoutPic/marshallLayout/JTM45ceriatone.jpg

[HotBluePlates]

I agree with Sluckey regarding the chassis thickness and layout issues.

Personally, I wouldn't want an amp made out of MDF. It's one thing for a home stereo speaker or car sub box, but an amp made from it with chassis, transformer and speakers would weigh a metric sh!t-ton. I also feel birch ply would be plenty stiff and strong, while being much lighter.

[TuBo]

Thanks for the replys!

hmmm, back to the drawing board.  What if I condense this to an 18" chasis instead of 24" bringing everything closer together, move the power & preamp tubes next to the can caps, and push the choke and board back? 

Why are the 10uf electrolytic caps on that layout showing the positive end going to ground, is that right?

I like the mdf wood, (even though heavy, my 212 cab is made out of it), it has a smooth surface, cheap easy to paint put a heavy coat of laquer on it with some bone corners, and its good enough for me, hehe

[HotBluePlates]

I like the mdf wood, (even though heavy, my 212 cab is made out of it), it has a smooth surface, cheap easy to paint put a heavy coat of laquer on it with some bone corners, and its good enough for me, hehe

Fair enough if you like it.

Why are the 10uf electrolytic caps on that layout showing the positive end going to ground, is that right?

Bias filter caps; the output voltage is negative.

What if I condense this to an 18" chasis instead of 24" bringing everything closer together, move the power & preamp tubes next to the can caps, and push the choke and board back?

The big problem (to me) is swapping positions of the input jacks and power/standby switch.

That puts the Presence control next to the input jacks, begging for oscillation. That's because relatively big signals from the speaker/feedback loop are right next to low level, high impedance grid wires for your first gain stage.

Also, you have most things on the chassis (tubes, jacks/switches, big part locations) in a mirror-image, but you didn't flip the board or the controls. Like Sluckey said, you'll build it and then wonder why you're having weird stability problems, buzzing or distortion on weak signals (if not outright howling). You're also having to run high voltage a.c. past all your controls to get to the bias circuit on the input side of the chassis, which will probably result in a hum you won't get rid of until you move the bias circuit back over near the PT and keep a.c. wiring away from the controls.

I STRONGLY encourage you to copy the known working plan. Did you just need input jacks on the right side of the amp instead of the left? Keep in mind that Marshall amps, when built with tubes hanging down in a combo like a Fender amp, have the input jacks on the left side just like a Fender.

[sluckey]

What if I condense this to an 18" chasis instead of 24" bringing everything closer together, move the power & preamp tubes next to the can caps, and push the choke and board back?

That amp should fit easily in a 18 x 6.5 x 2.5 chassis. I can't emphasize enough though... DON'T REARRANGE THE STANDARD LAYOUT.  There's a reason why everything is arranged as it is. If you need input jacks on the other end of the chassis then mirror image everything.

Why are the 10uf electrolytic caps on that layout showing the positive end going to ground, is that right?

Because that's the way they must be connected. They're in the bias supply. Those same caps on your board are wrong. They will quickly self destruct when you turn the power on, and likely cause some other damage too.

[TuBo]

awesome, thank you for all your help! workin on layout now

[TuBo]

Ok, think this will work??  heres the new layouts, thanks again for the help

[sluckey]

That's much better.

Here are some changes I would make...

1. Stack the input jacks 2 over 2. This will allow the other controls to move to the right a couple inches, shortening the wires to the board. That's a good thing.

2. Move the PT up closer to the front of the chassis to be inline with the OT and move the GZ34 and cap can to the back side of the chassis to be in line with all the other tubes.

Two wiring errors jump out at me. The bias caps need to be connected to ground, not the presence pot. And the presence pot needs to connect to the 27k resistor (move blue wire two turrets to the left). I didn't look for any other errors.

I'm curious as to why you want to build a mirror image layout?

[TuBo]

crap, thanks.  Im wanting to put this into a cabinet as a combo.  The layout I included is what I had in mind.  If I move the pt forward, it will hit the mounting boards, thats why I have it to the back.  Do you really think it will hurt that much if I leave it in the back?  What do you mean mirror, cause I kinda laid it out ass backwards, and have the text written in reverse?  Just the way I was lookin at it at the time, lol, didnt want to start over from scratch, however I pretty much did anyway.  Its fun though!

[TuBo]

I guess I can move it up, and move all 3 transformers left so it will clear the mounting board.  I still have some room to squeeze the jacks together, and move the pots closer to the board.  Easier for me to work with.  Im the same with working on cars, if its older than 1978, I dont want nothing to do with it.

[sluckey]

Do you really think it will hurt that much if I leave it in the back?

No. That's fine. I knew you probably had a reason. The important thing is that now you have the board, control panel, and tubes oriented in a logical order, but your layout is very workable now. My personal choice is to put all tubes and cap cans in line across the back of the chassis, like these examples...

http://home.comcast.net/~seluckey/amps/lightning/l4.jpg
http://home.comcast.net/~seluckey/amps/november/nov_06.jpg
http://home.comcast.net/~seluckey/amps/warbler/w_09.jpg
http://home.comcast.net/~seluckey/amps/revibe/revibe_12.jpg

The above examples are some of my more recent projects. I've kinda settled on that general style because I know the layout works well and it's easy to wire everything. No better or worse than other good layouts, just my preference.

Mirror image...If you displayed a standard layout on your monitor, then held a mirror up to the left side, the image in the mirror would be exactly reversed, left to right. Most graphics editors have a 'mirror' or flip option. I assumed that's how you ended up with some backward writing.

[TuBo]

Your the one that made that warbler, is that what you called it??? I was listening to   some clips of that just the other day, didnt realize that was you.  Loved it!!  My old man built me a hybrid univibe/reslie tone, and I have been in love with that type of sound ever since.  You ever build the tubo-vibe??  I think the guy that designed it was named scott swartz maybe.  THat is one I would like to check out too.  I saw the schematic in an old magazine, vaccuum valley or something.   Anyway, nice work!