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PCB design for guitar amps
#1
Hi Guys

Printed circuit board (PCB) use is not new to guitar amp fabrication; just look at Marshall amps built since 1965 or so, or to Ampeg amps from the same era, let alone countless other brands.

PCBs offer consistent unit-to-unit performance by eliminating variations of stray capacitance and stray inductance - the "parasitic" elements of the physical construction.

There is NO reliability difference between a properly laid out and assembled PCB and a properly laid out and assembled hand-wired amp.

We will discusss using PCB design software such as Eagle, and the basic things to be mindful of in using PCBs for guitar amp construction - really, for construction of any device.

Have fun
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#2
My main issue with Eagle is that in order to make a PCB that is large enough for a moderate sized tube amp, you have to purchase the uprated versions of Eagle, and they can get quite expensive. That is usually the case with that type of software though so I suppose it is to be expected. My version of Eagle I purchased in 2012 for school to do project PCB's, and while it works well for that, I am limited in the size of a PCB that I can do. Anything much larger than a notebook and it won't allow me to do it. Another frustration is with creating new parts in it. The process is sort of obtuse.

Greg
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#3
Hi Greg

"I feel your pain"

Eagle was designed back around 1985, as were most spice and PCB CAD systems, with continuous updates since then. Version-4 line persisted until about 2006 when they brought out Version-5 - now they're up to 7 or more. Fortunately, the version-4 that I bought still works and I think it was all you could get from them at the time, or it was the Pro package. Anyway, it allows massive board sizes, almost unlimited library, and so on.

The trial version was always crippled but maybe for the new versions they have crippled it even further? How big a board do you call a "note book"?

Making library parts can be a bit difficult for sure, because Eagle has an antiquated core. It was designed for laying out computer boards, so certain functions have names appropriate to that, such as "Gate swap". This allows you to swap identical sections within a part in the schematic, allowing an easier layout. For example, in a tube amp, a 12AX7 has two triodes and they will be dropped into the schematic in the order of A then B, and these might correspond to pins 1,2,3 and 6,7,8. In the actual layout depending on which way the tube faces, you might want the signal to hit the B section first then the A side, so doing Gate-swap in the schematic fixes that for you.

As far as making the parts, that is a whole thread.

As far as laying out your board to fit the card sizes you have, you might have to split the circuit into smaller portions that could be used as functional blocks to build a variety of overall circuits. For example, one card with just the output tubes. Another card with just the splitter. Another card with a 2-stage preamp. maybe you can have more stuff per card, but it depends on the size limit you are dealing with

To lay out a full-size board might mean having to shell out $$ for the non-crippled program. Good thing once you do that is you have a licence and can use it to acquire future versions.i
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#4
I paid the annual subscription for 'eagle standard' (CAD$130), which limits board size to under 24.8 cubic inches. I've used it for some small 'mod boards' as well as a power supply board, but at CAD$665/year, I can't afford to use the unrestricted version of Eagle to design my main board(s).

I wonder if it would be possible to purchase a license as a group?
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#5
Hi Guys

I believe Autodesk who owns Eagle would prefer if you bought a multiple-user licence for big bucks if there are multiple users working towards commercial ends, as most of us do. Even as a hobbyist, it is easy to slide into doing things for friends "at cost" or with "shared costs" and before you know it you're an amp builder.

It's certainly worth investigating, though.
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#6
Hi Kevin,

The largest size I could make was 100mm x 150mm. The version I purchased of Eagle Cad was one step up from the demo I think. It allowed me to make my senior project which was a pro-quality microphone preamp, but the tube amp boards are usually long and skinny and I could only use a modular approach as you mentioned. I've considered it but it would be nicer to have a full size board capability if possible.

My preamp project worked but I made a couple mistakes and Rev 2 of the board will need a couple layout changes and I have to make a Molex connector, which I was having trouble figuring out at the time. That was 2015 and I haven't had the time to get back to it yet!

Greg
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#7
Hi Guys

100 x 150mm is definitely small (4 x 6"). That is 15,000 square millimetres, or 150 square centimetres, or <24 square inches.

24.8 cubic feet? Did you mean 24.8 square inches? That seems minuscule for $600+ per year. I can't say I'm a fan of subscriptions for software.

I believe the full version (4) allows 32 x 32" boards (812 x 812mm, 6600 sq.cm, 1024 sq.in). I haven't looked at the current version but it should be similar. There are alternatives to Eagle but I've never tried them since my old Eagle works even with its couple of bugs.
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#8
(09-13-2018, 11:02 AM)K O\Connor Wrote: Hi Guys

100 x 150mm is definitely small (4 x 6"). That is 15,000 square millimetres, or 150 square centimetres, or <24 square inches.

24.8 cubic feet? Did you mean 24.8 square inches? That seems minuscule for $600+ per year. I can't say I'm a fan of subscriptions for software.

I believe the full version (4) allows 32 x 32" boards (812 x 812mm, 6600 sq.cm, 1024 sq.in). I haven't looked at the current version but it should be similar. There are alternatives to Eagle but I've never tried them since my old Eagle works even with its couple of bugs.

Yes, I meant inches; I normally think in metric, so I goofed Smile
And to clarify (here and in my edited post): the version of Eagle I use costs something like $130 per year; the size-unlimited version is $660 per year (Canadian dollars).
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#9
Hi Guys

How old-school am I? Or you?

I still use inches when laying the board even though every component you buy today has measurements in mm. When you make the library part, you can either convert the dimensions into inch and go from there, or lay the part out on a metric grid and let the board editor deal with it when you are laying traces.

I have hundreds of projects laid out on an inch grid and ponder changing to metric (as I really should do some time). The old projects are unaffected by doing new projects and parts in metric, until you go back to one of those projects and add a new part, or a new metric version of a part. The only issue will be whether Eagle thinks the pins are connected or not on the board, in which case you just make the grid small enough that it gets the trace centre as close as it needs for a "connection".
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#10
(09-13-2018, 11:50 AM)K O\Connor Wrote: Hi Guys

How old-school am I? Or you?

I still use inches when laying the board even though every component you buy today has measurements in mm. When you make the library part, you can either convert the dimensions into inch and go from there, or lay the part out on a metric grid and let the board editor deal with it when you are laying traces.

I have hundreds of projects laid out on an inch grid and ponder changing to metric (as I really should do some time). The old projects are unaffected by doing new projects and parts in metric, until you go back to one of those projects and add a new part, or a new metric version of a part. The only issue will be whether Eagle thinks the pins are connected or not on the board, in which case you just make the grid small enough that it gets the trace centre as close as it needs for a "connection".
My version of Eagle plots in mils by default. I try to remember to switch to metric whenever I start a new board.
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#11
One of the reasons I haven't upgraded is that I paid a set amount for my slightly better than demo version of Eagle, and I paid it once and that was it. Now they have a certain amount you have to pay per year, which for the equivalent version is $10 less than I paid for no yearly subscription. So it will work for me for smaller boards and I can make modular boards and make them work in an amp. It is fine for now but I'm always on the lookout for something better.

Greg
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#12
Hi Guys

KICAD might be something for you to look into. It is an open-source PCB design software that some people I respect use although I've never explored it myself so do not know the limits of board size, etc.

I'll post more threads about designing parts in Eagle and other things related to layout.

Have fun
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#13
Some popular amps have PCBs like the one pictured below. Note the following:
- there are multiple components stuffed into the same through-hole pads
- different components share pads. In many places, one component is inserted 'diagonally' from the lower pad of one component to the upper pad of a different component. I'm not sure how this was done, but I know that one builder who designs such boards uses Eagle.
- the pads are generously oversized. 

Thoughts?

[Image: C0VlYAs.png]
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#14
Thanks for the KiCAD tip Kevin. I'll check it out tonight.
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#15
Hi Guys

John, you correctly stated how the above board was designed - with over-sized holes.

Many builders who go from eyelet or turret board construction to PCBs try to copy their hand-wired layout as closely as possible in an effort to not lose any "mojo". One way, is to do as your photo shows and have very large holes (drills) in the pads, which themselves should be generous in diameter to provide a large solder area. This mimics eyelets.

In other cases, like Mickey Corrieri's amps, he rotated the components to match their physical orientation in the hand-wired prototypes, with all the familiar "V" shapes and "W"s and "L"s we see so frequently.

Have fun
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#16
(10-03-2018, 04:56 PM)K O\Connor Wrote: Hi Guys

John, you correctly stated how the above board was designed - with over-sized holes.

Many builders who go from eyelet or turret board construction to PCBs try to copy their hand-wired layout as closely as possible in an effort to not lose any "mojo". One way, is to do as your photo shows and have very large holes (drills) in the pads, which themselves should be generous in diameter to provide a large solder area. This mimics eyelets.

In other cases, like Mickey Corrieri's amps, he rotated the components to match their physical orientation in the hand-wired prototypes, with all the familiar "V" shapes and "W"s and "L"s we see so frequently.

Have fun

Thanks for the response, Kevin. 

In addition to preserving the "mojo" of turret-board construction, the PCB with oversized pads would also discourage builders from packing the parts too tightly. I think it was in TUT3 where you (KOC) said that an "airy layout" helps to make an amp feel and sound big. I've always tried to maintain a reasonably airy layout after digging into TUT3.
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#17
Hi Guys

Yes, John, the airy layout is a sure way to have an airy sound by reducing parasitic capacitances. It also assures that you end up with an easy to service assembly Smile

On the other hand, some assemblies will end up "tight" if there is a lot of circuitry or features packed into a relatively small space. This does not mean you cannot attain an open sound; rather, that you might have to be more careful with the layout of components and traces.

Laying out PCBs can be quite fun once you get over the hurdle of learning enough of the program quirks to do so. When I began learning how to use Eagle I also had to learn about the basic file management in Windows, so a double whammy. Fortunately, I've always had a whammy bar on my guitar...
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