Rack Chassis and Transformer Mounting

[Even Bigger D]

Hello All!

I'm starting a Soldano-esque power amp build, and I'm looking for a 4u rack chassis. I've seen the ones from Bud Industries ( http://www.budind.com/view.php?part=ch-rm ), and they look OK but I have no way to tell for sure. Anyone have any experience with them?

Anyone know of other suppliers? The Bud ones seem pricey - by the time I get a top and bottom cover and an internal partition to mount tubes on I'm looking at almost $200.

Also, any suggestions on how to mount the transformers? I'm planning to use these: C3Amps - O'Netics Transformers & Soldano Chassis - Onetics

If I just bolt them to the bottom of the chassis, will it cause problems racking up the finished unit because of hardware sticking out the bottom? If so, any suggestions?

[J M Fahey]

If the chassis proper where you mount them and the sockets, is similar to a regular tube amp chassis, it will be 2" or more from the bottom.
In that case, that free space absorbes bolt heads , the board, horizontal filter capacitors and more.
Over that chassis will stick out tubes, transformers, and clamp-mounted electrolytics, if you use them.
Chassis and metallic cabinet working is the most difficult task for homebuilders, because iron is not an easy material to work with and requires heavyweight machinery.
Maybe you can find an old surplus rack mounted transmitter or whatever, and you can adapt it for your own use.
Search for some custom laser cutting shop, that would simplify your task enormously.
4U won't let you *a lot of* elbow room, that's for sure, it's about the free space in a Marshall head.
EDIT: found out they are aluminum made. Anyway I guess you will use an iron chassis.

[Even Bigger D]

I'm not sure I understand what you're saying (which I'm sure is my fault).

What it appears Bud will sell me is an aluminium box with a front plate, sides, a back, a top and bottom, and then one or more internal partitions that will bolt to the sides if I want.

My thinking, and this is probably all very primitive, is that I would get one of the partitions, put it most of the way back, and mount the tubes on it running front to back like in a mesa 2:95 or whatever. Since I've got oodles of depth and not much height, this seemed like a good idea.

That leaves the transformers and choke. I need to bolt them to something, but I don't think that something can be the bottom of the box because the bolts would stick out. One thought was that I could get a second internal partition and mount them to that. I'd be worried about it bending though - I have no idea how strong .090" aluminum is, and I'm probably way exceeding the 25# limit Bud suggests for that chassis anyways.

It sounds like you are suggesting a "box withing a box" approach when you mention the "chassis proper"?

[J M Fahey]

Metallurgical work is a nightmare for the home constructor, unless you buy a nice kit with everything ready.
Marshall, Mesa, VHT, everybody (except perhaps Peavey) spends weeks designing their Rack Mounted amps, and have them made by big metallurgical OEM contractors, in at least 100 units each (better 1000 ).
Those aluminum rack cases are great for SS power amps, because you bolt a couple boards to the bottom, a couple large heatsinks to the sides, and your only problem is the PT.
On commercially made equipment, they bump the chassis bottom inside with a special die to level the bolt head (look at commercial amps)
For starters, you will have neither die nor hydraulic press, so you are forced to use a sub-chassis (think Marshall Plexi) that can hold the transformers and tubes on the top, and board , controls, etc. on its bottom.
That sub chassis must fit like a glove into the rack case, as a Plexi chassis fits in its cabinet.
This construction method is somewhat simpler (or even possible) for the DIY builder.
That's the general idea.
Personally, I would get a suitable pre-perforated commercial chassis, build my amp, and mount it later wherever I can.
The chassis should fit into a rack case like a glove, and, if necessary, be able to be mounted inside a plywood/Tolex enclosure if the other option gets complicated.
I make guitar amps commercially, for 40 years now, have shears, punch presses, special dies, coating and painting equipment, do my own silk screening, etc. and yet if I need Rack cases , must have they made in a Metallurgical shop.
Minimum order: 100 of anything, which can be sometimes be talked into 50, no less.

[Steve Conner]

There are a lot of different 19" rack mounting solutions. I won't recommend one, because I live in the UK, and prices and availability vary by country.

The cheapest cases are just a metal box. You are correct, bolts will stick out of the bottom, and they'll either scratch your other rack gear, or prevent rack mounting altogether. JMF talks of a special die to "dimple" the metal for mounting toroidal transformers, but you can do it with two different sized sockets and a large bench vice. For smaller things, I use countersunk screws. You can't bolt heatsinks to the sides, because then it won't fit into a rack any more.

Here's something I did based on one of these cases.
Hi-fi amp project


More expensive ones have a chassis inside that you can mount at various heights. I've used the Schroff modular racks for a few things, both at work and in my own projects, but I don't like them too much. The internal chassis is strong, but the top and bottom slot-in panels are flimsy. And the general construction doesn't feel like it could take the weight of a SVT-sized set of transformers, and since the total cost for one comes to three figures, they're not so attractive. I've yet to find a rack case that looks like it could take the SVT challenge.

This is a large 4U one.
Tesla Four - end in sight


Finally if you're feeling brave, you can make it yourself. Below is one of my favourite amp cases. I started with a dumpster-dived piece of lab equipment and made new front, back, top and bottom panels out of 1/8" hard aluminium alloy. For the front panel, I got assistance from a friend who owns a milling machine.
Toaster pics

I built this in summer break when I was a student, and have never had the time or workshop facilities to do another one like it since.

Finally, Bluebell Audio in the UK seem to do a nice range of rack cases with an internal subchassis, you may be able to find the same ones on Ebay or wherever.
http://www.bluebellaudio.com/enclosuresracking.htm

[Even Bigger D]

Thanks for the help guys. I think I've given up on the Bud chassis - the 25 lb. limit seems way too low to mess with even if it's overly conservative.

Here's my new plan: I buy a steel chassis with bottom from Hammond, 16x8x2. I put all the necessary holes in it (which I believe comes down to 4 power tubes, the wall power socket, and drilling a bunch of small holes) and build the amp. Then when I'm done I bolt one of hammond's rack plates on the front and put some handles on that and drill the 4 holes for the front controls through. I probably need some sort of bracket on the sides to take some of the weight too.

If I go this way, I see a two outstanding issues:

1) I need a tool to cut the power tube holes. I think I've seen some kind of die or drill accessory to do this. Can anyone point me to it?

2) I need some brackets for the sides. Anyone have an idea where to find that?

Again, thanks for all the help!

[J M Fahey]

I think you made the right/practical decision.
I'm living out of series made SS amps, but for the occasional tubed one-off when it doesn't pay to have commercial chassis made, I'm using Starrett hole saws, 29mm (1 1/8") for octal sockets and either 19mm (3/4") or 22mm (7/8") for noval sockets, depending on whether I top or bottom mount them.
Buy and measure your sockets first .
As for the brackets, Home Depot and similar shops sell some angle brackets for shelves that are excellent for this.
After all, you only need the chassis itself (with a bottom cover for safety) and the rack front panel ; sides are covered by the rack cabinet itself, the top may be the rack cabinet roof or some preamp you install over it, and the backside is open, you can mount a protective bar , Dual Recto style, and for transport the rack cabinet has a back cover.
You want a lot of air flowing there, even adding a small fan.

[Even Bigger D]

Those hole saws look fine, and I agree an angle bracket is what I need.

I think I have a plan. Thanks, folks.