Chassis questions for new Sunn amp build

[Boy Howdy]

I think I might be ready to tackle my first from-scratch build of a big amp. I want, basically, to duplicate my Sunn Sceptre. I have the transformers and most everything else, except for a chassis.

The Sunn is of course big and heavy. Two issues: I'd like to shrink it down a bit. And there's the matter of using an aluminum vs. steel chassis.

Most of the chassis I've seen advertised for sale are aluminum, and that's what I would prefer, but I'm concerned that it might be a little too insubstantial to hold up under the weight of those big-ass trannies.

I will not be including the reverb/tremolo circuit so that frees up a little space, but I will need more space for power supply caps since I won't be using cans ala the original. But I still think I may be able to cut down on the width by a two to four inches.

So what do you guys think?

https://us-browse.startpage.com/av/a...c848c4e819b1da

[nevetslab]

I think I might be ready to tackle my first from-scratch build of a big amp. I want, basically, to duplicate my Sunn Sceptre. I have the transformers and most everything else, except for a chassis.

The Sunn is of course big and heavy. Two issues: I'd like to shrink it down a bit. And there's the matter of using an aluminum vs. steel chassis.

Most of the chassis I've seen advertised for sale are aluminum, and that's what I would prefer, but I'm concerned that it might be a little too insubstantial to hold up under the weight of those big-ass trannies.

I will not be including the reverb/tremolo circuit so that frees up a little space, but I will need more space for power supply caps since I won't be using cans ala the original. But I still think I may be able to cut down on the width by a two to four inches.

So what do you guys think?

https://us-browse.startpage.com/av/a...c848c4e819b1da

One thought is to look around for a used amp head or combo amp, whose amplifier chassis is close enough to the size you're looking for, then, assuming you can find something dirt cheap that's not working, gut the chassis and start over with your build. You could always fashion a new escutcheon for the front panel to cover over excess control openings, jack openings to accommodate your build. Now, I haven't looked up the Sunn Sceptre to see what it is.

Most of the amp heads and combo chassis heads are made of steel.

Now, in my bone yard, I have a couple Blackstar 200's that were used for stage props. I had posted their availability in the flea market early last year, I think, for free, less shipping. I've attached some photos of these. They're in Burbank, CA at CenterStaging, LLC, which is still locked down due to the pandemic. I'm not crazy of their choice of putting both large xfmrs at one end, and a single handle in the center of the amp head cabinet (stupid re ergonomics).



Looking in their owner's manual, it's listed as 27.2 kg weight, unboxed. Anyway, just a thought.

[nevetslab]

Sunn Sceptre Amp Images

Just to add clarification to what you're asking advise on in selecting a chassis to build a Sunn Sceptre Amp Head, I googled the amp and found these images, along with a set of schematics, showing mod's that have been made.



sunn_sceptre_1971.pdf
sunn_sceptre_1971-1.pdf

This should give us a better idea of what sort of chassis you're looking to come up with to fit what's being built.

You're right....deleting the Reverb/Tremolo Circuits will save you the space Sunn used for their PCB assembly as well as the Reverb Drive Xfmr that feeds the tank.

One of the mod's I made on a Sunn 1200S was the input jacks circuit (same as on your Sceptre). The stock Sunn jacks don't have muting jacks, so it will hum if the volume pots are open. I've attached the schematic of the revised 1200S, which shows that input jack revision, along with the Switchcraft P/N's on two of the 'special' switching jacks used.

Sunn 1200S Guitar Amp-3.pdf

[Boy Howdy]

Yeah, I've actually modified my Sceptre a fair bit. I didn't mention it because some get upset about such things. But it was really a bass amp, I made it work better for guitar. (mostly cap value changes) And I did change the inputs around as well.

Just to be clear, I'm not looking to do a cosmetic reproduction of any sort. I want to make the same modded amp that I have now, but with better grounding, and a few tweaks. And I do want to use a new chassis rather than a junker.

[Mick Bailey]

Aluminium is fine for a chassis but it needs to be thick enough. I have my own folding machine and use 1.8mm and haven't had any issues over the years. Peavey used an aluminium chassis for their Mace series combos in the 70s and they deform around the transformer mounts. But this is a 40+ year old amp and the transformers hang down - a head is a much better bet for unreinforced aluminium. I've had a few Dr Z amps with aluminium chassis, too, as well as some really heavily built Blankenship heads and combos. I've done a fair bit of work on the ex-Mike Landau Twin Reverb built for him by Roy - this has very large Mercury transformers and an aluminium chassis. It's been carefully handled and shows no sign of distress.

If you had any concerns about the transformer mounting you could make a reinforcing plate to spread the load.

[catalin gramada]

I think I might be ready to tackle my first from-scratch build of a big amp. I want, basically, to duplicate my Sunn Sceptre. I have the transformers and most everything else, except for a chassis.

The Sunn is of course big and heavy. Two issues: I'd like to shrink it down a bit. And there's the matter of using an aluminum vs. steel chassis.

Most of the chassis I've seen advertised for sale are aluminum, and that's what I would prefer, but I'm concerned that it might be a little too insubstantial to hold up under the weight of those big-ass trannies.

I will not be including the reverb/tremolo circuit so that frees up a little space, but I will need more space for power supply caps since I won't be using cans ala the original. But I still think I may be able to cut down on the width by a two to four inches.

So what do you guys think?

https://us-browse.startpage.com/av/a...c848c4e819b1da

The secret to get a strong chassis is to weld the corners. If you chassis have not welded corners be aware to weld the aluminium thin sheets is pretty triky to get decent result.

[Boy Howdy]

Aluminium is fine for a chassis but it needs to be thick enough. I have my own folding machine and use 1.8mm and haven't had any issues over the years. Peavey used an aluminium chassis for their Mace series combos in the 70s and they deform around the transformer mounts. But this is a 40+ year old amp and the transformers hang down - a head is a much better bet for unreinforced aluminium. I've had a few Dr Z amps with aluminium chassis, too, as well as some really heavily built Blankenship heads and combos. I've done a fair bit of work on the ex-Mike Landau Twin Reverb built for him by Roy - this has very large Mercury transformers and an aluminium chassis. It's been carefully handled and shows no sign of distress.

If you had any concerns about the transformer mounting you could make a reinforcing plate to spread the load.

Ah, good info. I have some thick aluminum sheet. (it may be thicker than 1.8mm though, I'll have to check) I was thinking about trying to make my own chassis from it, but thought it might break because of the thickness. Of course if I did make my own I'd also have to make bender too. So maybe, maybe not.

I also thought about adding some reinforcement. a few sections of L-channel should help things nicely.

[Boy Howdy]

I'll keep that in mind. Thanks.

[Mick Bailey]

Whether it cracks depends on the bend radius compared to the thickness, as well as the state of hardness of the sheet (and of course, the alloy). Much of the sheet is supplied 'half hard'. You can get fully annealed sheet that's intended for pressings, but this is usually too soft for general use and is more inclined to deform.

Using a scriber will lead to an increased risk of cracking if the bend is acute. You can use layout blue and a brass scriber, or just a pencil to eliminate this possibility. When I've needed to bend a thick sheet I've sometimes had to locally anneal it by clamping a piece of steel bar each side of the line, rub it with soap and then use a blowtorch to uniformly heat it until the soap just turns black and then quench it in water, or throw some water on it. It folds much easier done like this.

I sometimes make an assembled chassis from a thick top plate and slightly thinner sides. You can use a variety of extruded sections, folded and bent pieces and make a decent job out of it. With countersunk screws everything can be made flush to take a front and rear panel. Extruded aluminium is available in L section and this makes good sides for a chassis. Cut it to length, mitre the corners and bolt it to the top. Then affix corner reinforcements and use angle to make the bottom flange. It's then strong enough to stand on.

[Boy Howdy]

Lots of good info there. Thanks, Mick. I'll give it some thought.

[Justin Thomas]

You'll have to search flr it here, but Juan Fahey had some good advice on reinforcing aluminum chassis way back when. I believe the ARRL Handbook from 1972 did also.

Justin

[nevetslab]

You'll have to search flr it here, but Juan Fahey had some good advice on reinforcing aluminum chassis way back when. I believe the ARRL Handbook from 1972 did also.

Justin

One of the chassis I had built in 1989 from Aluminum was when I was working for BGW Systems, and had access to their machine shop. I was only qualified to use the 10ft shear and the 16" cut-off saw for sawing the aluminum heat sink extrusions. I wasn't able to use the press brake nor the Weidemann 30Ton Punch Press, which had to be programmed to produce the flat pattern that then got folded up.

So, I cut top, bottom, side and rear panels from 16ga Aluminum (forget the grade), along with a 3/16" aluminum front panel. I then cut 1/2" square T6061 Aluminum stock, and drilled/tapped the blocks to join the chassis panels together. Came out extremely strong, but was a VERY tedious procedure. That was the first prototype for the BGW TriAmp, produced for Compact Monitor System's 3-Way Stage monitor, which used a JBL 2204 12" woofer, 2482 Mid-range Driver coupled to a short horn, less the slant plate acoustic lens, and a 2405 Tweeter. Cannon P-Connectors. So, the amp used a GTA Woofer Amp Module, a 7500 Midrange Module and a 150 Amp module for the tweeter, which was powered from a +/- 40V regulator to drive the tweeter. Internal crossover underneath the amp modules, where were in a separate compartment from the power supply. Pair of 80mm 41cfm AC fans on the rear panel, pulling air in thru the front vent panel across all of the heat sink fins. I still have that prototype, works great, parked in my shop at CenterStaging, along with one of the Compact Monitor stage monitor wedges.

So, many ways to build a chassis. Since then, when I needed a chassis, I'd draw up the plans and Vahe, our Mechanical Engineer who ran the shop would program it and punch/weld them up. We also had a Hager Press for inserting Pem studs and Pem Nuts, which I also made liberal use of. They also had a Time Saver...36" wide belt sander for both graining the Aluminum front panels, as well as running all of the flat patterns thru (to debur the tooling marks) prior to folding & spot-welding the steel chassis' together.