My latest project - KT88 SE tube amp

[evilskillit]

Re: My latest project - KT88 SE tube amp

Nice, yeah when I first started looking into building an amp I noticed the chassis and the transformer are a large chunk of the overall cost. However I bought a prosound amp recently "as is" for $20. In this case "as is" was code for broken. If I can't get it fixed then there's my chassis. Also found an old RCA receive in the attic which doesn't work. It wasn't anywhere near Hi-Fi back when my cousin bought it from K-mar in the early 90s, but it's got a transformer in it, probably the right voltage for a Gainclone. So I'm getting pretty close to having enough scrap around that all I'd have to buy are PCBs and some chips, or roll my own.

One of my first DIY amps was a rebuild of a Packard Bell tube amp, circa 1963. There was a LOT of help from the DIY Audio forum. I paid $25 for the PB console at a thrift store, gutted it, threw away most of it, and spent less than $75 on parts to rebuild it; mostly caps. The wood to make the frame was some scrap I had lying around. It turned out very well and I recently sold it for $100. It was pretty much a break even deal for me, plus it was fun and pretty cool. I also built a Peter Daniel Audiosector amp which is nice, a Velleman amp from PE which was OK, and a Chinese amp I got off eBay which never did work. When you're in the position to get one, you'll probably have a lot of fun with it.
Mike

[ttan98]

Re: My latest project - KT88 SE tube amp

I'm building this amp to drive the Stances sitting on the shelf over my basement workbench. Right now they are powered by a Sony boombox :o This single ended design should produce 10 clean watts per channel in UL mode. There will be a switch so I can change between UL mode and triode mode.

Mike I certainly can bring it to Iowa next year if you want to hear it. The kits Scott was selling were actually a really good deal at ~$800 but piecing it together myself and building the case I will only have about $300 total invested. I skimped a bit and went with Edcor's GXSE 15 Watt output transformers instead of the huge CXSE 25 Watt iron. I can always upgrade in the future if need be.

Project update: I applied 2 coats of the amber shellac last night. I'm really not happy with the color so tonight they get sanded back to bare wood. I'm going to try clear shellac and see if I like the blonde look better

Hi,
Can you tell the exact model no. of the Excor transformer you are using with the KT88. Thanks.

[TN Allen]

Re: My latest project - KT88 SE tube amp

"Project update: I applied 2 coats of the amber shellac last night. I'm really not happy with the color so tonight they get sanded back to bare wood. I'm going to try clear shellac and see if I like the blonde look better

You might want to try Minwax Antique Oil(red can). I've found it a very good, durable finish for Maple, easy to apply, forgiving if left too long before wiping it off(more oil and steel wool nicely cuts the gummy mess), and, if given a few weeks or so, depending upon heat and humidity, it hardens and sands as nicely as really good sanding sealer without entirely glossing over the woods natural surface. Given another coat after a generous first two applications, it provides a linseed oil & pumice like rubbed shellac finish, especially if lightly waxed with Butchers.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Hi,
Can you tell the exact model no. of the Excor transformer you are using with the KT88. Thanks.

GXSE15-6-5K

https://www.edcorusa.com/p/530/gxse15-6-5k

Mine should be here in about two weeks.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

This week has been crazy busy at work. But I was able to make a little progress on the amp build. The maple case's finish is done. The aluminum top plate is cut to size. Here is a picture of the top plate sitting in the case for a test fit:



Sorry for the cruddy phone camera picture, my wife has our nice DSLR with her out of town right now. I'm kind of disappointed that I chose to just build this case with **** joints and simple rounded over edges. But if this circuit sounds really good I plan to build another amp like this. Next time however it will have a much nicer case (best of my ability) and bigger/better output transformers.

[TN Allen]

Re: My latest project - KT88 SE tube amp

It's great that you are willing to share your progress, it looks pretty good to me. I'm really looking forward to seeing this project progress. I contacted Scott about buying a kit, they are all sold, and he needs another 19K + to put together another lot. Sadly, I don't have that, let alone 20+ friends to go in on a group order.

[bkeane1259]

Re: My latest project - KT88 SE tube amp

I think it looks fine with the boob joints. Why can we say boob and not ****?

Do you have a way to cut metal? How did you get the top plate fabricated?? One of the things that keeps me from doing these types of projects is my lack of proper tools.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Great questions. I use an old 60 tooth blade on my table saw to cut the aluminum plate to size. It's loud and throws some needle like shards (safety glasses, long sleeves, and ear plugs manditory) but works really well. For the small holes I obviously just use regular drill bits. For the larger holes bimetal hole saws (Milwaukee, Lenox, etc.) in a drill press or a corded hand drill work pretty well. For odd or square cuts my little Craftsman sabre saw with a metal cutting blade (fine tooth) works well. The key is to make the cuts slightly under sized and use files to get to your final dimensions. I will admit that it is a lot of work but I find it very rewarding when things turn out really well (almost CNC perfect).

I use a very simple drawing s/w program called MS Visio to lay my parts out. Then print it out to scale. Here is a picture of that template for this project:



I tape the template down to the aluminum plate and use it to center punch where the holes go.

Here is a shot after I have drilled most of the holes and used the hole saws on the big ones. I still need to file to final size and add a few smaller holes:



When all of the holes in the top plate are done I use my orbital sander with increasingly finer grits to finish the top. On two projects I ended up using Rustoeum "Hammered" paint. On my KT88 push pull mono blocks I used a red Scotch Brite pad after sanding to 220 grit for a natural aluminum satin finish, as seen here:

[[email protected]]

Re: My latest project - KT88 SE tube amp

damn Craig! those monoblocks are..................HOT!!!!





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[PWR RYD]

Re: My latest project - KT88 SE tube amp

Thanks arlis! I was able to file the two power supply capacitor holes to final size. These caps are huge. each about the same size as a soda can:

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Not sure why the picture is rotated...

[TN Allen]

Re: My latest project - KT88 SE tube amp

I'm curious about the KT 88 mono blocks, you may already have said, but were these a kit, or is the schematic available. They look great.

[Psycoacoustics]

Re: My latest project - KT88 SE tube amp

+1 on the monoblocks! More info please?

[PWR RYD]

Re: My latest project - KT88 SE tube amp

+1 on the monoblocks! More info please?

The mono blocks are based on a design that Bruce Heran of Oddwatt Audio sells. They sell complete kits at Tube Depot for $599 each. Here is link to a project build of those kits:

http://www.diyaudioprojects.com/Tubes/5751-KT88-Tube-Amp-Kit/

Not able to justify spending $1200 for a pair I decided to scratch build my own. By sourcing all the parts, building my own cases, and simplifying the power supply circuit I was able to build my pair for $505 plus the walnut wood. I got lucky was able to buy the 2 power transformers used, which saved me about $40 each. So I'd guess someone could easily build a pair for under $600 with all new parts.

These monoblocks sound terrific! They're only rated at 25 watts each but they totally blow away my 60 wpc Rotel amp. Both in terms of impact (dynamic power) and sound quality. The other cool thing is that this design can run a bunch of different tubes: 6L6, KT77, EL34, KT88, 6550, KT90.

[bkeane1259]

Re: My latest project - KT88 SE tube amp

This thread is totally making me want to go back to tubes. I must unsubscribe....must resist...must resist.