Cooking in Stereo

[TomMcNally]

Wow ! I finished up my second amp this morning ... all I can say is these amps and my big Cerwin Vega! speakers in the living room definitely rock the house ! Electro Harmonix KT-90EH's / JJ-12AX7/12AU7 / NOS GZ-37 rectifier - SDS Labs Cap Board - repro chassis from eBay - DynaClone transformers from Triode Electronics ...

[WA4SWJ]

Tom,

Looks awesome! I have not had time to finish a second one yet due to business travel. But I'm back now and I might get to finish one in a couple of weeks. I have to do taxes (I have to pay so that's why I'm delaying) and The AMSAT Journal before I can play with tube amps again. My first one is waiting patiently for the second and third one to be finished. And I can't wait to hear them.

I'm looking forward to seeing Shannon's measurements that he's done.

Have a great week!

Regards,

[skidave]

Tom,

Nicely done. I'm try to find the time to get my stuff together to get parts ordered. I don't know if I am going to use the reproduced chassis or not. However, I do have a quick question for you and the Group...what does everyone use to make the round holes for the tube sockets in a blank chassis? Are you drilling with a specific drill bit or using a punch set? I'm a wood worker, so metal is a foreign art to me... Any info would be appreciated.

Thanks!

Dave

[TomMcNally]

Hi Dave -

I wish I had some woodworking skills. I've been getting pretty good with the steel chassis though. Check out some of my amps at:
http://mcnally.cc/amps.htm
You'll see I like the pre-painted black Hammonds. The smaller ones are pretty thin steel though, the DynaClone transformers will bend 'em up pretty good.

Essential tools - get a Greenlee Chassis punch (not the electrician's variety) for the type of tube socket you want to use. Usually an octal is 1 1/8" - but there are lotsa variations. The Angela gold octals seem to be the best, although not cheap. Also, you'll want a genuine Uni-Bit - not an eBay import copy. They are great for switch holes, 9 pin socket holes, etc. They don't twist the chassis up like twist drills do. After you get used to using them, you can drill a pilot hole, then count the cuts as it gets to your target size. A rat tail file and half round file are good to have, as well as decent drill bits. I like to mark with a grease pencil, then use an automatic center punch, then drill a pilot hole. I haven't found a drill press to be very useful, lately I've done everything with a Makita battery powered drill.

I personally like to use stainless steel phillips hardware, it looks a lot better than the cheap zinc stuff. I do like the zinc nuts though, with integral star washer. They make assembly a lot easier.

Lately, I've been doing all of the metal work the first evening, cleaning up the chassis in the kitchen sink with dishwashing liquid to get rid of the cutting oil (I use that on the punches, Uni-Bit and everything) and then mounting all of the parts. Next evening I wire them up.

Sorry to be so windy - but I've been enjoying the metalwork as much as the rest of it. The MK-III clones were fun, but no holes needed to be drilled - I kind of missed that.

... tom

[TomMcNally]

Ed - Looking forward to a report on your amps ! I do have pretty much the same test equipment Shannon has, so I plan to run some reponse and distortion tests on my amps in the next couple of days. I just got a set of 8 ohm / 50 watt Dale resistors and mounted them on a heat sink with dual banana plus on them, so I can test the big stuff now. My 1957 MK-III's seem to have some HF oscillation, so if I can get rid of that, maybe I do some comparisons.

... tom

[EWBrown]

My fix for thin and flimsy chassis (like the Hammonds) is to use a length of aluminum angle bracket (3/4 or 1 inch) , running along the longer axis, iside of the chassis, and drill mounting holes for one side or end of all the heavy iron through the chassis and the angle bracket. This really beefs up the chassis (even the thin aluminum ones) and gives everything a good solid feel.

This necessitates that all the iron is arranged in pretty much of a straight line, and I do take care to position the power and output iron so that the cores are at 90 degrees to avoid inductive AC coupling.

FWIW, some of the electrician's Greenlee punches are applicable to our purposes. The punch for "half inch" EMT is actually 7/8 inch, good for some 9 pin sockets - and the "3/4 inch" is actually 1-1/8 inch, perfect for most Octals.

We have a set of these in the lab at work, and I've used them during lunch and "slow time" projects. The larger diameter fits the EMT fittings on the ends of the tubing, not the actual tubing itself.

/ed B in NH

[Shannon Parks]

what does everyone use to make the round holes for the tube sockets in a blank chassis?

Hi skidave,

I basically use two Unibits (#1 and #3), a Greenlee Slug Buster kit #7238SB (was a $80 BIN on Ebay, but I just looked and they are almost double that!), and Triode's Nibbler tool. I bought the Philmore Punch Kit early on, but it doesn't get used because of the Slug Buster these days. Honestly, these tools end up being using for much more than chassis making, and are worth the tool investment. Everybody borrows my Slug Busters around here. Yellow_Light_Colorz_PDT_06

Shannon

[dhuebert]

What do those KT-90EHs like for idle current?

Don

[WA4SWJ]

Tom,

How about a picture of your test setup? I'd like to get some test equipment but I truly don't have room in my condo. I do have a marginal oscilloscope but no true analysis equipment such as distortion analyzers and the like. I like the HP 8903B but most I've seen cost far more than I want to spend and for the amount of use I would give them.

I still haven't started on my other two Mark III's but I will next week. I'll let you know how they sound as soon as I finish the next one. I am anxious to here them.

Regards,

[Francois_G]

what does everyone use to make the round holes for the tube sockets in a blank chassis?

I wish I had access to a 2" Slugbuster -- need to cut 2 holes for BIG film capacitors. Yellow_Light_Colorz_PDT_20

[Shannon Parks]

what does everyone use to make the round holes for the tube sockets in a blank chassis?

I wish I had access to a 2" Slugbuster -- need to cut 2 holes for BIG film capacitors. Yellow_Light_Colorz_PDT_20

Ha ha!! I almost mentioned in my previous post a little note for you, Doctor G. Yes, my machinist co-worker got his dental tools out and cleaned out the piece of crud that was jamming up the 2" buster. Drop by this weekend, or I'll hand off to Thermion. Wait 'til Saturday afternoon and I should be able to give you your MKIII's, too!

Shannon

[TomMcNally]

The KT-90EH's are idling at about 65 ma ... nicely balanced, thanks to the trimpot on the Poseidon board.

I've used holesaws for the bigger holes - they do a nice job in steel, but make a mess of most aluminum. I prefer to use a deep chassis, either 3' or 4" and hide the oil caps underneath ... (oh and the ugly varnish coated Hammond chokes too) (oh yeah and the filament transformers) like this:



(for those of you with analytical eagle eyes - the delay tube was not yet installed, nor the meter and switch wired when that pic was taken. I have full voltage and current metering on all stages)

I have an HP-334A distortion analyzer I'd consider selling if anyone is in the market, whatever the average eBay final price is would be fair. Also a Leader LAG-125 generator that needs minor work - it's distorted - probably a blown output transistor. I got a Boonton 1120, the same as Shannon and Thermion have ... Shannon wrote a cool program that lets the computer (via GPIB) run a full set of measurements in a minute or two. I don't have a real "bench" just a dining room table that has only seen a Mountain Dew can as far as food goes in a few years. eBay has filled my little house with too much stuff ! I'll try to grab some pics soon - I need to haul some unused stuff out into the sunshine and take some pics for eBay !

... tom

[gogzhad]

I've tried a lot of methods for cutting chassis holes. I concur with the above; hole saws tend to be hard on aluminum, but generally work well on steel. For large holes, though, a hole saw can be a challenge in any material.

One thing that helps a lot is using a drill press. Another is using a piece of wood against the non-tool surface to back it up - especially helpful with flimsy material.

Another tool that works wonderfully in any material is a circle cutter (think machinists would call it a fly cutter). It makes a very neat cut and is less messy than hole saws. However, it can only be used with a drill press.

For smaller holes in aluminum and lighter steel, a unibit is the way to go.

For your 2" holes, with a hand drill the hole saw will work; with a drill press the circle cutter is a revelation.

[logos47]

Those Mark IIIs with the PIOs underneath and the filament transrformers look amazing. Can you describe the sound? I am looking at some similar mods, including the filament tranny to take the load off the power trannys.
Bob

[TomMcNally]

Bob -

The under-chassis shot is my 300B amp, not the MK-III's ...
I think I was showing the pic as an example of how to mount
the oil capacitors under the chassis.

... tom