Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Planning the 42-based tube amp

I need a name for this amp.  Usually folks just call it by the output tube type - like 6V6 or 300B or 2A3.  Since we are using a type 42, and the design resembles one that might have been engineered before WWII, I guess we just call it a '42 amp!  A nod to both the output tube type and the fact it's a retro design.

I spent a lot of time thinking about this amplifier build.  On a couple other builds I have gotten a bit squeezed into a corner (sometimes literally and figuratively) when I didn't allow space for a switch or something and it wound up a little wonky.

Not this time.

In addition to laying out the top of the chassis with the tubes and the transformers, I also did a full-size diagram for most all of the wiring.  I also did a layout for the power filter section, since that has the most components that will be in one area.  So I had these two layouts and the schematic to work off from.

I did all of the planning and thinking and then laid it all out on the chassis box.  I tried to be as accurate as possible, and as it turns out, I only made one goof, which was easy to recover from.

The chassis box is a Budd aluminum box I've had for at least ten years.  It was still wrapped in brown paper.

The aluminum was *real* thin, and I found out that the drill press may not have been the best way to drill the holes for the transformer mountings, the switches, the input and output jacks, etc.

I switched the belts on the drill press to run at the slowest speed, but I still got some holes that were a little less than exactly perfect.  The holes will be covered up with fasteners, so it's no big deal, but I was a bit surprised.  I drilled some small holes later with a hand drill, and they were perfectly round!  Go figure.

At any rate, here we have the (mostly) drilled chassis.  I was determined to make things line up nice and straight...looking at stuff that is mounted crooked makes me a little crazy.

After the mounting holes, I get to do the task that I enjoy a lot - punching chassis holes.  Like a lot of other amp homebrewers, I too faced the problem/dilemma of creating holes to mount tube sockets.  All of the holes for the sockets on our 42 Amp build are 1 1/8 inches in diameter.  I *have* used hole drill bits - the kind you'd drill a hole in a door with - and they do work, but they scare the heck out of me.  Not to mention the cut isn't the tidiest in the world.

So I wound up buying a handful of Greenlee punches off the dreaded E**y.  I have them in the most common sizes for chassis work.  If you are doing this sort of thing on a semi-regular basis, I strongly encourage punches.  Much safer and cleaner than using a drill (even a Uni-Bit).

Most all of my punches are vintage...and have these way cool boxes that say "Radio Chassis Punch."  Very neat.  Of course, they also say "Fine Tools For The Craftsman," which in my case, isn't exactly accurate.


You just drill a pilot hole and then thread the punch guide through that.  Then spin on the cutter head, and turn the whole thing with a socket wrench.

Here's our chassis ready to go.  I'll start by putting the transformers, on/off switch, input socket, output jacks, volume pot, and tube sockets all on before I start wiring it up.


Post a Comment 3 comments:

  • Toy Making Dad said...
    June 1, 2010 at 11:43 AM
    Very cool. I'd be interested in seeing a larger version of the schematics so that I can use them as the basis for a disintegrator ray I am working on.
  • Bigbargain said...
    June 2, 2016 at 7:58 AM
    did you ever finish this amp?
  • Yr Fthfl Blggr said...
    June 3, 2016 at 6:25 PM
    Yes, I did. There are 7 more posts describing the build. If you click on "Newer Post" under the posts you can see them in order.

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