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K-502 Tube Amplifier Kit and DelSol Speaker

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K-502 Tube Amp Kit and DelSol Speaker

I wanted to try a DIY audio project and I was looking for something inexpensive and easy to build in order to increase the chances for success. Looking around various websites for ideas, I came across these projects which seem to fit the bill nicely.

K-502 Tube Amplifier Kit with DelSol Speakers
Photograph 01: K-502 Tube Amplifier Kit with DelSol Speakers

With just a few modest tools and skills, you can make this "frugal" hi­fi unit. The following tools should do the trick:

  • Hand drill and a few drill bits.
  • 2-3/8" and 3" hole saws.
  • Jig saw and/or mitre saw (if your steady just a hand saw should do).
  • A small work bench (I used a Black & Decker workmate).
  • Four clamps that will open up to at least 12".
  • Power sander.
  • Soldering gun and rosin core solder.
  • Heat gun.

Electrical Tips

The resistors will stay in place before soldering if you bend the wire legs back when you mount them on the board.

The electrolytic capacitors (the 3 big ones in the kit) must be mounted with the correct polarity. The negative leg is the one under the white stripe running down the capacitor. Usually it has the longer lead.

I chose to put the amplifier inputs on the back panel. To limit the potential for picking up unwanted noise, use shielded cables and keep it away from the transformers.

The kit comes with a cheap switch that looked like it would be a hassle to connect. I used a regular switch in the front.

I'll leave the technical details for assembling the Amplifier to the instructions that come with the kit. All you need are some basic soldering skills.

Tube Amplifier Case

The case I made for the amplifier is about as easy and straightforward as you can get. As far as a cover goes, I'll leave it up to you. I used a Plexiglas type because I like the look of the tubes. One thing to keep in mind is that the tubes and primary transformer get hot and need good ventilation.

K-502 Enclosure Ventilation
Photograph 02: K-502 Enclosure Ventilation

The K-502 kit comes with everything you need to get up and running but has only a pine board to mount it on. However due to the dangerous voltages on the board I wasn't comfortable with the idea of having it sitting open like that. So I just picked up some cheap 1/4" poplar boards (3" x 2' and 6" x 4'). For the cover, I used a small piece of 1/8" thick Plexiglas I had lying around.

Before you start, it's a good idea to sand the pine board and poplar panels smooth. You should also drill the mounting holes for the circuit board and transformers. Vent holes through the bottom of the pine base are also a good idea.

Take the 6" poplar and cut 3 pieces, two for the side panels and one for the back panel. The two for the side panels should be the same length as the width of your pine base. Mine was 7 5/8" but measure yours, as they are all probably a little different.

Clamp these two sides together when you cut your side profile and drill the vent holes. It helps to keep both sides consistent. Keep in mind the height of the tubes if cutting a side profile, as you want good clearance for the cover because the tubes get hot.

The back panel is going to as long as the length of the pine board +1/2" that your side panels have added. You want the back flush with the sides. Mine turned out to be 9-1/8" + 1/2" = 9-3/4".

Cut your front panel from the 3" poplar. It needs to be the same length as the back panel because it will be flush with the sides at the front.

Glue the two sides to the pine board and clamp along the bottom, use a damp cloth to remove excess glue. Let dry over night.

Glue the front and back on and clamp along the bottom, use a damp cloth to remove excess glue. You are going to have to be creative to ensure that the top of the back panel has contact with the sides. At least long enough for the glue to set. If all else fails you can just try and keep steady pressure on it with your hands until the glue sets. Then let dry over night.

Any wood dents and dings should now be taken care off with stainable wood filler. Especially if that's the finish you want to use.

When you drill your hole for the volume control, keep in mind that your board is sitting on 1/4" spacers. Measure carefully and drill a tiny hole first to ensure you have the right spot.

Drill the rest of your required holes, and give the case a final sanding using at least a 220 grit. I sanded the front and back panels vertical edges round, to give it a more finished look. Dry fit your boards and transformers and bits and pieces now for a final fit check. If everything looks good you can now stain. I used MINIWAX PolyShades natural cherry gloss for the case and front of the speakers.

One problem I ran into was mounting the knob on the volume control. Because the 1/4" thick front panel uses up space the volume knob wouldn't fit. To get around this just sand the back side of the knob so that the black plastic base is flush with the internal brass nut.

Rear View - K-502 Tube Amplifier
Photograph 03: Rear View - K-502 Tube Amplifier

Plexiglas Cover

For the cover make your vent holes or slots in the area of the four tubes and the primary transformer. Keep an eye on it when you first power it up and if any part of the cover gets above warm, make some more vent holes. You'll have to experiment a bit here.

The Plexiglas will bend if heated, I used a heat gun and clamped the cover down where I wanted the bend. If you haven't peeled off the protective cover from the Plexiglas you can do it now.

In a well ventilated area, move the heat gun back and forth along where the bend will be on both sides of the glass. Move fairly quickly and don't get too close or the Plexiglas will blister. You have to be patient here as you'll have to slowly heat up the Plexiglas to a point where it's soft enough to bend. Once it's pliable bend into desired shape, test fit it on the case. Use gloves as it will be hot! You'll end up probably having to make a few adjustments, as it's hard to get it perfectly flush with the case.

K-502 Front Ventilation
Photograph 04: K-502 Front Ventilation

DelSol Speakers

Pekar's "Delsol speakers" are inexpensive, easy to build and sound great. The bookshelf speaker that uses the Aura NS3-194-8E fullrange driver. Since I didn't have access to a table saw, I had the hardware store cut the MDF down to a more manageable size. I used a 3/4" thick half sheet (4' x 4') of MDF from Home Depot and had the attendant cut three strips (one 5" wide the others at 8-1/4"). This should be ample to make the speakers. Most hardware stores can cut your wood for free. For the front of the speaker, I couldn't find 1/2" maple, so I used oak instead.

When cutting the holes with the hole saw go slow, and clear the chips out at least every 10 seconds, or you'll start smelling burnt wood. When you are about 3/4 of the way through, turn the piece over and cut it a bit on the other side, so that it wont chip when the saw brakes through. Make sure the piece is clamped well before you cut as any movement can result in an oversize cut. It is best to try on a piece of scrap first.

When you glue and then clamp your wood, double check that the wood is still in the correct position. The wood tends to shift around under initial clamp pressure.

DelSol Aura NS3-194-8E DIY Speaker
Photograph 05: DelSol DIY Speaker

Summary - K-502 Tube Amp Kit and DelSol Speaker Projects

The K-502 amp and DelSol speakers sound great in stock form, but there are a lot of modifications you can do to improve them. Check this site for ideas. My original plans were to use this case as quick fix until I made a metal case. It was easy to make and the results, to me anyways, look nice so I may not bother. One problem with a wood case is that it's susceptible to interference from outside electrical noise.

Finally I'd like to thank Bruce Heran, who wrote the article More Modifications for the K-12M Tube Amplifier Kit. He was extremely helpful to me in tracking down a few problems I had. I plan on trying his Auricap capacitor upgrade next.

For more information about the K-502 and to see other similar tube amplifier kits, see the S-5 Electronics K-12 and K-502 Tube Amplifiers Kit page.