Our industrious friend Pete Millett has released another build of note, the Jonokuchi
headphone / speaker amp. The Jonokuchi, like the Engineers Amplifier
, makes use of a compliment of very reasonably priced Edcor transformers
. Unfortunately, or fortunately depending on your tastes, the Edcor iron is only available with Royal Blue powdercoated bells, so if you'd prefer another color, be prepared to pay for your local shop to sandblast it off or purchase some unpainted bells of the same size.
The Jonokuchi, like the Engineers Amplifier, makes use of old television tubes that are cheap and readily available. It's a single-ended affair that uses 13EM7 tubes with 13V heaters. As stated on Pete's page: the amp design was optimized for headphone use, but still makes a fine amp to drive small speakers to reasonable levels.
This is a relatively low cost build at approximately $200 in parts, just add a chassis. Assuming you make use of the PCB, you'll want to go with an 8" x 12" chassis as it should fit everything perfectly. Hammond makes a cheap one for ~$30, or you could get something more exotic like...
|... a Hammond Chassis with Walnut Side Panels|
|...or a custom chassis from ebay seller po1019|
Pete also put up plans for a Front Panel Express Chassis that comes with all the holes already trimmed out and labeled, but for nearly $300 for the chassis alone, I think I'll pass ;)
|Jonokuchi with custom FPE panels|
I was able to pick up my red board just recently from Pete and I've dropped a few parts on it so far.
|The lovely red Jonokuchi board|
|Mostly populated, a few things left to source|
I'll update in the coming weeks as the remaining parts arrive. Should be a fun little project. Now which film caps should I use... Obbligato, Mundorf, Sonicraft... oh the possibilities :)
I decided to go with the Hammond Chassis and AmpOhm Paper in Oil caps. Yes... the AmpOhm caps are a bit large, but with some creative mounting, they didn't pose a problem. Pete has the hole layouts in PDF on his website, so download and either print out yourself on a couple pieces of paper and line them up with tape or let Kinkos or Staples handle it on larger format paper.
|Chassis with drilling holes laid out on top|
|Drilling out the holes on the drill press.|
I had Staples print the CAD drawings on the 11x17 paper, but despite saying not to resize in the instructions, they did anyway and I killed the first chassis, so be sure to measure the drawings first no matter what.
|Mouting the AmpOhm PIO Caps|
I finally finished up drilling the Hammond chassis, there were quite a few holes for tubes, transformers, a pilot light, the IEC inlet and the PCB mounting standoffs.
|Front of the Chassis|
|Rear of the Chassis with oversized holes for Cardas RCAs|
All holes were drilled with a trusty low-cost drill press and de-burred with a dremel tool. The rectangular opening in the back of the chassis was cut using a dremel metal cutting disc and then filed manually. Once complete, I gave the whole thing a once-over with 180 grit sandpaper.
Finally got the chassis back from my overly busy powder-coater. Now things are finally starting to come together. In place of the 5 watt 220 ohm resistor I placed a Hammond 157J filter choke (205 ohm) that was primed, painted and wrapped with leather. Cardas RCAs were installed on the back panel, along with some nice thick gold-plated binding posts that Parts Connexion was clearancing. I didn't want to splurge on Cardas posts as this amp will likely see a lot more duty as a headphone amp.
|Board fitted in the powdercoated chassis|
I designed a nice custom plate with the Japanese Kanji from Pete's Front Panel file and copperplate font spelling out "Jonokuchi". These little touches make the amp feel a little more polished and complete. The screws connected to the aluminum standoffs surrounding the tubes were depth-mounted as I'll be placing some decorative metal surrounds around them.
|Custom plate embellishes the top of the chassis|
The bell-ends are done being powdercoated in lovely a copper color and have been reassembled to the Edcor iron using my typical brass hardware and acorn nuts. I actually take the time to trim down each screw with a rotary tool to ensure that it's the proper length for the acorn. A few extra minutes of effort here and there goes a long way on the finished product.
The transformers were then mounted and the set of copper colored tube surrounds were screwed in place using very short screws as there is very little clearance between the chassis and the board. A jewel pilot light with a 12V LED was mounted on the left and connected to the tube heater pins on one side.
One the Jonokuchi was completely assembled and measurements checked for D.C., the amp was plugged in to verify the LEDs were lighting and the tube heaters were warming up. I then plugged in my trusty Panasonic tester headphones and had a listen. Unfortunately there was a bit of static noise depending on the position of the potentiometer. I ran a connection from the back screw of the Alps Blue Velvet pot to the chassis ground to ensure proper grounding, but that did not solve the issue. In Pete's design, the JP1 connection is used to connect AC ground to signal ground. After shorting JP1 with a short piece of buss wire, the noise was gone.
The amp itsself is on the lively side in my humble opinion, it possessed a detailed sound with taut bass and no rolling off of the high frequencies that I could detect. Unfortunately I was only able to source one pair of 13EM7 tubes (Raytheons), they tend to be a difficult tube type to find in the usual places. I feel that with the right compliment (some warmer tubes), I'll be pretty happy with the Jonokuchi.
I was able to get some custom engraved brass plates for the front of the unit detailing the operation of the push buttons, so those will be the final touch. Here's some final photos:
Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.