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JAN6418 Valve (Tube) Preamplifier / Headphone Amp Kit

 Mark Houston   Australia Flag   To email Mark, type out the email address.

JAN6418 Valve (Tube) Preamplifier / Headphone Amp Kit

Oatley Electronics in New South Wales Australia makes a few kits based around the Raytheon JAN 6418 sub-miniature valve (tube). I decided to try out the $27AU K272A Stereo Tube Preamplifier - Headphone Driver kit. [Note - the K272 has been replaced with the K272C Stereo Tube Headphone Amplifier Kit which costs $40AU (December 2013).] A photograph of the Oatley Electronics tube preamplifier kit is shown below. The kit is complete but comes with very ordinary (generic) components which is not surprising considering the very low cost of the kit. The printed circuit board (PCB) is plated through, dual layered, solder masked and screen printed. The kit can be run off a 9V battery which makes this a safe and ideal kit for those who are new to electronics.

Some of you will likely be turned off by the very low cost of this kit. Before you pass judgment on this little kit, do note that Audio-Technica uses a JAN 6418 sub-miniature tube in its model AT3060 condenser microphone which sells for about $600US. Music professionals often describe this microphone as having a warm classic tube sound.

6418 Sub-miniature Tube Preamplifier / Headphone Amp Kit
Photograph 1: 6418 Sub-miniature Tube Preamplifier / Headphone Amp Kit

These 6418 valves look like small Christmas Tree lights. Hard to believe that they are actually pentodes. The USA made Raytheon JAN6418 sub-miniature valves are low power consumption pentodes that are used as triodes in the circuit. I built one straight out of the packet and loved the sound so much I decided to get a few more of the kits and try some builds using better parts. It took just over an hour to put the kit together.

JAN6418 Tube Preamp / Headphone Amp Kit
Photograph 2: Stock Build - JAN6418 Tube Preamp / Headphone Amp Kit

The 6418 tube filaments are 1.2V and draw just 10mA. You can barely see the tubes glowing in a dark room and the are just slightly warm to the touch. From a single 9V battery the entire preamplifier draws just 12mA. For a longer run time I used two 9V lithium batteries in parallel. The 9V supply is the B+ (or laughingly HT+) while a small 5V regulator chip (L4949) and 270R resistor feed the filaments which are wired in series. The second stage is a low distortion CMOS headphone driver IC (PT2308) that is set to unity so the preamp can drive both high and low impedance loads including headphones. The gain of the circuit is adjustable from 0 to 7 times and can be increased further if required. A LED lights for the first few seconds when the device is turned on to show the device is on then it only re-lights when the battery voltage falls below 6.8V. The schematic is shown in Figure 1. Please note that this circuit is © Oatley Electronics and permission to host the schematic on this site has been provided by Oatley Electronics.

6418 Miniature Tube Preamp / Headphone Amp Schematic
Figure 1: 6418 Miniature Tube Preamp / Headphone Amp Schematic
(Schematic © Oatley Electronics)

For the next build I swapped the all carbon resistors with metal film types, the 0.1uf MKT input and inter-stage capacitors were replaced with 0.47uF polypropylene film types, the PT2308 headphone driver chip was swapped out for a Burr-Brown OPA2134 op-amp, and the 220uF electrolytic output capacitors were also replaced. The valves are extremely microphonic and will "ring" if they are struck so rubber grommets are used to help damp the tubes. This does work and removes all audible noise from mechanical vibrations.

Upgraded JAN6418 Tube Preamp / Headphone Amp Kit
Photograph 3: Upgraded JAN6418 Tube Preamp / Headphone Amp Kit

With the original PT2308 IC the kit is a capable headphone driver. The PT2308 has only slightly higher distortion figures than the OPA2134 but can drive more demanding loads such as 32 ohm headphones. Because I am using this build strictly as a preamp (feeding a valve power amp) the OPA2134 is a better choice.

For the gain control I used are two PCB mountable potentiometers (pots). The kit comes with 1M trim pots which mount direct to the PCB which will accommodate either vertical or horizontal mounting trim pots. The trim pots would work for me as my Oppo 980H (modified by Custom Analogue) CD/DVD player has a volume control but there are times when twisting a knob to get desired volume level has its advantages. So I used two small knobs extending out the front of the enclosure to facilitate gain setting. They are not intended to be used for continuous volume adjustment but there is no reason why you could not do this but for me the remote is much easier.

JAN6418 Tube Preamplifier / Headphone Amp Kit
Photograph 4: Rear - JAN6418 Tube Preamp / Headphone Amp Kit

I was recently reading about a $5000US preamp that is housed in an expensive plastic enclosure. You would think that for $5k you would at least get a metal box. According to the designer, magnetic issues with metal enclosures has him building his high-end audio gear in very expensive plastic cases. Who am I to argue? For my enclosure I used a rigid ABS box which I lined the bottom and the lid (internally) with flashing (bituminous aluminum foil). I lined the box to help suppress mechanical vibrations and to help block RFI. It also added some good weight to the box.

6418 Tube Preamp / Headphone Amp
Photograph 5: 6418 Tube Preamp / Headphone Amp


Testing Results

The documentation indicates that the preamp is good to 35kHz which is well beyond the audible range. So I decided to find out for myself. Using a 47k resistor for the load and a input of 2.82V (which is higher than what the preamp would normally be expected to handle) I checked the output an oscilloscope. Sure enough the high frequency extension was there but I was not expecting the -3dB point to extend to 50kHz and the lower -3dB point was below 10Hz which is more than enough bandwidth. Low frequency square waves can be difficult for valve amps to handle, but not his time. There is some slight tilt but quite good. One the screen shots, the bottom trace is the output.

100Hz Square Wave Response
Figure 2: 100Hz Square Wave Response

The square wave response at 1kHz is excellent and even at 10kHz the square wave was good but with some rounding.

1kHz Square Wave Response
Figure 3: 1kHz Square Wave Response

10kHz Square Wave Response
Figure 4: 10kHz Square Wave Response


Sound

The very first build (stock kit) blew me out of my favorite listening chair. This subsequent (upgraded) build seemed a little restrained. Or is it just better balanced? I did feel the first build was a bit foreword which may make this build sound a little reserved by comparison. Also I had a few hours of listening time on the first build and as any thermionic fanatic knows, new valves can have an extended break-in period. Because of their size I would guess that the JAN6418 needs no more than ten hours of continuous play to "settle" them. They do not have a huge life. Setting the filament voltage at a low 1.2V should give one to three years of hard play with little detriment to sound quality. Higher filament voltages will drastically shorten tube life. The tubes are about $2 each and I bought an extra ten tubes.

Anytime you can dig out micro detail and recorded nuances you WILL get a richer, wider, fuller more detailed sound stage. That's what this preamp can do in spades. Good depth, wide breadth and sharp detail revealing fine recorded detail with no hard edges. Every track I listen to now delivers fine detail previously hidden. Tone and resonance in voice now has an extra layer, finer, breathier than I have heard before. Massed strings sound like the space about them was recorded rather than the instruments themselves, airy almost thin. Oboes donít sound like I remember and every key touch is transported into the room. Piano is a little more "sweet gong" like than all-out bell like. But no register appears to dominate. Bass / rhythm (left hand (lower notes)) are in perfect balance with right hand melody / lead line (higher notes). The piano sounds balanced-tonally, addictive.

The base kit from Oatley Electronics is well worth the $27AU price tag. The original kit may sound too forward though. For about $20 more you can transform this basic kit into a very tonally balance high performance preamp. Or just build yourself a retro-thermionic portable valve headphone driver to go with your space-age mp3 player, iPod or steam driven portable CD player. For 27 bucks, how novel?

Mark Houston
DIY Convener, Melbourne Audio Club


More 6418 Valve (Tube) Preamplifiers and Headphone Amps

I've built a number of these kits now and below are two more. A portable headphone amplifier in a plastic enclosure along with a Cowon S9 media player and the headphones are Phiaton MS400.

6418 Tube Headphone Amplifier Kit
Photograph 6: 6418 Tube Headphone Amplifier Kit

6418 Tube Headphone Amplifier
Photograph 7: 6418 Tube Headphone Amplifier

The final build is a preamplifier in a wooden jewellery box similar to the upgraded build mentioned before.

6418 Tube Preampamplifier Kit
Photograph 8: Jewellery Box 6418 Tube Preampamplifier Kit

6418 Tube Preamp / Headphone Amp
Photograph 9: Jewellery Box 6418 Tube Preampamplifier Kit


Update Feb 2010: Oatley Electronics has introduced an updated version of this kit, the K272A. See the 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A) thread for more info.

Update May 2010: Oatley Electronics has introduced a phono preamp kit, using the same 6418 valves. See my build of the Oatley Electronics K282 6418 Valve RIAA Phono Preamplifier Kit.

Update November 2010: The December 2010 issue of audioXpress includes a favorable review of the Oatley Electronics K272A tube based Headphone Amplifier Kit. The full review is available for download and here are some highlights of the audioXpress Reliable Review: Oatley Electronics K272A Headphone Amplifier Kit.