Project: DIY More Tube-based Pre-Amp Kit (6J1 “Fever” Clone)

Ever since the vinyl “revival”, I’ve been bitten by a vintage audio bug of sorts. Of course, the old formats always come with their headaches and frustrations, but they also have a character and charm associated with them. One thing that’s always puzzled me is why people seem to have an affection for tube (or valve) based amplifiers. A lot is often said about the “warm” character of such amplifiers, which is merely a euphemism for some form of (potentially agreeable) harmonic distortion. As a person who has often been critical of audio compression and the (mis)use of graphic equalisers, it seems strange to me that people would seek to knowingly reproduce the audio in a less-than-ideal way.

Then it occurred to me that … before I tried vinyl records, I thought they were silly as well. After I tried them, I collected a small amount of them because the experience of putting on a vinyl record was rather amusing, but also because they didn’t sound half as bad as I had imagined. Maybe I should try a tube amplifier before I consign them to the pile of trash.

Of course, my budget (and power budget) really doesn’t stretch to buying a high-end Class A piece of Hi-Fi equipment – I don’t even use fancy speakers, often preferring my headphones. Instead, my thought was that perhaps a taste would be good enough. Many years back, I recall a “pre-processor” which had two tubes which you would run your audio through to give it the warm and fuzzy tube sound, despite using a regular transistor-based amplifier to actually drive the speakers. The unit was about AU$200 which was expensive in my eyes.

But a few years later, I spotted the DIY More kit which seemed to do the same thing, sold on eBay as a 6J1 tube-based pre-amp kit, a clone of the “fever” design which is very popular. The whole kit cost just AU$8.87 including the tubes, which I thought was a bargain, so I promptly ordered it and let it sit in my kit drawer.

Years passed, while it continued to sit in the drawer, without ever being assembled. Perhaps it was the fear that it was too complicated and I would screw it up … or perhaps the fear I would be wrong about my perception of tube-based amplifiers … but the day of assembly has now arrived.

The Kit

The familiar zip lock plastic bag makes a return, with all the components haphazardly thrown into it. I’m rather surprised at the lack of care given the fact they are shipping 6J1 tubes in the bag which are made of glass and have a fragile “nipple”, along with the ceramic tube bases … I was just fortunate that nothing broke in transit despite the lousy packaging.

The package consists of a choice of barrel jacks for power input, RCA plugs for input and output, a potentiometer to adjust the volume, a stack of Hyncdz capacitors (<sarcasm> what a premium brand! </sarcasm>), four axial capacitors, some diodes and resistors, two LEDs, four transistors and the two tubes. No instructions are included.

Luckily the silkscreening on the board is very comprehensive, allowing for easy construction despite some of the labelling being in Chinese. While this kit is branded DIY More, this PCB design is almost universal amongst the “Fever” 6J1 clones.

The double-sided PCB has green solder-resist on both sides and white silkscreening on both sides as well. The exception is the ground track which is uncovered – possibly to allow easier grounding with the chassis in a kit with an enclosure. Test points for filament voltage and the split voltage-doubled rails are clearly marked.

One concern I had was the two tubes I received were not identically marked. Ideally, in a stereo system with two channels, having them as evenly matched as possible is one of the aims and starting with a dissimilar pair of tubes is a bit disconcerting. I decided to build it anyway – this is probably just a consequence of the low price.

Assembly

Before starting, I always like to do a quick count of how many solder joints are necessary to complete the project, as that’s a good idea of how long it might take.

Barrel Power Jack - 3
2x RCA Jacks Assemblies - 8
2x Tube Holders - 14
9x Electrolytic Capacitors - 18
4x Axial Capacitors - 8
4x Transistors - 12
Dual Channel Potentiometer - 8
5x Diodes - 10
2x LEDs - 4
21x Resistors - 42
Total: 127 joints

As a result, this kit is probably among the more involved of the ones I have attempted thus far taking slightly more than an hour and a half to complete. As they are all through-hole components, it was easy to just populate and solder in almost any order. While the board has a HASL finish without any soldering-specific thermal management of the pads, the vias did make soldering a breeze and rather enjoyable.

This time around, I was running out of my preferred 0.8mm 60/40 solder with crystal clear flux and instead opted to complete the kit with a beefier 1.2mm 60/40 rosin core solder which leaves some flux residue. I didn’t bother cleaning it as the solder claims that it doesn’t need to be cleaned – however it does make the board slightly less neat in appearance.

It didn’t take too much effort and the board had all components populated. The need to mount the axial capacitors vertically was not implied by the silkscreen, but seemed to be the most sensible way to complete the kit. I suspect other types of capacitor may have been supplied with the other kits.

Then it was a case of installing the tubes which takes considerable effort and care to ensure you don’t break them and you get them fully seated. Unfortunately, I started mounting one of my tubes when I noticed it had a bent pin preventing it from properly seating. I had to extract the tube carefully, manually straighten and re-seat. It was a tense moment.

The other issue was that I had mounted the tube sockets not perfectly flush to the board, and as a result, the two tubes point slightly outward rather than being nice and straight. I guess that just goes to show that this one was “built at home”.

Testing

One of the biggest issues was to find a 12V AC/1A power supply, as many products now use switchmode regulated DC supplies due to efficiency regulations. I did manage to find one from some long decommissioned old telephony equipment, which I lopped the plug off and fitted a suitable plug.

It seems either the line voltage was high (it usually is) and the transformer output was high (quite probably) as the +/-28V rails were measuring 34V – just one volt shy of the capacitor’s maximum working voltage rating. I measured the current consumed with my clamp meter and it seems that the amp really only drew 390mA while driving a high-impedance input, so being below the 1A rating of the transformer probably led to the voltage floating upward somewhat.

The appearance of the kit is rather unusual – the blue glow from the LEDs under the tube bases is probably something I could do without as it obscures the orange-yellow glow from the heater filaments. Speaking of the heaters, the tubes come to life very quickly on applying power, taking just about 10 seconds to warm up.

I decided to hook it into my Rohde & Schwarz RTM3004 to see what it was like – the first test was just to see when the amplifier would be overdriven.

With an input (C2) of 5.5V peak-to-peak, on maximum gain, it’s clear that the output (C1) is clipping on the positive excursion about 12V above the ground level, whereas the negative excursion is easily reaching about -18V. I suspected I had an uneven voltage rail or built something wrong … but I couldn’t see my mistake and the rails were balanced when measured by a multimeter. I chalked this down to possibly the biasing point of the amplifier – I mean, you would not reasonably expect a line level input to be so “hot”.

Dialling back to a more appropriate line-level input of about 894mV peak-to-peak, the output achieves ~7.23V peak-to-peak. It’s rather smooth looking, with the tube amplifier being obviously an inverting amplifier. The left channel checks out …

… so I repeated it for the right channel. Despite the mis-matched tubes, the gain is roughly similar, with the maximum output of ~7.15V peak-to-peak for an input of 894mV peak-to-peak.

With shorted inputs, I was wondering what the noise output would look like. Driving a very high impedance, we see about 274mV of peak-to-peak noise, mostly 50Hz hum with some spikes. I wonder if some of that is being conducted into the amplifier in some way. This looks quite bad – but in reality when driving reasonable impedances (rather than an oscilloscope with 10:1 probe), it seems the hum falls quite dramatically.

As I have the Bode Plot option on the RTM3004, I decided to give it a shot and graph the frequency response throughout the complete range available. Rather interestingly, the unit has about 18dB of gain at 1kHz, dropping to 0dB at 982kHz which is very much into the AM-band … it seems like it’s probably quite a decent audio amplifier.

At maximum gain, the frequency response is quite remarkably flat, well within 0.5dB between 10Hz through to 24kHz. Left channel showed a gain of 18.39dB at 1kHz, dropping by 0.41dB by 24kHz. Right channel was 18.27dB and 0.42dB respectively, being fairly closely matched. Even the phase deviation of -12.42 and -12.79 degrees respectively is commendably similar, and still nowhere near danger of oscillation.

Configured for unity gain, it doesn’t seem that much is gained with very similar results – down by about 0.4dB at 24kHz with a phase deviation of -13 degrees. As a result, it doesn’t seem we are slew-rate limited in any way.

Putting the input into the left channel and plotting the bode from the right channel with the input shorted gives us a slight idea of what the cross-talk is like. The output is 18dB amplified, with the maximum cross-talk in the bass frequencies and upper treble. The output being -9dB to -18dB implies a channel-to-channel separation of about 27dB to 36dB which is sort of like FM-radio. At the best, it was around 50dB of separation. These figures are probably not the best, but may contribute to the character of the amplifier.

To explore how it actually sounded, I connected its input to my Asus Xonar Essence STX and the output into a Zoom H2n Handy Recorder. With this combination, I played a variety of music through the amplifier with it configured to amplify as much as possible while remaining below clipping on the H2n, reducing the output from the Xonar as necessary to keep the system happy. The recordings made at 96kHz/24-bit were then downloaded to the computer and listened to through the Xonar through my AudioTechnica ATH-M50x headphones.

From my experiment, I can definitely hear some sort of sound-stage altering effect which may be a product of the crosstalk which may be related to the design of the voltage-doubling power supply stage on the amplifier. This has the effect of having a slight cross-feed effect which didn’t seem objectionable. The main difference I heard was a slightly warmer vocal with increased mid-range oomph, but it was slight enough to be easily missed as using a different sound card/output op-amp. The amplifier didn’t seem to have any great negative traits such as hiss or noise aside from a hum that didn’t seem too severe and is to be expected from such a crude AC-power input stage and low-quality capacitors.

I actually began to like it somewhat – I didn’t feel like it was something I needed to have permanently attached to my system, but I didn’t find it to be anywhere near as bad as I had imagined it to be. It’s actually a rather competent little amplifier.

Conclusion

While I don’t think I could judge the idea of tube amplification from the results of an AU$8.87 kit, I was pleasantly surprised at how competent the kit was despite its low price, mismatched tubes and somewhat “hum-prone” design. The unit had 18dB of gain at the top end, channel-to-channel gain figures that were very close and an excellent frequency response. It may have been down on the channel-to-channel isolation and hum, but considering that it’s taking 12V AC and pushing that up by voltage-doubling to +/- 28V (nominal) and driving the filaments from half-wave rectified AC smoothed through a capacitor, I think that is to be expected. It’s a compromise which makes powering the kit simpler, not helped by the choice of cheap capacitors which may not have stellar ESR characteristics.

While the lowest-cost bare-bones kit may have been cheap, the packaging and shipping were a bit poor and it was a miracle that the tubes survived intact. That being said, without an enclosure included, it’s a little impractical to use as the tubes could easily be victim to a stray impact from an object, or the whole board might slide around on a table and expose potentially dangerous voltages (measured up to 68V non-ripple-free DC). Without an included 12V AC supply, it could be a little challenging to find an appropriate supply with the popularity of DC-output switch-mode supplies due to efficiency regulations.

On the downside, because it is so competent, the sound of the kit didn’t sound anything like I imagined. It wasn’t hissy, it wasn’t even “singing along” as I would have imagined. Instead, it seemed to sound like a crossfeed of certain frequency ranges and a slight increase in mid-range warmth with vocals sounding slightly fuller. Nothing that would jump out at me like silly EQ presets on a 90s stereo system.

In the end, I’ve got to say that I rather like the kit and how it turned out but I don’t think it’s something I would use everyday. I suppose I could always modify it to make it even better … maybe.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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