Dinky little 2-valve Chinese pre-amp

[Yorkie]

Our younger son brought a little 2-vale (or should I say 'tube'?) pre-amp to show me this weekend, which he's just bought on e-bay. It uses 2 x 6N11 (12AU7) noval twin triodes. Built on a good quality PTH PCB, with a Perspex top so you can see the 'innards'. The power supply is 18V AC input, and it came with an 18V transformer (220V input), and a rather messy multiple adaptor to adapt it to 13A UK plug. He asked if I could neaten that up for him, so I mounted it in a Maplin plug-top 'wallwart' enclosure. Not sure about the longevity of using a 220V rated tranny on 240V but it didn't seem to get hot in use.

He suspected that one channel might not be working as the output was low. I squirted a 20kHz sine wave into it, and true enough, that channel wasn't working. Checked the valve filament, which was fine - swapped the valves over, and yes, it's the valve that's duff - not one channel of the amp. Whether or not the supplier expects some valves to fail, I know not. Suffice to say that the amp came with a spare valve, so when our son gets home, he'll substitute the valve and hopefully, that will sort it.

Not expensive at 50$ US (£30 GBP) 'Buy It Now' with free post and it came in ten days.

http://www.ebay.com/itm/6J1-x2-HiFi-ster...0514046949?

6N11 valves are plentiful too, at about £6.00 each post free, both new Chinese and Russian old stock:

http://www.ebay.com/itm/NEW-Shuguang-Chi...0649841315 £6.00 each

http://www.ebay.com/itm/NEW-NOS-RUSSIAN-...0797482407

There seem to be lots of variants both one and 2 valve min amps, the one-valve ones being described as 'headphone amplifiers'.

There are tiny blue LEDs beneath the valves which light them up blue - gimmicky perhaps, or some might feel naff even, but hey - it looks quite cheerful!

I wound the sine wave generator up to 25 kHz and there was no sign of any distortion on the scope trace.

It remains to be seen whether it proves to be reliable in use - only time will tell.

The design needs to provide for the heaters and HT - anyone's guess how it does that, given the paucity of components!

The smallness of size can be seen when compared to the 'wall-wart'. (It's about 3" x 4" x 2" high).

Hope that's of interest.

[Joe]

Those adapters with the 13A pins on are great as they'll take both 2- and 3-pin 5A plugs.

Don't forget that our mains in now nominal 230V so 220V is within tolerance. Whether the transformer is is a different matter.

- Joe

[Steve_P]

Interesting. But check the markings on that mains adaptor - they are Chinese after all.

Cheers,

Steve P.

[Yorkie]

Interesting. But check the markings on that mains adaptor - they are Chinese after all.

Cheers,

Steve P.

Actually, the adaptors do look quite well made Steve, but I won't be using them as I've housed the transformer in a Maplin 'wallwart' box which plugs straight into a 13A socket.

I guess that since the seller sells the amps worldwide, he supplies adaptors which cater for all eventualities in various countries.

As plugs and sockets go, To my mind, the UK 13A design is far superior to any others.

[Nick]

...or should I say 'tube'

No

[tiv48]

HT is 12 volts.
Rob T

[Yorkie]

HT is 12 volts.
Rob T

Yes indeed - the 12V AC is no problem with the 18V AC input Rob - it was just that the component count was so low that I wondered how the upped it for the HT, rated at 300V for that valve though no doubt the valve will work with lower HT than that. I can see how they could uses a 'diode pump' to up the voltage, and to rectify it to DC for the HT, but given its compact size and low component count, it's an impressive little thing. I can only see one diode - a 1N4007, four transistors (presumably?), nine electrolytic and three little brown square jobbies which are 1uF. Can see any transformers/inductors at all.

If I'd have had the time - not that it's mine to play with, I might have taken the Perspex base off and taken a few Voltage readings, if only out of curiosity.

Time will tell as to how it performs and if it proves to be reliable.

[Nick]

On a more serious note, though you might not easily see distortion on a 'scope, it'll be there...

At low B+ voltages, these tubes will work but create a lot of even-harmonic distortion which, as it happens, are not too harsh on the ear.

The original design was by a Korean guy called "sijosae" on DiyAudio years back - lots were built in the mid 2000s based on 12AU7s and 6DJ8s - most of the eBay ones are variants of this...

The 6N11 is similar to the 6N1P (actually a Russian 6Н1П) which is in turn similar to the 6DJ8, so all these designs are related.

A typical schematic is shown below:
   

[Yorkie]

Thanks for that Nick - very informative! So from the circuit as shown, it does basically run on 12V DC HT. Presumably that's what the 1N4007 does - rectifies the incoming AC to supply the DC. I think it will provide more than 12V in this particular instance, in that the AC input is 18V rather than 12V. Yes, I appreciate that just looking at a sine wave on the scope isn't in itself a very sophisticated means of checking for distortion, but at least there was no flat-topping of the peaks with the gain control turned up to maximum. In the final analysis, the ears are the only instruments that really matter!

Incidentally, plenty of valves sloshing around in cyberspace - this seller has 49,847 NOS Chinese make, made in 1984!

http://www.china-hifi-audio.com/en/psvan...-nos-p-605

[Mark Hennessy]

Surely the even-order harmonic distortion is the whole point of such a thing