• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6J1 China preamp thoughts

I normally build high voltage tube preamps but couldn't resist trying the China 6J1 kit even tho I didn't expect much at all. There are few versions, I got the blue one in the pic here. I must say I'm quiet impressed with the sound, it is very clean, no top end loss (that I can hear), smooths out audio little (for a single stage) and can double the output so it's more than a buffer. It is useful little preamp.
But I'm running it at 16V because that's all I had and on the back of the board is printed plate Voltage 28V. I'm reading 38V. Either they made a mistake or maybe it sounds good because it's getting more Voltage. Yes I did drop the heater Voltage resistor to 6.3. Anyone build this thing? What plate Voltage do you get at 12V and what are general thoughts on these 6J1 buffers?
An externally hosted image should be here but it was not working when we last tested it.
 
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Hi, low voltage mode is not very good, better use PCB 6J1 with +B 250 V. I tried variant 6J1 Valve Pre-amp Dual Channel Tube Preamp Preamplifier Board / PCB new hot | eBay with very good sounding. I put 6J1 in pentode mode /You can use triod mode, but mu is about 40-50/ for preamp before phase splitter for PP KT88. I put also 6J5 without changes instead 6J1 with better result
I also used for preamp ECC88 stereo tube preamplifier with power supply PCB DIY
and SRPP Vacuum Tube SRPP Headphone Amplifier Pre-amplifier AMP Board PCB for 6N11/6DJ8 | eBay
 
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These low voltage tube boards from China often use a voltage doubler to get the plate voltage.
I wouldn't be concerned with your plate voltage, but do check the voltage ratings of the electrolytics to make sure you are not about to fry them.
Of course, if the are any opamps in there, you better check the voltage rails on those as well.
 

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These low voltage tube boards from China often use a voltage doubler
It does have voltage tippler, those two transistors take care of that. Other than than it is a straight tube signal. I'm getting another kit just to see what I can do connecting them in serial or chaining all 4 tubes. Depending on total gain and the noise levels it might be useful for mixer, adding tone control, etc.
Other than that I can't see any radical mods being done on the board it's self, it's double sided so I'll explore what can be done with it as it is.
I'm using this one for recording music off the Net, it helps to smooth out some of the grittiness of mp3 broadcasts, it also makes great level control that's why I thought of a mixer. It's interesting fun anyway, for the price one can't go wrong.
 
It's also important to mention that there are tons and tons of low voltage Chinese tube preamp PCB's that ignore the issue of grid current at such low plate voltages, and they may specify a 100K input impedance, but with the actual triode factored in, this is usually much lower.
 
I got the FX Audio 6J1 fully assembled preamp with case recently. I really didn't need another preamp,
I already have three preamps, two built from kits and the third purchased fully assembled. These cost
between $180 and $350. I like all three and two of them I like very much.

I purchased the FX Audio on EBay just to hear what these dirt cheap Chinese preamps sound like.
It was only CAD $38 including shipping, about USD $28 at the current exchange rate and I didn't have
to pay Pay Pal the exchange rate fee because it was listed in Canadian dollars.

It arrived well packed in cardboard box with moulded foam padding
The package contained the preamp in its case, the two 6J1 tubes and the power adapter.

I hooked it up to my Tubelab SSE via a direct input connection and powered up.
Firstly I should mention that this preamp is practically dead quiet, with the vol. pot. at maximum there
is just a barely audible wisp of electronic noise with my ears pressed against the speaker.

For USD $28 this little preamp sounds quite impressive, it sounds almost as good as my other much more
expensive preamps. There is quite good detail and clarity of sound and it matches very well with my Tubelab SSE.
There is no schematic, just a slip of paper with "some instructions". I'm sure the two tubes are essential to the
circuit and are not there for show. I'm not too appreciative of the LED lights in the tube sockets that light up on power on.

So, to sum up, if you don't have the money to spend on an expensive preamp or if you just want to hear what a
dirt cheap Chinese preamp sounds like then this preamp would be a suitable choice.
 

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6J1 has linear good mode with +B more 100-150V /+U about 250V, after plate resistor on the tube +B will about 100-150V, inner resistance of 6J1 in triode is big and Your headphones must be with res. about 1-2 kohm, ....headphones with 30-300 ohm are not suitable.Your headphones must have res. about 2-3 times more inner res. of tubes. If +B is 12-20 V, mode is not linear/. Amp for 28 $ will sound for 28 $....
 
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This is the schematic dragram of the board on the picture at the beginning of the this thread.
Fever 6j1 Tube Pre-amp Circuit - CircuitLab
It is clearly a preamplifier. I paid 7 euro and 19 cents for mine (DIY kit without case); the two tubes and the shipping cost from China are included in the price. It is still on transit so I dont'have tried it yet. The total B+ voltage would be just right for a dry battery radio tube, it is fairly low for a regular 6J1. By the way, the coupling capacitors seems to be way too big.