• 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 found that the hum fix posted earlier reduced the hum a little but it still hummed annoyingly. Difficult to interpret the instructions as all the photos have disappeared. I will try to beef up the capacitance multiplier but I don't hold out much hope. The FX Audio unit that I have is so much better than these eBay kits.
 
I have tried a different looking one, but the schematics is the same. I noticed some hum and it seems like it provides some gain, so not a buffer, as I had initially expected.
I didn't take the whole thing seriosly, I bought it because it was cheap and because I had also played with some class D amps (tpa 3118) which I found to be a bit harsh sounding, then somebody wrote that it can be "mellowed" by using the aforementioned tube preamp, the whole point being getting a good sound for little money. I still regard the whole combination as just "toys" just to play with. Unfortunately I don't have enough free time for toys :) On the other hand, if you are on a budget and have lots of free time, this combination (this tube pre+ tpa 3118) should not be easily dismissed :)
 

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I too have bought the FX Audio valve pre-amp and use this one on my hobby room system. It sounds fine and exhibits minimal hum. The circuit for the amplifier part is pretty much identical to the 6J1 kit (with the voltage doubler type of supply). I do not have the circuit for the DC-DC converter. Would like to find a similar module to drive my 6J1 kit and replace the voltage doubler. I run mine with a pair of GE5654W's.

I put some GE565Ws in my FX Audio valve pre-amp and this happened - that lovely fizzle sound with additional smoke! Unfortunately I hadn't tried the 6J1s first so now I don't know where to begin to try to find the cause.
 

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Electrical standards for lighting applications are slightly different than the general use standards. A lighting transformer installed in a audio amplifier is technically a violation of the electrical code. But tube heaters are similar to halogen lamps, so there aren't functional issues. Toroidal transformers may vibratate mechanically: this is the main practical downside. Check the insulation between primary and secondary windings before reusing any old transformer.
 
6J1 Pre-amp from Aliexpress humming removal

Hello fellow nerds!

I bought my first Tube amplifier a few weeks ago! It is a 6J1 preamp. I bought it on Aliexpress and when it arrived it didn't take long to assemble it! Was so exited to finally own one but i gotta say.. It hums alot
So i began investigating, adding 4700µF capacitor all around the PCB to see if it helped, but didn't!
After almost giving up i got 1 last thought.. It could be the layout. So after looking at the layout i noticed the 12VAC was making a run over to the switch in the Potentiometer and it was running directly beside the audio signal for left channel. So what i did was adding a 270K resistor between the tube Pin 1(GRID IN) which took care of it without a problem so tried listening to it but there was a difference in the sound so added one to the other tube and that did it
I also did add a jumper wire to make sure it has a good ground connection as the Ground also passed though alot of components that was close to the AC signal!

Here you can see the diagram with the resistor added! Notice i also redrawn the 100K as it was wrong on the orginal diagram mentioned in the black box in the picture
MPtiKE5.png


Here is my backside in full view
LdKZrLD.jpg


AND a closeup to show you where the problem is
IyeqmxK.jpg


Only thing you can notice now is harmonic sound and a low level humming but the harmonic is stronger!
Hopefully this will help someone!