Chinese tube preamplifier - It beeps terribly from the speakers - what can you d

Hello. I don't know much about tube technology, and recently I bought a Chinese 6j1 preamp for funny money. I replaced the lamps with 6ż1p-ew, but except according to better sound quality for me - the brum remained. Here is a diagram, a description that I got for this amplifier and its photo:


I am asking for some advice on what to replace to make silence ... I will add that I grounded the zero and it was the same ... And I noticed that when I disconnect it from the power supply, the amplifier sounds for a while and does not hum. I power it through an ordinary transformer from 230 V to 12 V.

Hello,
At first, something did not suit me here, because 6J1 is a pentode, but after analyzing the schematic, you just "connect" it in the triode circuit. From what I remember, 6J1 = EF95.
Coming back to the problem, analyze the board and the connections for correctness of connections.
Measure the voltage, since you have no current after disconnecting the power supply, it would be appropriate to take the power supply. Measure electrolytes. There is not much difference with this circuit to non-tube circuits.
best regards

zbyszek_k007 wrote:

I am asking for some advice on what to replace so that there is silence ...

Increase the capacity of C2, C4, C6, C8.
Did you measure the voltage +/- 28 V?

I measured the voltage and at + 28v it is 28.9v, at -28v it is -28.9v (+/- 0.1v), but where it should be 12.6v there is little, about 10.13v (+/- 0.02v) . You can hear the noise after turning it off, but before I thought it was not audible. I'm inserting a noise recording. first turning it on, then I increase the volume 4 times. When I touch the potentiometer, I can clearly hear the noise increase, but when I touch the potentiometer and the ground, I cannot hear them anymore. Link to the mp3 file.

zbyszek_k007 wrote:

where it should be 12.6v there is little, about 10.13v (+/- 0.02v).

This glow voltage - is not really a hum.

zbyszek_k007 wrote:

You can hear the noise after turning it off, but before I thought it was not audible.

Something about the mass?
What do you think about this strange power supply?
Symmetrical power supply, voltage multipliers, and still only 56 V for the lamp.

Adamcyn wrote:

What do you think about this strange power supply?
Symmetrical power supply, voltage multipliers, and still only 56 V for the lamp.

Like any weird power supply ... It's weird
The advantage is that you use a regular 12V transformer (and you have safe voltage on the plug). The symmetrical power supply results from the doublers used, although my experience shows that tube circuits do not like "floating" ground.

Adamcyn wrote:

This glow voltage - is not really a hum.

It can be because it's made sloppy. One-way rectification and low capacity is a solution from the beginning of the semiconductor era. Here you need a Graetz bridge (e.g. from Schottky diodes to have a lower voltage drop) and possibly a larger capacitor.

First, however, I would suggest finding out where this hum comes from. This information:

zbyszek_k007 wrote:

You can hear the noise after turning it off, but before I thought it was not audible.

says that possibly the error is in guiding the mass, or is related to that unfortunate mass from the multiplier. I suggest soldering two switches there - one that cuts off the glow circuit, the other one for the anode power supply - and test the hot lamp, cutting off which circuit causes the hum disappearance.

This project is a complete pathology, a typical Far East invention. The author of this strange thing probably thought that if he made a symmetrical power supply, the circuit would act as an operational amplifier. If you were to make a single power supply, you could simplify and give up the input capacitors C9 / C11, and yes - they must be. Due to the presence of the negative power supply, the input ground and minus of the system are uncertain, although in an resistance amplifier both potentials must be "rigid", preferably the same. Because of the R16 / R21 resistors, we still have -28V in the output .
There are no benefits of such a power supply, only the disadvantages.

Not to mention the fact that if we consider the whole circuit as a complete preamplifier, we have a preamplifier that inverts the phase and has an unacceptably high output impedance ...

As for me, asymmetry. As if to omit the D1 diode, or to add a second diode behind the tubes at the other end of the branch, which would balance it. Obviously a Schotky diode? One diode is the path to asymmetry. Such a generator is created. The internal capacity of the lamp is different on both sides and then it is amplified further with a hum. Normally it symmetrizes to ground in the case of AC and here it is massacred.

If the gentlemen, the designers of the chopsticks, wanted to supply this amplifier with safe voltage, that is, with those unfortunate 12 volts of alternating current, what is D1 for? You can raise 12 VAC to these tens of volts with a small transformer, as God commanded, a bridge and a decent capacitor behind it. Or even use a well-shielded and filtered step-up converter (and supply it with a DC power supply). was. Ehhh .. That only a lamp from the housing protrudes to the delight of the Audiowoodo sect . Like a similar "miracle" - "Taga Harmony" with control tubes MOSFET end, or power amplifier placed on its head.

They could put TL071 and the glow of the lamp itself to the delight. Try to supply with a slightly higher voltage. There are sometimes no mass loops here? End of power, probably this lamp also has a 3-wire cable. Controllable from a laptop that also has 3 wires.

Hello
In my case, the mains hum caused the lamp glow to be powered,
I used a rectifier and a 12V stabilizer to supply the glow.
best regards