Help! 6sn7GT

[Matthewong]

JojoD818, I've found the schematic diagram but I can't understand what is the actual value for the resistor in the 1st stage cathode as the pic in below, there stated 402, is it a ceramic cap or resistor?But I think there should not use a cap, but use a resistor there right?

 

[SY]

402 ohm resistor, I'd guess.

 

[JojoD818]

I agree with SY, it's a 402 ohms resistor indeed.

 

[Matthewong]

if really a resistor with 402 ohm, then where can I find it? I don't think so you can find this kind of resistor in the market.

 

[ilimzn]

402 ohm is a standard 1% series value, but there should not be much of a difference if you put 390, or indeed 390 and 12 in series

BTW this circuit has partial positive feedback to counter the effect of an unbypassed cathode resistor on the first stage, without the need for a cap. It's gain will be similar to that of your first design, and you said that one had too high a gain. You can lower the gain by connecting the 30k cathode resistor of the follower stage to ground instead of the cathode of the first stage.

 

[Matthewong]

Thank you, hope that i'll success this time.

 

[Brion55]

How long is the wire going from the pot wiper to the grid of the first stage? Is it shielded? Does it still hum with the pot all the way down?

 

[JojoD818]

a picture would really help troubleshooting your preamp Matt.

 

[Matthewong]

Actually I run my wire quite long and messy , that's why I need to rebuild a new one with shorter wire and as little wire as it can. I think it will really help a lot to reduce the hum. When I finish my pre-amp, I will attached a photo to you and you can have a look at it and then give me some commend. I think my pre-amp will be finished in around 1 to 2 week time. Anywhere, thank you all for the helping hand, I very appreciate your help. Thank you.

 

[Matthewong]

I've forgotten to ask you guy something that I don't understand, why all the branded amplifier, no matter tube or solid state. Why the RCA socket did't just fix in the chasis body? They will divide by something like plastic or anything else, ans at last, they still connect a wire to chasis body for grounding. What is the different between fix the RCA socket directly to the chasis body and divide by something and connect it thru a wire? Can anyone tell me? One more thing, my pre-amp will come out a weak 'pop' sound after I switch it on for about 3-5 seconds, and it causes my power amp 'protect', how can I solve this little problem?

 

[ErikdeBest]

Hi

The problem you mention...if I am not mistaken is occasionated through a very low frequency at the output of the pre amp when you turn it on. After things settle in the pre amp this is over...but true, this few seconds can be bad for speakers and our poor 'audiophile' heart.

What you can do...if you use SS power amp turn it on after the pre-amp (when you have both pre an power with tubes, you can turn them on simultanly, because the power amp also has to settle) or, and that may be better, install a 'mute' switch that connects the output of the preamp to ground. Turn your pre amp on, wait some seconds and turn this switch off. A more elegant way, used through some people and available in kits is to use a relay for this work. With a timer or so he automatically 'disconnect' output from ground after some seconds. (or put your volume control at the output of the preamp and turn it way done before you turn on the amp..this way the output is also grounded)

 

[Matthewong]

that's mean the only way, the most easy way is to add a timer relay to the pre-amp output. One more thing, can I straight away fix the RCA socket to the chassis body, without anything such as plastic (avoid the RCA socket ground direct contact with the chasis body)?