4x6N3 preamp board from ebay

[PALUSE]

😀
Hi Koonw.
Let's go "Feedback".
R14 is really needed to apply R17 (feedback), right?😉
But I think is the only way to get both channels stable.
The are some resistor values that are different from original Aikido and Chinese version. The voltage dividers values at the V stage and I stage are quite different, from the Aikido version as the cathode resistors at the V stage and I stage.
Can you advise me if the Aikido original resistor values works fine with feedback?

Cheers
😛

 

[Koonw]

😀
Hi Koonw.
Let's go "Feedback".
R14 is really needed to apply R17 (feedback), right?😉
But I think is the only way to get both channels stable.
The are some resistor values that are different from original Aikido and Chinese version. The voltage dividers values at the V stage and I stage are quite different, from the Aikido version as the cathode resistors at the V stage and I stage.
Can you advise me if the Aikido original resistor values works fine with feedback?

Cheers
😛

Yes R14 is needed, together with R17 forms the feedback ratio. When R17 is open the level at junction R14/R17 is same as input level, no drop of level, open loop gain is 23.5db

With 6N3p on board, when R14 is 100k and R17 is 120k, the level at R14/R17 is exactly -23.5 db, which is the amount of NFB, so net gain is 0 (23.5-23.5) db. When R14 is 100k and R17 1M, the the junction level is -8.7 db, so net gain (23.5-8.7)=~15 db net gain.

Different value of Rk will not change the gain as long as the two Rk are of same value, (some tubes need to work on higher voltage say for 6H30p needs 90V) so a change in Rk does not change the gain. It is the 1st stage gain of tube that is accounted for the operation, 2nd has no effect on the gain. So with 6H30 on board, open loop gain is 16.5 db, with R17 120k, the junction level is -17.4 db, result net gain -0.88 db. With R17 1M, it is 4.85db down, net gain is 11.65 (16.5-4.85) db, which is about 3.5 db lower than if 6N3p is on board due to different gain (mu).

There is some maths involved here.

 

[cl1238]

Hey dude,
First let me tell you that, it was not a kit, but a assembled board.
All the compunents already soldered, and i like to learn and experiment,

Hi, quassar
I'm looking at this board as well but an unassembled kit. Can you tell me if the PCB component label show just R1, C1 etc.. or actually show the value of the component? Because I've previously bought tone control kit which does not show value of the component and no schematic.
Thanks.

 

[Koonw]

.....
There is some maths involved here.

Happy New Year!

Here are the calculations for 6N3 with Aol=15, it can be adopted for any other tubes. You can do some calculations and test with sim attached.

Suppose that Aol is 15, to give 1.5V output required 15/1.5=0.1 or 100mV at J (grid). The input level is reduced while the output is held constant at 1.5V


Gain db V Input(1.5/X) X R17 K

0 1.5 1 120.3
6 0.75 2
9.5 0.5 3
12 0.375 4
14 0.3 5
15.6 0.25 6 1119
16.9 0.214 7
18 0.186 8
19 0.167 9
20 0.15 10


For 0db(1X), voltage between Vin,Vj = 1.5-0.1=1.4, Vout,Vj=1.5+0.1=1.6V
I=1.4/105=0.0133, R17=1.6/0.0133=120.3K
For 15.6db(6X), Vin-Vj=0.25-0.1=0.15, Vout-Vj=1.5+0.1=1.6V
I=0.15/105=0.00143, R17=1.6/0.00143=1.119M