LEDs used as Cathode Bias

[acorn]

Hi All


As any one used LEDs as cathode bias in the first stage of the Kel84 or 6550 Wad amps, would it be possible to use them in this position to replace the cathode bias resistor. and would decreasing the 1000uf to 470 alter the working conditions of the ECF80 I know you would have less bass but would it also increase the distortion, as this first stage been designed to be optimised with regards to the components implemented in this stage.

I would be intrested in any comments. I understand that the signal path goes through that 1000uf cap, so is there another way around this circuit so that I dont have to use it or reduce this cap to a much smaller value without adversly effecting the frequency response.

Regards Acorn

[Andrew]

-- Hi Acorn.

If this is an input stage.

LEDs should be OK but I would try battery bias first.

Isn't there something on the old BB about this?

cheers,

-- Andrew

[Paul Barker]

I haven't found the right led yet ymmv try it.

[Andrew]

Need to get one with a good impedance curve, most are all over the place.

LED bias seems to work OK on the "Seduction" phono, haven't tried it myself.

Seduction uses an HLMP-6000, you could try that.

But I think battery bais might be a better idea.

cheers,

-- Andrew

[IslandPink]

I tried it on a phono , first stage, and it was close to the R + Black-Gate cap I was using in that application, but not quite as good . After a few days, I went back . I'd expect it to be less good if there's significant swing on the bias , as in your case, but if you can find suitable LEDs , it's easy and worth it for educational purposes .
For battery bias, I definitely thing grid-battery bias is better than cathode battery bias - the current modulation is a lot less and the battery has less sonic effect . Battery bias in the cathode is 'tight' but toneless and lacking in low-level , to me : think of the battery as a big electrolytic cap .

Mark

[Andrew]

Yes, Mark, is right. I would stick the negative battery terminal on the grid, ground the positive and ground the cathode. I think I got that right.

This should give you a fixed bias stage. Virtually no current should be drawn from the battery so it will last the "shelf life" of the cell.

If the voltage on the cell is not correct, use a couple of high value resitors to form a voltage divider.

Of course, please try all three as you might like one of the other alternatives.

cheers,

-- Andrew

[Richard]

Originally Posted by Andrew

Yes, Mark, is right. I would stick the negative battery terminal on the grid, ground the positive and ground the cathode. I think I got that right.

This should give you a fixed bias stage. Virtually no current should be drawn from the battery so it will last the "shelf life" of the cell.

If the voltage on the cell is not correct, use a couple of high value resitors to form a voltage divider.

Of course, please try all three as you might like one of the other alternatives.

cheers,

-- Andrew

Well it would certainly sound different . The battery pos should go to cathode and the neg to ground. Now what about the feedback connection?

Rich

[John Caswell]

Hi all,
I think a little bit of confusion has crept in here.
For battery CATHODE bias +ve to cathode -ve to earth, as Richard says
For battery GRID bias -ve to grid via the grid resistor (47K-2.2M as specified in the cct diagram, don't leave it, out he no function otherwise) and +ve to earth, as Andrew says. The battery should be at the earthy end of the grid leak resistor.
Of course if you use battery grid bias and earth the cathode then you don't have anywhere to apply NFB if you are so inclined, tricky!

John aka Dr John

[Richard]

Of course if you use battery grid bias and earth the cathode then you don't have anywhere to apply NFB if you are so inclined, tricky!

If you use battery cathode bias I cannot see a way to apply negative feedback to the cathode.

If you use battery grid bias then you could apply feedback via a divider using 100R between cathode and ground and connecting the loop at the cathode. (Leave g2 connected to k as before.)

Assuming Kel84, try 3k6 in the feedback loop and adjust from there (set amount; dB or sensitivity back to standard, and hf compensation on SW). There would be .26v cathode bias generated by the 100R to adjust the battery voltage for.

Rich

[NealG]

Sure you can Richard, I did it here and also did it the same way on my WAD 300B's