Reduce tube gain.

[vmanj]

Hello everyone.
My preamplifier uses the circuit shown in Image 1.
If I understand correctly, this is done to reduce the tube gain.

But if you change this scheme as in Image 2, will it be just a “cold bias" of the tube?

 

[abbey road d enfer]

Hello everyone.
My preamplifier uses the circuit shown in Image 1.
If I understand correctly, this is done to reduce the tube gain.

No. It increases the input impedance, which goes from 1Meg to about 10Meg. It maybe necessary for some applications.

But if you change this scheme as in Image 2, will it be just a “cold bias" of the tube?

Yes. However, gain would not change dramatically, because in both cases, the un-decoupled cath resistor introduces NFB, which limits gain to a maximum of about 10.

 

[Philbo King]

Assuming you want more gain, a 1 uF cap across the 20k0 would boost the gain significantly.

For less gain, increase the 1k0 resistor.

 

[abbey road d enfer]

Assuming you want more gain, a 1 uF cap across the 20k0 would boost the gain significantly.

You probably mean the 20k resistor...?

For less gain, increase the 1k0 resistor.

That would change the operating current by a factor about 0.7, as Gm, so, with the large amount of NFB provided by the 10k cathode resistor, it would not change gain very much. It would also reduce headroom, which may or may not be a concern..

 

[Philbo King]

You probably mean the 20k resistor...?

That would change the operating current by a factor about 0.7, as Gm, so, with the large amount of NFB provided by the 10k cathode resistor, it would not change gain very much. It would also reduce headroom, which may or may not be a concern..

Yeah, the 1k would need to go up to 5k or 10k to have a significant effect on the gain. And it would throttle back the tube current a bit.

 

[vmanj]

Then it turns out that in image 2 there will be less headroom, i.e. more distortion than in image 1 ?

 

[abbey road d enfer]

Then it turns out that in image 2 there will be less headroom, i.e. more distortion than in image 1 ?

Likely...

 

[vmanj]

I see, thank you.