Q: Cathode LED BIAS & Global NFB

[LinuksGuru]

Hi !

I have an idea to replace cathode resistor and electrolytic capacitor with LED or SiC Shottkey diodes in amplifier with global feedback in PP amplifier. The only question remain how to re-connect global negative feedback resistor. One of the options is to connect it the grid of first input tube but it will yield high EMI noise pickup. So better to keep where it belongs to - in cathode circuit.
I attached 2 variants - which one is correct?
Thanks in advance.

 

[rayma]

Either will work, but bias1 is better.

 

[euro21]

.. but bias1 is better.

++

 

[rayma]

The bias2 circuit includes the (nonlinear) LED as a component in the NFB gain set network, not good.

 

[jhstewart9]

One of the options is to connect it the grid of first input tube but it will yield high EMI noise pickup.

Reverse the plate leads of the PP output section to the OPT.
With the proper network the FB to the grid is now negative.
The first grid becomes the summing point, just as in an Op Amp.
The input impedance is usually low. Should be no problems.
Not often done tho.🙂

 

[mdpaudio]

If onlyna LED is used without a resistor then bias2 is the only option, problems with that?

 

[rayma]

The feedback is not useful if the lower element is only a LED.
Its dynamic impedance is small, nonlinear, and will cause distortion.