vmanj

Triode gain reduction / bias
« on: January 20, 2019, 10:09:11 AM »
Greetings to all.

In image 1, the standard triode inclusion.
In my microphone preamplifier, a circuit is applied as in image 2.

I have a few questions.

- If I understand correctly - R4 added to the circuit to reduce the gain of the stage?

- Cathode bias (hot / cold) is adjusted by resistor R3.
Does R4 have a similar effect on cathode bias, or does its magnitude solely affect the decrease in gain?


ruffrecords

Re: Triode gain reduction / bias
« Reply #1 on: January 20, 2019, 03:07:02 PM »
The bias is set by the current flowing in R3 which is the same value in both cases. The additional 10K resistor introduces negative feedback which reduces the gain. In this case the gain is roughly the 100K plate resistor divided by the additional 10K resistor.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

vmanj

Re: Triode gain reduction / bias
« Reply #2 on: January 20, 2019, 04:37:58 PM »
Thank !

For example, if R3 - 1K is the so-called “warm bias”, in option 1, then it will remain “warm” in option 2, but with less gain.

I hope I understand you correctly ?

ruffrecords

Re: Triode gain reduction / bias
« Reply #3 on: January 21, 2019, 09:17:12 AM »
Basically yes but I don't know where you get the idea that 1K is a 'warm' option whatever that means. The quiescent (bias)  current in the tube is held more or less constant by the dc negative feedback due to the 1K resistor. The actual bias current depends on the resistor value but also on the plate voltage.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

vmanj

Re: Triode gain reduction / bias
« Reply #4 on: January 21, 2019, 12:55:36 PM »
Thank.

Mylithra

Re: Triode gain reduction / bias
« Reply #5 on: January 25, 2019, 09:03:52 PM »
Generally speaking, decreasing the value of the plate resistor, or increasing the value of the cathode resistor will result in a decrease in gain.

abbey road d enfer

Re: Triode gain reduction / bias
« Reply #6 on: January 26, 2019, 06:19:40 AM »
- If I understand correctly - R4 added to the circuit to reduce the gain of the stage? 
Yes, but the main purpose is to provide an input for negative feedback.

Quote
- Cathode bias (hot / cold) is adjusted by resistor R3.
Correct.

Quote
Does R4 have a similar effect on cathode bias, or does its magnitude solely affect the decrease in gain?
It changes the bias point only a few percent, mainly because the voltage across the tube decreases a little.
But remember that the input signal must be DC decoupled (or floating) because the grid sits at about 10V above ground.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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