07-11-2016, 12:58 PM
Reference the bypassed or unbypassed cathode resistor, another reason to bypass it is in distortion reduction.
Roughly, negative feedback reduces percentage distortion by the same amount as gain.
So if you have a high-gain, ECC83 first stage, you'll have more gain than you need, so you can lose it by applying NFB and get the benefits of distortion reduction.
If you reduce that gain, such as by using an ECC82, or by leaving the cathode resistor unbypassed, there won't be as much gain to play with and you can't have as much NFB, so you won't be able to reduce distortion as much.
This DOES assume that the high/low gain stage is itself only a minor contribution to overall distortion. But it's a valid assumption - most of the distortion would be in the EL84 output stage.
Roughly, negative feedback reduces percentage distortion by the same amount as gain.
So if you have a high-gain, ECC83 first stage, you'll have more gain than you need, so you can lose it by applying NFB and get the benefits of distortion reduction.
If you reduce that gain, such as by using an ECC82, or by leaving the cathode resistor unbypassed, there won't be as much gain to play with and you can't have as much NFB, so you won't be able to reduce distortion as much.
This DOES assume that the high/low gain stage is itself only a minor contribution to overall distortion. But it's a valid assumption - most of the distortion would be in the EL84 output stage.
