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kalee20 · 4th Jan 2019, 9:35 am

If you move the connection of the coupling capacitor to the LOWER end of the 560Ω resistor, then the upper triode and 560Ω resistor becomes just a 2-terminal bit of circuitry, a constant-current load for the lower triode. This allows it to give the full gain equal to the μ of the valve, and at low distortion. Just as Radio Wrangler and others say.

However, the grid resistor of the following stage rather spoils this by reducing the load from near infinity to 270kΩ.

Taking the output from the upper end of the 560Ω resistor gets a bit of this back, in a way which I can partly see, but I'm missing the essential paper and pencil to work it out right now!

I wouldn't, however, call it a cascode - the upper valve isn't operating as a common-grid stage.

4th Jan 2019, 9:35 am	#16
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Puzzling audio circuitry If you move the connection of the coupling capacitor to the LOWER end of the 560Ω resistor, then the upper triode and 560Ω resistor becomes just a 2-terminal bit of circuitry, a constant-current load for the lower triode. This allows it to give the full gain equal to the μ of the valve, and at low distortion. Just as Radio Wrangler and others say. However, the grid resistor of the following stage rather spoils this by reducing the load from near infinity to 270kΩ. Taking the output from the upper end of the 560Ω resistor gets a bit of this back, in a way which I can partly see, but I'm missing the essential paper and pencil to work it out right now! I wouldn't, however, call it a cascode - the upper valve isn't operating as a common-grid stage.