• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

VA/CF 6SN7 preamp input points?

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Hi,

Is this OK, or would I need to add something to the VA stage output to keep it stable when not in use? A 1M to ground or something?

Is like to have a switch which allows to select between a zero gain CF/buffer and a preamp with gain.

Thank you!

Blair

 
I just looked at my switch. It's all wrong. Not enough coffee yet. The question is still the same though. Can I break the circuit in half there with a switch without consequence, or do I need to add something to the VA stage while it is "hanging" in the off position?

Thank you!
 
I don't know why Broskie claims that adding an extra resistor to the cathode follower improves linearity. It doesn't. It does isolate the CF from capacitive loads, which is why people may add it.

A quite separate issue is using a resistor in the same position to provide bias, as you appear to be trying to do. For this you need a grid resistor too, to the junction of the two cathode resistors.

To get rid of the bang, you need to ensure that all AC voltages being switched sit at identical DC voltages. This often requires extra coupling caps and high value resistors to equalise the voltages. In your circuit you would need to be careful about adding too much stray capacitance between grid and anode of the voltage amplifier, which then gets multiplied by the Miller effect.
 
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