Tubes as Diodes.............but in a VC Filter Circuit...........Possible?

[frequencycentral]

Here's the Stenier Parker diode VCF:



Anyone know if a 12au7 could be used for the diodes, presumably using pins 1/3 and 6/8?

Also can anyone explain how this schematic works as a VCF?

[kurtlives]

Don't know if its possible

but here is how to use a 12AX7 as a clipper as shown by Dano. (This was posted and discussed on the first page of his tube boost thread).

[R.G.]

Anyone know if a 12au7 could be used for the diodes, presumably using pins 1/3 and 6/8?

Whatever for?

Also can anyone explain how this schematic works as a VCF?

They act as current controlled resistors. For small enough signals, in the range of 25mV, the curvature of the diode's forward characteristic is not so bad as to cause gross distortion. The slope of the forward characteristic varies from essentially open circuit down to a few ohms. Raising the control voltage increases current in the input side of the lower diffamp, causing current to flow through the six diodes. This current changes the small signal resistance of the diodes.

The 2N2222/2N2907 pair is a variable gain inverting amplifier. The resistances and caps make it very similar to one of the standard active filters once you do some reduction and generalization of the circuit.

[frequencycentral]

Don't know if its possible

but here is how to use a 12AX7 as a clipper as shown by Dano. (This was posted and discussed on the first page of his tube boost thread).

Yeah, I looked at that but the pinout seems wrong comparing it to the 12au7 spec sheet.

Anyone know if a 12au7 could be used for the diodes, presumably using pins 1/3 and 6/8?

Whatever for?

Also can anyone explain how this schematic works as a VCF?

They act as current controlled resistors. For small enough signals, in the range of 25mV, the curvature of the diode's forward characteristic is not so bad as to cause gross distortion. The slope of the forward characteristic varies from essentially open circuit down to a few ohms. Raising the control voltage increases current in the input side of the lower diffamp, causing current to flow through the six diodes. This current changes the small signal resistance of the diodes.

The 2N2222/2N2907 pair is a variable gain inverting amplifier. The resistances and caps make it very similar to one of the standard active filters once you do some reduction and generalization of the circuit.

Whatever for - I want to make a VC filter in which the audio path is tubes. Thanks for the explanation R.G. - do you think this would work effectively using tubes as diodes? If not - any other ideas?