Re: Valve preamp.
I should add that I used ECC83 (12AX7) for all positions as I did not have a spice model for the ECC81 handy.
I will also note that I only modelled the neons/zeners in the dc feed back loop to keep it as close as possible to Fred's original design.
If I ever build it I will drop these and use straight forward resistive division, ie a single reisistor of 560K from the anode of the last valve to the junction of R111/R110 (Fred's numbers) and R110 is 270K to -120V
One of these resistors is then trimmed to set the voltage balance on the diffential pair. (Could make one of them a resistor + pot)
The only reason for the neons in the first place is because in the original design R110 goes to 0V and the resulting division to get down to the 27V of the grid of the differential amps would reduce the gain too much for effective dc control. Use of the -120V for the division removes the need for the neons.
Peter
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