Best place to put it is in parallel with the V1 stage supply cap, is that called C12/whatever?
The other thing you need to be sure of is its power rating because of course the current it sinks is multiplied by the volts drop = Watts.
For example I'm using 5W 100V + 5W 150V in series; that limits the current sink to 30mA max. because more than that and the 150V one will go 'pop'...
Could of course be 5 x 50V is series and so on
If the supplied voltage drops below the zener's it will stop conducting and it would just be like it wasn't there at all. Like where you were before without it.
What you need to do then is find out what is the supply volts for V1 stage, after the 10k resistor wot you just put in
then subtract say 10V from that, then that's the total zeners' voltages you need to add up to that. Then it's simply a question of finding X number of them to make it up.Say if it had to be 230V, could be 2 x 100V zeners + a 30V one, if 215, 2 hundreds and a 15, and like that.
Hope that makes sense
Er, the problem with regulating the whole supply is the power rating heat dissipation thing. Zeners can't do it on their own unless it's low voltage and quite low current, so you start getting into power supply regulators that use a zener as a reference but then a power transistor or something that actually provides the output. Starts getting complicated ~ I found it was sufficient to stabilize V1 stage, which stops the remainder amplifying its fluctuations. That was/is good enough.
) but i know alot of people consider boutique resistors to be audio foolery 
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