Tube Mic PSU Heater Voltage Mod...

Okay, so the noise of the 317 is broadband-type noise, right?
Wouldn't an RC filter after quell that (down to the corner frequency at least)?

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Yes, that's what I tried to tell.  And it doesn't have to be anything special or substantial. I use 4.7 Ohms and 2200u but that leaves some low freq noise. (which will be totally buried in capsule noise)

Jonte, like the idea of the big cap for soft start. Just plop it there in series with the 220R resistor right? You have me worried about noise. Would the above mentioned post 317 RC filter be enough? Gotta start building now to play with this and buy and learn to use a scope...

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No, just increase the value of that bypass cap for the trimmer you have there. Or was that what you meant? Also, if you want a slower start you can always increase that 220R (and trimmer) a bit. Those values are just a good starting point from data sheets.
The post RC is fine, but why not use a bigger cap?

Does your mic have a voltage divider network for grid reference? That is the place where the heater supply noise will most easily get into signal path. I'm paranoid enough to still install some RC filtering there too. 1Meg and small 1u poly will do.

Zener diodes are very noisy, (and caps were big and expensive) therefore that 33R series resistance was originally needed. It does create kind of current limiter, but that was not its primary purpose. How ever I don't see anything wrong in it. A stiffer modern supply with 317 and smaller resistor RC will however make heater voltage adjustments because of tube aging or changes unnecessary. Matter of taste mostly. I have also built heater supplies with Zeners. I like everything passive

OK, the latest...

Ok,

You can leave out that one extra RC from front of 317.
I would put that protection diode for adj. no matter what

Otherwise, I can not see any problems. Oh wait, you have 25V AC ? Isn't it possible that you will exceed the maximum voltage difference for 317 ?

I tend to skip that poly cap from the input pin myself. Keep wires short, I guess you are bolting the 317 to case?

Here we go Jonte.

The data sheet says it can handle a voltage difference up to 40V. My transformer outputs 23.7 unloaded. Figure we get up to about 30V at the bridge rectifier, then lose some through the first RC filter. Say 28V - 6V output = 22V. But it's not really a 6V output from 317 because of the filtering afterward, so it should be less than that.

Am I okay or am I off-base somewhere?