Pop when power on

Finished a 12B4 preamp, voltages good and signal passing just fine.

Only issue is there is a nasty loud pop when the preamp is first turned on (nothing on power off). I am using a SPST to break the 120V on the primary of my PT.

I tried a 0.01uF 1000V cap across the switch, no luck. My IEC jack is the filtered type so I think technically there is a cap across the Live and Neutral output jacks of the IEC.

What else can I try to remove this nasty pop when the power is turned on? How do I select a thermistor to try?

If you check out the thermistor data sheets, I think what you'll want to look for is something that will give you decent resistance cold, but allow you to draw a good percentage of its max current to bring that resistance down in operation. That might be difficult because the pre won't be drawing much at 120VAC (assuming thermistor on the primary of the power transformer).


2. Could a thermistor be used in series with the B+?

I've always put it on the primary side because it then also serves as a back up fuse (yes, I use fuses too). If it blows in your secondary, more of your amp will potentially remain energized. When going solid state rectification, I've always included one. I haven't noticed excessive turn on pops, but then I do always turn the pre on before the power amp.
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The data sheet for NTC inrush current limiting thermistors is here.

I tend to look at the CL-90 and CL-130. When the "steady state" draw of the signal circuitry is not large enough to create enough heat, encapsulating the thermistor in wool or acrylic yarn plus heatshrink can be tried. Retain what heat is generated, to cause the resistance to decrease towards 0 ohms.