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Anyone heard of "in-rush" limiter resistor
Anyone heard of "in-rush" limiter resistor
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Old 18th March 2006, 01:33 PM   #11
andy2 is offline andy2  United States
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Join Date: Nov 2004
Location: Los Angeles
I've just installed a NTC Thermistor CL-70 at the input of the main transformer and it seems to do the trick. There is absolutely no noise at turn-on. The CL-70 has a 16ohm rating at 25degree and can handle current up to 4A.

Thanks to everyone who helped out.
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Old 18th March 2006, 01:38 PM   #12
ErikdeBest is offline ErikdeBest  Switzerland
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Join Date: Mar 2003
Location: Zürich
20x60=1200mv thats just 1.2v!
This happens when the circuit is already dragging some current. But at start up there is almost no current drawn, so even 1k wouldn't help much.

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Old 18th March 2006, 11:23 PM   #13
ivegotmono is offline ivegotmono  United States
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Location: maryland
Here's a schematic with a "surgistor"


Click on schematic at top of page next to HF87

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Old 24th March 2006, 03:26 PM   #14
commsysman is offline commsysman  United States
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Join Date: Mar 2006
Location: california
Smile surge resitors

One thing to keep in mind, is that IF you use the surge resistor right in series with the cathode or plate of the tube, you need to put a bypass capacitor in parallel with it so that is does not cause a non-linear SIGNAL voltage drop during normal operation. Failure to do this will cause gain loss and possibly distortion problems.
It is generally preferable from a design standpoint to put a surgistor in series with the power transformer primary or secondary to bring up the power supply slowly rather than put something directly in series with tubes.
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Old 31st March 2006, 02:39 AM   #15
Hector Ruiz is offline Hector Ruiz  United States
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Join Date: Mar 2006
Location: Metairie, LA
Default In-Rush Current Limiters

I have used them as others have states somewhere in series with the primary winding(s) of the unit's power transformer. I used to purchase them from Digi-Key. they were made by Keystone Electronics. The chart given in the Digi-Key catalog showed the cold vs. the hot resistance along with the safe steady-state current draw that the limiter was capable of handling. Totally unrelated to audio, I used them in series with my household incandescant lamps. I like the color balance of GE's REVEAL series of lamps. The Keystone deveices limited the in-rush current of your typical household lamp. As an experiment, measure the filament of a 60-watt incandescant ligh bulb at room temperature. Apply Ohm's Law E/R = I. You will be surprised at how much initial current flows through the filament of a lamp when it is first turned on! Therefor any tungsten load, such as a tube's fialment can benefit from the same control of in-rush current. With respect to an earlier question about current limiters in the anode circuit of a tube, consider the plate current doesn't flow until the filament begins to "boil" electrons off of the surface of the cathode. You can therefor control plate current indirectly by controlling the filament temperature. I would google search "Keystone Electronics" for distributors of their products.
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