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andy2 20th February 2006 02:44 AM

Anyone heard of "in-rush" limiter resistor
Has anyone heard of this?

It is suggested that adding this to the anode of the 300B would limit the initial "in-rush" current at start up.

This resistor when cold has very high resistance but after warm up has very litle resistance.

Where can I purchase this?

Tubelab_com 20th February 2006 02:58 AM

Go to and search for " inrush current limiter ".

I use the GE ones. I have used the CL-70 in series with the power line input. This softens the shock to the filaments when the power is switched on.

dshoaf 20th February 2006 02:58 AM

Check this out for more details:

I've not seen its use in the anode of a tube amp's stage but they are common in the AC line in amps with SS power supply rectification. A good example would be the McIntosh MC-225/240/275 series of amps. Also, Jim McShane's upgrades for the HK Citation II has them inline just after the high voltage rectifiers to slow the rise of the high voltage line before the 6550's filaments get hot.

Hope that helps.



andy2 20th February 2006 04:34 AM


Originally posted by
Go to and search for " inrush current limiter ".

I use the GE ones. I have used the CL-70 in series with the power line input. This softens the shock to the filaments when the power is switched on.


Could you share the schematic where you used them?



Mark A. Gulbrandsen 20th February 2006 04:47 AM

I use em in my PAss Labs and Krell Klones to keep the mains fuse from blowing. Its caused by the inrush current durung the initial rail capacitor charge up. I generally use CL-60's. They can also be shorted out with a time delay relay to take em out of the circuit after the amplifier is operating.


Tubelab_com 20th February 2006 12:49 PM

I simply put it in series with the power switch. It has a relatively high resistance when cold, so it limits the initial current. As it gets warm, from this current frowing through it, the resistance drops to a low value.

Some people also put them in series with the rectifier tube so they can get away with running a larger than normal input cap. Use one in series with the cathode, OR one in series with each plate (2 total).

This is shown on the page in the digikey catalog, though they show a solid state bridge.

Tom Bavis 20th February 2006 01:55 PM

I installed a CL70 (or was it CL-170?) in the AC input in my Fisher 800C and Stromberg 6L6 amps... too late to save the power switch on the 800 though...

protos 20th February 2006 03:20 PM

It is not going to be much use as a delay on the B+ since the resistance when cold is very low- around 5-20 ohms depending on type- so you are not going to drop many volts with e.g. 60ma.
20x60=1200mv thats just 1.2v!
They are more useful as current limiters on big ss amps where the switch on current might be hundreds of amps for a few milliseconds.Of course it won´t hurt in the primary of a tube amp or in line with filament secondary.

kevinkr 20th February 2006 03:27 PM

You don't want to put an inrush suppressor in series with the anode of your 300B, their linearity is not very good. You want to place this in series with the primary of your power transformer.
Incidentally this will not provide a significant amount of delay to your B+ coming on, but will reduce the current flowing when the power is first applied which is kind to capacitors fed by silicon diodes, the transformer, power switch and fuse. If you need slower B+ use either a delay relay or a tube rectifier like the GZ32/5AQ4 or GZ34/5AR4 which have long warm up times.

Tubelab_com 20th February 2006 06:44 PM

If you use a power transformer with a very low DC resistance, like a 500 or 750 watt 480 volt industrial transformer, and a SS rectifier with large filter caps, the surge current at turn on can be 10 amps or more with NO load on the power supply. A CL-70 in series with the power switch makes the difference between smoking diodes, and a happy power supply. This is an extreme (but very real) case, but it goes to show that these things have a place in tube amplifiers too. They also limit the initial current surge caused by cold filaments. If you are using expensive DHT's, these are like cheap insurance.

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