CL-90 Inrush current limiter for ST-70

Can anyone provide any insight as to how to properly install inrush current limiters (CL-90) on an ST-70 and Mark III. Is this a good cost effective way to reduce incoming AC line voltage? Thanks.

I had to rebuild several Weber WZ-68 solid state rectifiers and they have an inrush current limiter in them.
I had some difficulty with them blowing out due to voltage spikes where I live. I swapped out the IN4007 diodes for IN5408 that were a little more robust and kept the same current limiter that came with it from Weber. I have used them for 6 months with no problems.

Thanks for your help!

I've been playing around with a CL90 and am wondering about its efficacy. I wired it into a switched outlet and hooked up my multimeter to the outlet. The voltage today at the house is around 122.6V. When I flipped the switch, the multimeter read 118V for a split second and then immediately read 122.6V. I figured the voltage would start out at something much lower and take its time to get up to maximum which I figured about 120V. I thought the resistor would "eat" 2-3 volts. The CL90 never got warm to the touch. Out of the circuit I get around 140R reading.

Any ideas or help? Thanks.

Captain Coconut wrote:I've been playing around with a CL90 and am wondering about its efficacy. I wired it into a switched outlet and hooked up my multimeter to the outlet. The voltage today at the house is around 122.6V. When I flipped the switch, the multimeter read 118V for a split second and then immediately read 122.6V. I figured the voltage would start out at something much lower and take its time to get up to maximum which I figured about 120V. I thought the resistor would "eat" 2-3 volts. The CL90 never got warm to the touch. Out of the circuit I get around 140R reading.

Any ideas or help? Thanks.

Hmm. I'm not sure why your CL-90 isn't working as intended, Captain. A defective CL, or something to do with the way you have the current limiter and switched outlet wired perhaps?

I posted elsewhere on this forum last night regarding the CL-90. I will expand on the details a bit here. As explained in that post, I have a CL-90 mounted in an external electrical box, which contains one duplex outlet. I wired the CL-90 in series with the hot line.

My meter probes were inserted into one outlet, while my VTA ST-70 was plugged into the other. With the amp off, the incoming voltage read 122.6 vac . When I switched the amp on, the AC voltage dropped to around 115 vac and sat there until the 5AR4 began its output. Once the amp began drawing more current, the AC feeding the amp slowly began climbing over a couple of minutes. It settled in at 119.7 VAC and that's where it stayed. If your CL-90 never got warm, something is wrong. Mine gets so hot it will burn the tip of your finger if you touch the disc. (Don't try this at home, kids. 120 VAC can kill you.)

As Bob has mentioned, the quality of new 5AR4's isn't what it was fifty years ago, and the voltage feeding our homes is higher than it was back then too. For me, a $3 component is cheap insurance. Certainly not a miracle cure, but it helps.

I've also used them in another application, courtesy of Eli Duttman and Jim McShane. When I did a few modifications to my SCA-35 (upgraded diodes, EFB bias mod) I also added a CL-90 in series with the first leg of the B+. The DC to the boards was running a little high, even with a CL-90 on the incoming AC. Sure, I could have simply added more resistance to the power supply. But the CL-90 has the bonus of adding some time delay to the B+, so the tube filaments get a bit of a chance to warm up. It's not nearly as good as a separate filament transformer, or one of Roy's delay boards, but it is easier to implement.

I wouldn't give up on current limiters if I were you. As you can tell, I really like them.

Shovel, I'm a complete idiot and didn't have the amp plugged into the circuit. There was no current draw. My readings were just with the thermistor installed and nothing else.

Captain Coconut wrote:Shovel, I'm a complete idiot and didn't have the amp plugged into the circuit. There was no current draw. My readings were just with the thermistor installed and nothing else.

Now, now, we'll have none of that "complete idiot" talk around here, Captain. Besides, if I were to post all the complete idiot moves I've done, I'd eat up too much bandwith on this forum! That's why we're here, to help each other out, and to (hopefully) learn from each other's mistakes and successes.

As you noted, a current limiter needs current to work. As it heats up, the resistance drops. Also be aware that the heat is a factor. If there were a momentary drop of the incoming AC (from a thunderstorm or other cause), the thermistor isn't going to work as intended, because it's hot and at its lowest resistance.

Give the CL-90 another try, and please let us know how it worked for you!

Just a note on the tube rectifier side ive been using JJ 5AR4s in my vintage mk3s with SDS cap boards and have had no problems with them so far. theyve been in my amps for about 5 years and get a far amount of use.