Dealing with tube amplifier inrush current // Thermistor install // Before and after

[@danrussell9357]

I like the explanation that light bulbs would generally blow when first switch on because of the current inrush and that tubes often fail in the same way. The thermistor mitigates this issue.

[@MikeFreda]

Thanks for commenting. Yes, that is a great explanation! Cheers.

[@guitagious9464]

I will definitely be installing one of these before I install my second replacement transformer in my Egnater Tweaker 40. Thank you for a great video that answered many of my questions, Mike!

[@MikeFreda]

Thanks for commenting. I’m installing these on all my amps now. It’s a low cost solution for protecting the amp from high inrush. Cheers.

[@guitagious9464]

@@MikeFreda Right!!! (Seems like amp companies would include these in their original designs. If only Egnater had, I wouldn’t be faced with having to purchase yet another Mercury Magnetics PT!)

[@WestCoastMole]

Mike, something to consider. In a steady-state condition after power is supplies the thermister will still dissipate power. Continuous operation will cause the thermister to age until they reach a point where they reduce the standing voltage to a point that is unacceptable. For that reason it is a good idea to keep an eye on them. A way to extend the operational life of a thermister is to place a relay that shorts out the device that has a one half to one second delay in the coul line. At one half second the thermister's job is done. Reducing the steady-state heating will make a big difference in the life of the device.

[@mobicus1]

Thanks for the great explanation of inrush current and how to manage it!

[@MikeFreda]

Glad it was helpful! Cheers.

[@russellcolon9114]

Thanks!

[@MikeFreda]

You’re welcome, thanks!

[@kanabapuka]

Thanks, great to see the real life application af these components.

[@MikeFreda]

Glad you liked it! My thoughts exactly for doing this video. Cheers.

[@jimnewman5596]

Flattening the initial current spike will extend the life of the tubes and power transformer.

[@MikeFreda]

Thanks for commenting. Yes, I would agree with that statement. Cheers.

[@newtronix]

Ah I should have watched this one first!

[@jeffjeff370]

Well thanks a lot!! great info for my tube amp

[@MikeFreda]

Glad to help! Cheers

[@Jenisonc]

I'm late to the game, but I'm subbed going back to catch up!

[@MikeFreda]

Thanks for commenting! Cheers

[@janinapalmer8368]

The best place to place the thermistor is between the positive output from the bridge rectifier and the first 100uF filter capacitor....

[@MikeFreda]

Thanks for commenting. I'm not sure if that the best place to install them but its a place to install it. Part of the reason there is inrush is the Power Transformer is resisting the state-of-change for a few cycles. This happens before the bridge rectifier and caps excite. If you just want to guard the components after the rectifier, sure install it there. cheers.

[@Stu66orn]

What about both positions before PT and after rectifier?

[@janinapalmer8368]

@@MikeFredathese thermistors are rather old school nowadays ... they are hard to come across and are unreliable as they get older . A delayed soft start circuit in the power transformer is the only way to go ...

[@MikeFreda]

@Stu66orn I do not know. I've never done that. I'm not sure why you need that sort of double protection. It would make your B+ lower though. Cheers

[@janinapalmer8368]

Substitute a 27-33 ohm wire wound resistor for the thermistor and let the delay circuit and relay short out the resistor after the first 100uF is gradually charged up

[@alfovebraseth]

Thanks - I learned a lot

[@MikeFreda]

Glad it was helpful! Cheers.

[@WassimBoudja]

excelent explanation, thank you .

[@MikeFreda]

Glad it was helpful! Cheers.

[@doubledigitdreams]

All the little upgrades are great. I’m waiting on parts for the constant current mod. Is there any way to add a headphone jack to this amp. My amp is virtually dead quiet and want to see if I could use a pair of headphones for some late night listening.

[@MikeFreda]

Good to hear. Yes, if you have low impedance headphones you can tie right into the outputs. If you have multi-tap OT then you can wire the headphone Jack to the highest impedance setting. You would want to make sure the headphone Jack had a switch built into it. There is a “trick” to help lower noise floor a bit by installing some low value resistors at the headphone Jack. These amp make a fair amount of power to drive almost any headphones. Cheers.

[@ericklassen742]

My supplier of electronics components could not provide CL 90s and suggested CL 60s for my typical AA5 vintage radio restorations. Is that unwise for a CL 90 substitute? Can you help me understand the down-side of using the CL 60?

[@MikeFreda]

Thanks for commenting/asking. For the thermistor for operate in its ideal parameters, it should be sized to the amp load. What you can do is measure the inrush like I did and measure the steady state current of the amp. Review the chart and see what falls into that range. Having too large of one has less effect on capturing the inrush and having one too small will exceed its capacity. If you chose one with too high of resistance, the B+ will be slightly lower as well. Unfortunately, I'm not familiar with AA5 Vintage Radio so it's hard for me to tell you want to use. If it was me, I'd just measure.. or even get a few different sizes and try them out like I did in the video. Cheers.

[@pastamatt]

If I have an amp that wants a 5 amp fuse, can I use a a CL-80 (3A) and a CL-90 (2A) in series to handle it for inrush protection and mains voltage reduction? (because it's an early 70s amp and I often get 125VAC at my house.... Do Thermisters work that way? Can they handle the larger current by being in series? And if so, do they need to be right next to each other, or ought they be one on each of the PT primary wires?

[@MikeFreda]

Thanks for commenting. I've never done this and I cannot find anything on the data sheet to say it can work in series as described. I think it should work. They are basically a resistor that once it warms up its resistance decreases. A listed 5 amp fuse may not be an indication on how much the amp draws or what the inrush is. You should measure that (like I did in the video) It may be a good idea to get matching ones and keep them both on the line side on the hot lead. I would not split them up to either side of the PT wires. They can be used to lower wall voltage. If it was me, I'd just use one that is large enough. Cheers.

[@Maxxzinno]

Hi, I have a question about installing a thermistor in my Audio Research Tube preamp. It’s an SP-6A. I bought it used and checked the fuse and it was wrong. There was a 1A 250v slow blow instead of the rated 0.5A 250v slow blow. Not good. I put in the Rated fuse and it blew. The original owner recapped the electrolytics but he didn’t use 1 for 1 values. There’s extra capacitance in each bank. Example 6x 200uF was the original. He put in 6x 220uF. I’m assuming the extra capacitance is causing a current surge on start up. I installed a 60ohm 0.8A thermistor on the hot ac wire to help mitigate this. But the 0.5A still blows. But now better than before because a 0.75A was also blowing before the thermistor. now it’s not. Maybe I need to try a 0.6A thermistor to get the 0.5A not to blow on start up. I measured how high the surge is with my multi meter and it’s surpasses 1A then comes down to 360mA steady state. Will need to experiment with this. Really need to have the rated fuse in there. If u have any tips on how to select the proper thermistor for this please let me know. Thx

[@MikeFreda]

I would recommend running this altered power supply on the Duncan Amps Power Supply Simulator and see what it actually draws for a few cycles. I suspect your inrush current is a lot higher that what you seeing on your multimeter. Standard Multimeter screens can't react fast enough to really gauge what the current is. I have a Fluke 115 that has a max setting and it can just capture what my inrush is on my amps. 200 to 220 is not that big of a deal but it really depends how they are wired (series/parallel/RC/RL.. etc) Without seeing the schematic it's hard for me to determine what is going on with your amp. None-the-less, that is a fair amount of capacitance to charge up and to be honest, the difference between 0.75A to 1A fuse is not something I myself would be too concerned about. Cheers.

[@Maxxzinno]

Hi, this is tube stereo preamp. And it’s not just 1 cap of 220uF. It’s 6x 220uF instead of 6x 200uF. 2x 100uF instead of 2x 75uF, 2x 330uF instead of 2x 300uF. When it was recapped the capacitor values were upped. Maybe he couldn’t find the exact values. All that that extra capacitance must be causing the current spike on start up. The stock fuse is supposed to be a 0.5A. There was a 1A in the unit when I bought it. This is not good. So I put in the 0.5A and it blew. that’s when I realized the capacitance must be the issue. So I added a 60ohm 0.8A thermistor. The 0.5A still blew. So I tried a 0.75A fuse and it didn’t blow. A little closer to the stock 0.5A but would still like to get it to stock fuse. I see some specs for thermistors that mention max capacitance. Does this come into play when choosing the thermistor ? Would going lower in amps and higher in resistance get me closer ? Let’s say 0.5A 120ohm thermistor ? Thx

[@MikeFreda]

The new cap values are probably what was available to the tech. It's hard to give advice on this without knowing exactly the max inrush current. They way I see it, the stated fuse rating is based on the original components as designed. These components have changed so the stock fuse value is not really valid anymore. Yes, you can mitigate the issue with a larger Thermistor that will "absorb" more current. If you are unable to get an accurate max amp reading you could just purchase a couple different ones in that range and try them out. I'd still simulate the PSU on the Duncan Amps PSU designer. It's surprisingly accurate! Cheers.

[@carlosedwardos]

Even 1.1 amps seems like a really big spike of current compared to 60mA operating current, why not use a CL-90 and really clamp down that inrush current, to even better protect expensive transformers, tubes, etc.?

[@MikeFreda]

Thanks for commenting. Yes, a CL-90 may work for reducing the the inrush a bit more, however, the resistance of the CL90 is 3-4 times higher than what I used so you may see a lower primary voltage and a bit more heat. I was also taught for a margin of safety, you should at least double the values of the anticipated amperage rating so the CL90 would just suffice. Cheers

[@carlosedwardos]

​@@MikeFreda lower voltage due to the inrush limiter would be fine, as all of the old radios, TV's and other electronics were designed for either 110, 115, or 117 volts (depending on how old they are) and these days line voltage in USA averages about 123V, so you need some voltage drop, if you want to protect these old components.

[@MCVixenVee]

@@carlosedwardos I concur. He should use a CL-90 or even better the - Ametherm MS22 12104 - Inrush Current Limiter.

[@DerekRonin]

excellent explanation and tip!

[@MikeFreda]

You’re welcome!

[@dakata2416]

Great video Mike, but the audio volume is low. ❤

[@MikeFreda]

Thanks for the feedback. I’ll review the audio setting on that video. Cheers.

[@intelcorei0364]

will these resolve the popping sound of amplifier when i turn on the ups?

[@MikeFreda]

I would need more information about the amp to determine if it would or not. Is in new? Old? Tube? Solid Stat? what sort of PSU is in it? That being said, popping usually occurs when the power supply is trying to stabilize and/or the output coupling capacitor needs to charge up. Sometimes it can be a bad switch or even bad power supply caps. As mentioned in the video, power supply surge is another issue and to clam that you can use the Thermistor. They are cheap enough to try. Cheers

[@davidnguyen5028]

Hello, I’m trying to replace a CL100 NTC 16A on my Adcom Amplifier 5800. I purchased a CL101 NTC 16A from Mauser electronics, since they don’t have the CL100. The resistance for the new CL101 is only at 0.9 ohms at room temperature. Does this sound right to you? I believe I may have selected the wrong replacement thermistor. Shouldn’t the resistance be higher or as long I select the correct type, NTC, at 16A then I should be good to go and not worry about the resistance reading?

[@MikeFreda]

Hello, the CL100 seems to be obsolete and I can't find and concrete data on it so it's hard for me to say if it is okay. If you can find it, just match all the properties as best you can. It won't affect the sound of the amp since its out of the system once it is hot. That being said, 16 amp rating for a amp seems high to me. Most 120v house circuits are rated for 15amps so that must a pretty big amp and your lights must flicker when it gets turned on! lol Cheers

[@davidnguyen5028]

@@MikeFreda I went ahead and installed the CL101 NTC16A anyway. It didn’t look right when I checked the resistance at room temp and see 0.9 ohms on my multimeter, that’s why I was wondering. The amp produces 250 WPC, it’s a nice amp, I just didn’t like the inrush current producing a loud pop on my speakers. The loud pop went away after I installed the CL101. One more question, other than the high resistance, is there something else on how thermistors operate that prevent the current from rushing in so fast?

[@MikeFreda]

Good to hear. I would make sure it is spaced away from any other components. They do get hot. These devices are just a temperature controlled variable resistor. They come in negative or positive. When the negative ones get hot the resistance goes down and are technically removed from the incoming voltage. They are only effective when they are cold. Cheers.

[@brandoncochrone5582]

@MikeFreda hey I have a old Scott's marine navy morale radio and I wanted to convert it to a guitar amp I don't know how much this would cost or if you do this sort of thing or have time to but I figured it wouldn't hurt to ask

[@MikeFreda]

Thanks for commenting. I guess anything is possible. Old Navy radio's tend to use obscure tubes that don't lend itself well to guitar amps. The first thing would be to find a schematic and to see what tubes are actually in it. Sometimes the only thing that is usable is the chassis, tube sockets, Power Transformer and possibly the the Output Transformer if its has the right specs. Good luck, cheers.

[@brandoncochrone5582]

@@MikeFreda I appreciate your reply I will look into finding a schematic and do some more research before I get into taking it apart. many thanks...

[@theoloutlaw]

How do they fair in Transistor amplifiers that 'click and pop' with power On/Off?

[@MikeFreda]

Thanks for commenting. SS amps are not really in my wheelhouse but this is common especially in older amps. There is a myriad of reasons why this could happen. Poor or arching power switch. DC offset in the PSU. (ie =/-45V) One side can raise or collapses first causing a pop upon the inrush. Dirty/staticy controls. You can try a Thermistor to see if it helps. I would also try a good cleaning. Cheers.

[@theoloutlaw]

@@MikeFreda Cheers and Thanks for your reply! :)

[@frankgeeraerts6243]

Can you use them also after the rectifier ?

[@MikeFreda]

Thanks for commenting. I've never done that and haven't really given much thought of doing that either. If you had a Tube Rectifier, it probably wouldn't do much. Inrush is mostly caused by the Power Transformer resisting the state of change. After the initial ramp up the spike is gone. It takes many cycles for anything to happen on the DC side so I'm not sure if there are any gains installing it that way. It may work a bit but it would affect your rectified voltage for sure. Cheers.

[@frankgeeraerts6243]

Thank you .....@@MikeFreda

[@300poundbassman]

Great deal. Nreeds me that chart. ™️❤️😎

[@zdzisiek1979]

👍

[@Auxbeam]

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[@snuffelsuf]

could this guy talk any quieter and softer.....i guess he doesnt care about being HEARD

[@MikeFreda]

Thanks for commenting. I do wear a Lavaliere Mic and set the volume to 1/2 way when editing. I’ve rematches the video on my iPhone and it sounds fine ti me. Honestly, you are the first person to say it’s too quiet. What are watching the video on? Phone? Computer? TV? If this is indeed an issue, I’d like to sort it out. Thanks.

[@marks7663]

@snuffelsuf - Don’t be a jerk! Mike is awesome for sharing this info.

[@johnstuchlik5828]

​@@MikeFredahey Mike the audio on this video is a bit low.the commercials were noticably louder.iwonderd why and looked at volume on phone at 90percent.normally set to 60to70percent to listen to KZbin. I could hear you and good content.thanks for the chart.

[@MikeFreda]

@@johnstuchlik5828 Thanks for the feedback. Audio levels can be a bit hard to figure out. I did get some new mics recently. Hopefully the level are better for the listening audience. cheers