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Soft-Start for Dummies

Halogen bulbs ...

Halogen bulbs – with a Positive Temperature Coefficient

Soft-Start for Dummies

A recent question was as follows:

I am not a specialist in electronics, but I wish to implement some soft-start mechanism for my tube amp, so as to preserve the life span of my tubes. Is there anything I could do ?

Yes, there is. You can purchase a 230VAC / 1000W halogen lamp. That is my view of a spectacular ;) Hiend-Audio soft start device. YES, exactly, just like the one’s that your painter’s crew used when they were painting the walls in your apartment last summer. The ones that you can purchase dirt cheap in almost any House and Garden supermarket. If you are a bit of a handyman, It suffice that you purchase the bulb with a set of mounting fixtures, and create a safe enclosure for them. Be cool (don’t worry). The box need not be heat resistant. The bulbs will be fairly cool, so do not expect any exaggerated amounts of heat emission to be a major problem.

Next, I suggest that you “modify” a normal power extension cord, in such a manner so that you “route” the current in series: from the mains “live”, through the halogen lamp, and then to the amplifier, and then back to mains “neutral”.

Within the amplifier, the current goes to the primary of the transformer. Such a solution is of benefit to all those of you who do not yet feel confident in “fiddling” in the insides of your freshly purchased tube amplifier. We do not interfere with the equipment. Besides, this is a genuine occasion to “observe” the current impact that happens during switch on of the amplifier. For some brief seconds, you should actually see those halogens glow quite intensively. But that will gradually fade away. Gradually means … within a few seconds, or few tens of seconds, depending on the amount of filter capacitance that needs to be loaded up to you operating voltage within the insides of your amplifier.

You can really “see” how your system behaves. Fist comes a “stroke” of bright light, the “impact”, but then it gradually disappears.

Of course it would be best if the halogens were inside your amp. Then, after say 10 seconds, you could activate a relay to short them totally out of the circuit. A time constant of 10 seconds seems reasonable, or any other that you find appropriate, and you will have a feeling for the optimum value, as you will have the results and the knowledge from your “extension cable experiment”.

Once the relay activates, it essentially detours the halogen lamps, so that the latter pretend that they do not exist. However, the truth is that the filaments of say, 2x500W halogen bulbs, working in parallel …. they will be bored to death by the small little trickle of current that will be drawn by the amplifier during normal operation. They shall lazily and slowly raise one eyebrow, look around, shrug, yawn, and then they will go back to sleep again.
These bulbs will be cool. You could theoretically touch them with your fingers, although quartz glass does not like human fingerprints, so maybe don’t do that.

Here I must admit that I did not implement this idea in my OTL, and I regard that as a true pity. I’ve got resistors that limit the current surge when loading a zillion micro-farads of filter capacitance. Now, please try to count how long is the time constant RC, if you multiply 10 kilo-ohms by a zillion micro-farads. It turns out that it rounds up to 10 zillion microseconds. Simply speaking, it is frustrating to wait at least for two minutes, before the device is fully powered up and ready to rock and roll.
The aforementioned idea with the halogen bulbs is fully compliant with the KISS strategy. Keep it Simple and Stupid, with just a touch of Brutal Force. My favorites.

The fairly “Dumb” halogen bulb(s) perfectly solves the soft start problem.

At switch on, the glowing bulbs heavily restrict the current blow, by taking all of its “force” and converting it to Light. The power rating of the halogen lamp determines the maximum current that can possibly flow. For a few seconds they may be fairly bright, but then they start to gradually fade. Within a few seconds they fade, cool down and eventually seem not to be doing anything at all.
When “hot”, they have a resistance of about 230V * 230V / 1000 Watts = 53 ohms. When cool, measured by an ohms-they have a minuscule resistance of about 3,9 ohms. So they can simply stay for good in the primary circuit of your amplifier and they will not do any harm at all. As soon as they cool down, they will have a resistance of probably 8 ohms at a temperature of 30 degrees. Close to nothing, in terms of the transformers internal winding resistance. The halogen bulb’s resistance is obviously a function of it’s temperature. The relationship is “positive” in nature, i.e. the higher the temperature, the higher the resistance.
A halogen bulb may be treated as a ‘power’ version of a Positive Temperature Coefficient device, just like thermistors. (“PTC”).

This solution of a soft-start on such bulbs – has another important advantage. It e does bring a spectacular feeling upon the guests visiting your home, during a party. You turn on the equipment, and there, suddenly, there is startling brightness all around. A moment of consternation. But then, you smoothly comment: “I’m firing up the Equipment. It is time to rock and roll …”

 

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