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Q4: Tube amplifier blows Fuse on turn-on .. why ?

Q4: Tube amplifier blows Fuse on turn-on .. Why ?

4. When I switch on my tube amplifier, sometimes the fuses blow. At turn on. After replacing them – all is functioning just fine. Sometimes it even blows the fuse for my whole apartment. What’s wrong ?

I would say, that this is a perfectly logical behavior of your fuses, although highly inconvenient from the user experience point of view. To understand what is happening, let’s try to “envision” the process of “switching on”.
a). Your transformer, at the very short moment of actual switch on, has no magnetic field within it’s core. It simply did not “build up” yet. At the same time you have the “full thrust” of 230VAC (or 115VAC – whatever!) on the two ends of it’s primary, and that primary, believe me or not, has no more than something like 50~70 ohms of resistance. Divide the 230VAC by those poor little 50~70 ohms, and you arrive at the first major heart attack: the peak current surge, that can potentially travel through the primary coil, prior to it “building up” the magnetic field within the core, a prerequisite that is necessary for normal operation. This “build-up” process can take as long as 1 or 2 miliseconds.
b). The secondary winding of your transformer is connected to a rectifier bridge. Producing the High Voltage for your anodes. But guess what! It is dirty, full of ripple voltage, so it needs to be filtered out. And your amp uses a fair amount of Capacitors to get rid of this “ripple” and to smooth out the voltage, so that you don’t get that annoying hum in your speakers. Alas, but there is no free lunch. The process of loading a capacitor to its “full voltage” requires energy. Significant current is needed to load up you capacitors. What you may, or may not realize though, is that within the first 3 miliseconds after switch-on, those caps have a voltage ZERO, and suddenly get a full thrust of say 400VDC on their ends. The process takes place according to an exponential decay function. Within a short period of time, at the very beginning of the process, capacitors, especially those BIG ones, constitute a short circuit, similar to a thick copper rod.

expotential decay

expotential decay

An unbelievable impact current flows within a short period of time. As the “cap” gains on “voltage”, the current gradually subsides … after some 15 seconds everything is in balance, loaded, and an equilibrium is reached. Just to give you an idea, It is not uncommon to blow 10 Amper recifier bridges, when loading a filter, that is supposed to “deliver” a mere 200mA of current to the output tubes. The bigger the capacity of those caps, the stronger the impact of the surge current, which can reach a value of even 50 Ampers.

Therefore, it is perfectly logical, that the fuses blow. But what can we do about it ? Obviously, it would be of benefit to find a means to “limit” the maximum surge just after switch-on, but in a fashion so that it does not interfere with the quality of our listening experience…. We need a “soft-start” mechanism.

A “soft” and gradual entry, rise, up to the full anode voltage, with a limited current. The easiest way to do it is the way I do it on my workshop, when playing with new constructions. Ok, what I am about to say is certainly controversial, and even might arouse disgust and contempt between some of the purists … but I will venture this route anyway.
I have a riddle for you:
What is the “optimal” self-regulatory element that will limit the peak current of a device being switched on, so as to limit the current surge to no more than say 5 amps, a self regulatory element, that after a few moments later, as if “touched” by a magic wand from a fairy tale ….. simply “disappears”?For now, I shall leave you with only but a hint:
It’s power supply: 230VAC
It’s power consumption: 1000W;
Resistance (while in use): 230VAC * 230VAC / 1000W = 53 ohms.
Resistance (cold filament): 4.3 ohm.

Yes, … you have guessed correctly. This is a very brutal solution, but … a very quick and easy one.

Obviously, much more finesse solutions exist, but … sometimes, when you are in the midst of a project that is just about to be powered up, the Keep-It-Simple-And-Stupid aproach, refined with a little bit of Brutal Force, allows to achieve wonders …

 

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