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## Re: [Amps] Use of toroids in tank circuit of tube amp?

 To: amps@contesting.com Re: [Amps] Use of toroids in tank circuit of tube amp? Manfred Mornhinweg Mon, 08 Sep 2008 00:55:58 +0000 mailto:amps@contesting.com>
 ```Hi Dave, Peter, > I'm looking for information on applying toroidal inductors in the > plate tank circuit of a (large, 4-1000 based) tube HF amp. Are there > special considerations in estimating core size and material type to > avoid saturation, or other electrical considerations? There are many consideration, but saturation is NOT one of them. The tank inductor in a classical tube amplifier sees no DC, and at HF all magnetic materials in existence will melt down or otherwise degrade from loss far before they saturate. So you need to design the inductor such that the magnetic flux stays below a critical level set by loss, and that will "automatically" result in a level far below saturation. > You need to use powered iron cores. That's right (except for the typo of the "d" going amiss in "powdered"). While some ferrites have lower loss, they are not stable enough for a relatively high Q application like this. Also, many ferrites are more touchy than powdered iron when it comes to heat tolerance, due to a lower Curie temperature. > The limitation is the voltage across the windings. Yes. And there are two things here to get it right. First is to understand the ratio between flux density and the other parameters. Flux density is directly proportional to applied voltage, and inversely proportional to frequency, number of turns, and cross section of the core. For sine waves, like we have in an amp, and using metric units, we get the very simple relationship: Flux density [Tesla] = Volts / (turns * Hz * cross section [m2] * 4.44) The 4.44 factor in this formula is given by 2 * 2 * 1.11, where one of the two's stands for the fact that you have two half cycles in each cycle, the other two is for the fact that you will magnetize the material in both polarities, and the 1.11 reflects the difference between the RMS value and the average value of a sine wave. So this equation gives the peak value of magnetic flux density. The peak-to-peak value instead, which is used in some data sheets, is of course twice as high. So, for example, if you have 4000 volts RMS across your tank coil, at 3.8 MHz, the core has 5 square centimeters of cross section, and you have 100 turns, then the flux density would be: 4000 / (100 * 3800000 * .0005 * 4.44) = .0047 Tesla, or 4.7mT, which might be fine with some powdered iron mixes, but not with others. And this brings us to the second thing you need to care for: How much flux density a given core will take, at a given frequency, for a given amount of loss. This is a figure you have to take from graphs or equations in the data sheets provided by the manufacturers of the cores. Typically these data sheets also give the heat rise for a given core size at a given amount of total core loss, and also they tell what's the maximum temperature the material can safely work at. So you need to consider the ambient temperature inside your amp housing, see how much headroom you have from there to the core's limiting temperature, and that gives the acceptable heat rise, which in turn gives the acceptable flux density. In this you can apply factors for intermittent service, if you like, but you should also consider the loss of the wire, which might contribute a very significant amount of heat. This wire loss is calculated from the current going through the wire (in turn calculated from the applied voltage and the reactance of the coil at the frequency considered), from wire diameter, skin depth at the given frequency, and resistivity of the wire material (at the worst case temperature!). Once these things are clear, you need to start some juggling with core sizes and number of turns, until you arrive at a combination that provides the correct inductance and also will tolerate the applied voltage without excessive loss in the core. And of course you need to repeat the calculations for each band. Which brings me to a side note: It's not good to short out turns on higher bands, like many designers do with air coils. The toroids couple all the turns excessively well for that. If you short turns, a really big loss results. So you need to switch bands by switching in or out entire coils, each with its own core. Typically you would use toroids for the lower bands only, and air cored coils for the higher ones. All the above might sound a bit overwhelming when you first read it, but it is nothing terribly difficult, once you put numbers into it and start understanding the matter. Manfred ======================== Visit my hobby homepage! http://ludens.cl ======================== _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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