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Using a microwave PT as a B+ choke
Hi, Vacuumlanders, here's a somewhat strange topic...
I may have found a use for salvaged microwave power transformers. I exhausted all the usual notions, e.g. spot welder, fishing sinker, landfill (this is close, these transformers are not very useful!) So, how about using the HV secondary as a choke in a tube B+ supply? Problem (but easily overcome): One side of the microwave power transformer HV secondary, the inner end of the winding, is attached to the core, i.e. must be at chassis voltage (don't even think of floating the mount!) Even if disconnected, the winding has poor insulation at that end, so we don't want to expose it to the full B+ at the capacitor end of a CLC filter, nor the 120 Hz FW rectified Vpeak at the tube cathode end of a choke-input filter. However, in a FW rectifier with CT transformer, we can move the choke from the rectifier cathode and put it between the secondary CT and ground where it will do exactly the same job. If you want a CLC filter, just connect the reservoir capacitor between the rectifier cathode and the CT (but cap. negative will be live so don't use a can type.) Now we can leave the HV secondary winding connected to the core with its HV end connected to the B+ transformer CT. The 115 VAC primary is left open. Before we go on, here are some characteristics of a typical 1 KVA uwave power transformer... quite ugly, I fear: Core centre limb: 1 3/4 x 1 1/2 inches cross section (that's the good news, but it quickly gets worse!) The 115 VAC primary is only about 144 turns of quite thick wire - this is only 1.25 turns per volt. Resistance is 3 ohms or so. At no load the magnetizing current alone is close to 3 amps! The 2000 VAC secondary is about 2500 turns of thin wire, pile wound in layers, no interlayer insulation except the enamel covering (but it works at 2 KV so who's arguing.) Resistance is about 115 ohms. BTW, for other experiments, I had already pushed out the laminated iron blocks used to increase the leakage reactance for magnetron service, leaving a straight transformer with a 17:1, or so, ratio. I see no merit in leaving these magnetic shunts in for choke service. Question: what's the inductance of the HV secondary at zero DC and how would it change under, say, 100 mA DC? I looked for an equation for inductance as a function of turns, core size, etc, but could only find solenoids and toroids, no EI lamination equations. Any ideas? So, to go on with the experiment... I tried the HV secondary as a B+ filter choke (as above) in my "Scratch 10" amplifier, a 10 watt P-P amplifier using two 6V6's with LC (choke input) B+ filter configuration. Doing this the power trans. secondary winding CT gets the Vpeak voltage stress to core but, as a B+ winding it is well insulated - there was no problem. I worked, no hum, just a somewhat lower B+ suggesting the inductance was larger than I had before at the rectifier cathode. So that's about it. Do we have a source of "free", albeit quite bulky, tube PS chokes from the uwave landfill? One last question... How could we best use the uwave PT primary? It could just be wired in series with the HV winding for about 12% more inductance (or 12% less, I suppose). I tried putting a FW rectifier on it to get a "free" bias supply but the result was only about 15 VDC peak. Pity. All comments welcomed... Cheers, Roger Disclaimer: Don't do any of this unless you competent to build high voltage tube power supplies. They contain lethally high voltages with high current capacity. Also, don't play with microwave power supplies unless you know what you are doing. The transformers are around 2,000 VAC RMS and the rectifier/voltage doubler can still deliver a lethal voltage from the capacitor after the unit is switched off. |
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