Re: Halicrafters SX17 transformer
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John Cathode current flows through the cathode resistor developing the cathode voltage.? Knowing any two one can calculate the third using Ohm's Law.
Using the maximum plate / screen grid currents from the tube manual I find the same 120mA that you report.
Using the cathode voltage and cathode resistor combinations I calculate about 100mA.? This might be a bit high since I substituted tube manual plate current for those tubes that have a grounded cathode.? So the actual plate current is most likely between these two limits.
Summing all of the heater currents, I calculate 3.9 amps.? Need a 4 amp rating or better for the heater winding.
Using 300 volts @ 120 mA for the plate winding, I found three power transformers in the various catalogs.? Using the lower plate current should allow a few more candidates.??
Still need to know the hole spacing for mounting the power transformer to the chassis. Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Wednesday, May 29, 2024 at 01:25:02 PM CDT, John via groups.io <siloam@...> wrote:
Thank you for sharing the images.?
I have been looking at the table in the manual and diagram as well as valve data sheets to determine the spec of the transformer. The rectifier is connected to its own heater winding and its heater operates at 5vac. The remaining heaters operate at 6.3vac. Finally, the anode voltage at the 6V6 is stated as 300Vdc. Most of the remaining valve anodes are driven via limiting resistors at 260V which is curious given that the datasheet states the maximum operating voltage is 250V, although there is a 10% leeway. In any case.
The three secondaries must therefore be rated at 5Vac, 6.3Vac and 300Vac RMS.
I also looked at the maximum permitted anode current of each valve and added them all up together. It comes to 144mA. The rectifier is rated at 225mA, so that would seem to be well within its capability, assuming all valves were driven at their full current potential, which I dare say, they are probably not.
Rounding up a little for simplicity, 300V x 0.150A = 45W. One also has to allow a bit for lamps etc.
I am not sure how the cathode voltage factors into the calculation of the current each valve is drawing, and have been looking at some valve reference books to understand, but so far have failed to understand. perhaps someone could kindly enlighten me, then I might be a able to arrive at a more accurate calculation.
Thank you for all the feedback and comments so far. They have been very helpful.
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Jim Whartenby