Re: Halicrafters SX17 transformer
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Comments in RED
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, May 28, 2024 at 11:41:30 AM CDT, John via groups.io <siloam@...> wrote:
On Tue, May 28, 2024 at 06:37 AM, Jim Whartenby wrote:
John
The SX17 manual says that the measurements shown in the tube voltage chart were made at a line voltage of 115 VAC.? That does not mean that the SX-17 was designed for operation at 115 VAC, it just means that the voltages measured are most likely within +/- 10% of the values given in the chart at that line voltage.? Note too that the voltmeter was specified at 1000 ohms per volt.? Modern DVMs have a much higher input impedance so the voltages you measure will also be higher then those listed in the chart.
Thanks. I got my information from a couple of online articles which mentioned 110V as well as 117V, but I appreciate that mains voltages vary and measurements vary depending on the instrument used. I am using a DMM, but do occasionally check with an AVO 8 for comparison.
I had to look up the AVO 8 meter specs, it is a 20k ohms per volt meter movement.? It will still read a bit high because it will load the circuit less then a 1k ohms per volt meter will.? This is not really a problem as long as you are aware of this issue.? This higher voltage measurement adds to the myth about high line voltages over time.??BTW, 110 volts is the ancient Edison DC specification, funny how things get mixed up.? About the power transformer rating.? You should be able to calculate the cathode current for most tubes in the SX-17.? The voltage given for the cathode is listed in the tube voltage chart, the cathode resistor value is given in the parts list.? Or one can just look in your favorite tube manual for plate and screen currents at the plate voltage given.? Same can be done for heater current for all tubes except for the power rectifier.? When all of this is done you should have a good idea of the requirements for the power transformer.
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I thought about that? myself and will try and do some calculations. I am curious about your exclusion of the rectifier from the heater current calculation. It is obviously heated so must draw some current?The heater for the rectifier is a separate winding on the power transformer since it is at B+ potential so it's requirements are already accounted for in the power transformer, about 5 volts @ 3 amps .? ?The other tube heaters are in parallel so their current requirements are additive.??
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I may have a suitable power transformer here in Arkansas for the cost of postage.
Regards,
Jim
That's a kind offer. I will do the calculations first and see what is needed.?No problem, I have saved more components then I have a need for.? I am not aware of what books and aids that are available on your side of the pond but I recommend that you download a pdf of the "Radiotron Designers Handbook," 4th edition.? It is a great resource and it has the answers to many questions that come up in tube equipment.? Take what you read online with a grain of salt.? If it contradicts RDH4 then it is misinformation, IMHO.? Your replacement power transformer that meets the heater winding current spec will most likely have a B+ winding that is a bit high.? It is not a problem to just live with the higher voltage since any resistance you add will affect voltage regulation with the change in B+ current as the radio plays and the audio sections draws power.? Some resistance in the rectifier plate circuit is OK to limit the current seen by the rectifier tube cathode.? See page 99 of the RDH4 for more information on this point.
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Jim Whartenby