hot varistor - Printable Version
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hot varistor - tab10672 - 11-06-2010
How hot does the (tube saver} varistor usually get? I am restoring an aa5 that is working ok now eccept the varistor gets very hot, hot enough to sizzle water. I understand the princible of holding back the surge but if the high heat is not normal can it be replaced with higher wattage resistor without damage to the tubes?
Thanks, Todd
Re: hot varistor - Radioroslyn - 11-06-2010
What model Philco is it? Don't know of any that use a varistor in the circuit. If it is truly an aa5 the heater string should operate at or about 121v@150ma. +10% isn't going to be a problem for the filaments.
Terry
Re: hot varistor - tab10672 - 11-06-2010
53-565, so far all the aa5's of that year that I have found use a varistor befor the 12ba6 for a slow start
Re: hot varistor - Radioroslyn - 11-06-2010
Well after take a quick look at the diagram it looks like there using it to drop the filament voltage by using it as a 100 ohm resistor. The string in this set equals 106v It's going to have to dissipate about 15W that's why it's getting hot. They're calling it a "tube saver" I would say if it bothers you put a jump across it and replace the 35C5 with a 50C5.
Terry
Re: hot varistor - rghines1 - 11-06-2010
Adding up all the filament voltages comes to 106. Assuming a typical mains voltage of 122 VAC, leaves 16 VAC across the resistor. Assuming a nominal 150 mA for the filament string, the varistor wattage would be 16VAC x 0.150A = 2.4 watts and a resistance of 16/.150 = 106 ohms. The schematic appears to be showing a hot resistance of 100 ohms for the varistor. Curious if you could measure the voltage drop across the varistor. Also would interested in the mains voltage and the filament voltages across a tube such as the 35C5. If you really want to pick it apart, break the filament string circuit also to get a current reading. Makes for a good ohm's law exercise. The varistor at 2.4 watts will burn your finger. Richard
Re: hot varistor - Radioroslyn - 11-07-2010
On second thought you could just drop in a 50c5 in place of the 35c5 that would bring the string up to 121v reducing the load on the varistor. Philco must have had a gut of 35c5's in '53. Most of the time you won't find in an AA5. Typically they where used in sets that used 6 tubes like a AC/DC set with a RF stage or a AM/FM set. The engineers at Philco must come up with this scheme to use up some overstocked tubes.
Terry
Re: hot varistor - Don Lind - 11-07-2010
The purpose is to limit filament current during turn on to extend tube life and reduce warranty costs.