EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I run my Scotts on either a variac or a step down transformer set about 105 volts. Scott chassis have a hi / low voltage switch on the amp. Up is >110 Volts and down is <110 volts. I have been running my Scotts at 105 volts with the switch in the UP position.

Should I actually flip the amp switch DOWN since I am only running 105 Volts into the radio ??

What say all ??

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Bruce,

At 105 line, the switch in the down position will give more correct filament and B+ from the power supply ( ideal operating range). You also save a few watts and some unnecessary heat at 105. Be sure to switch it before plugging it straight into the line at some point. These sets have so much gain, you probably wouldn't notice the difference much either way, but definitely don't want to plug it into a 120 outlet with the switch down. I've seen line voltages hover near 130 at times which could lead to overheating and cap breakdowns.

regards, Bill

Using a Variac is a good idea, but seems like a lot of effort as I like to run one or another Scott about every day, often with a CD player or IPOD on phono input.

In our area, central Ohio, line voltages have been 122 volts or so. For Scotts with the high-low switch, I run in "High" position , meaning the tubes glow less bright. Scott manuals say to run in the up position unless house voltages are always below 110. Some early 1930's manuals say they are designed for 110 to 120 volts. My early AW-23 manual says it is designed for 110 to 120 volts ac. And to run in the down position (low) for house voltages below 110 and "up" for voltages over 120. I figure Scotts are rugged and house voltages over 120 are not a problem in the "high" position. For later models like the Super XII or my Zenith 12-A-58 that lack the hi-lo switch feature, I think today's voltages are a bit excessive.

During a restoration, and as I have occasion to revisit my older restorations, i am adding a thermistor for a softer start and cut 3 or 4 volts off the house voltage "seen" by the radio. From Mouser I use the CL-80, with a 3 amp rating for radios of 10 tubes or more. For lower tube count radios I use the CL-90 with a 2 amp rating. On antique Radio Forum, some tout using a 6 or 12 volt transformer to buck the house voltage so as to reduce voltage to the power transformer primary, but I have never tried it.

The thermistors I use present a resistance of around 100 ohms when cold. I Install one after the AC switch in series with the transformer primary. They get hot fast when the radio is switched on. As they get hot, the resistance falls towards zero. And they do get hot and need air space and some ventilation. Look a bit like a disk capacitor and about the size of a quarter coin. They act as an inrush limiter and so the dial lights start dim and brighten over 10 or so seconds, reduces the initial shock to the tube filaments and reduces the initial B+ voltage peak as the filters are charging up.

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