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 Post subject: Tube Voltage Values
PostPosted: Feb Wed 07, 2018 5:24 pm 
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At the top of a schematic for the Farnsworth radio I am working on there are representations of the 8 tubes. Along side the pictures are a list of voltages and resistance values for each pin. Are these values that should be present when the radio is on? Can they be used to diagnose problems? Are the resistance values between the pin and ground when the radio is on?


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 Post subject: Re: Tube Voltage Values
PostPosted: Feb Wed 07, 2018 5:27 pm 
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Voltage readings are taken with the radio on. Resistance measurements with the radio off.

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 Post subject: Re: Tube Voltage Values
PostPosted: Feb Wed 07, 2018 5:37 pm 
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The more complete schematics and service manuals usually contains such a diagram. In the documentation, there should be a notation on where the negative lead of the meter should be connected (i.e. "ground" or "chassis").

Yes, they are intended for diagnosing problems. Usually a variation of 20% from the indicated value is acceptable.

If this radio is unrestored, the usually recommendation is to replace all the electrolytic and paper capacitors first. There's no point in diagnosing potential problems with these old components still installed.

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 Post subject: Re: Tube Voltage Values
PostPosted: Feb Wed 07, 2018 6:58 pm 
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AJJ wrote:
Usually a variation of 20% from the indicated value is acceptable.

There's usually a note about the meter resistance used for the measurements.

Most sets of the tube era (pre-WWII) used 20,000 ohms-per-volt meters like the Simpson 260. While this will not matter for measuring B+ at the output of the power supply, it can make a large difference in readings at points separated from the source by a significant resistance.

If you use a high-resistance (10 Megohm input) meter for your measurements, you can match the 20Kopv performance by connecting a resistor across the test leads. The value will be 20,000 times the range the meter was originally set to. That may require some guessing. For example, on a 100v range, the resistor would be 2,000,000 ohms (2 megohms). On higher ranges you don't need to do this.

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