AC Measurement And Resistive Line Cords.

AC Measurements and Resistive Line Cords ~ By Bill Jones

   

I found a small Monarch with a nice looking cabinet. It is a TRF with only four tubes in a series string. The main problem with the radio was with the resistive line cord.  Even if the resistive cord is good, they are usually   ragged and not presentable.  This cord was bad so I had to decide on how to repair the set using some means to drop the voltage.  There are several ways to drop the voltage and after some consideration I decided on the  “AC-diode” voltage dropping means.  It only requires a small diode and resistor in series with the filaments.

    

These resistive line cord radios often have nice cabinet features, but perhaps limited electronics.  They seem to be neglected because of the cord problems. It is not that difficult to get the sets working, but it is difficult to measure the voltages of the rectified waveforms. After repairing the set, I decided to measure the filament voltages and soon realized that it is not possible with the usual multi-meter.  I found this of some interest.

It is because multi-meters do not have AC reading meters.  They have DC meters. Therefore, the multi-meter must rectify the AC voltage, converting AC to DC.  Then rectified voltage then measured by the DC meter.  These meters rectify the AC and then usually filter it. This gives a DC reading of the peak value of the waveform or if not filtered it will give the average DC value. The meter scale is then falsely calibrated to read what is called the AC value of the voltage being read. If you only use the meter for AC waveforms there is no problem.   Digital meters often have a “true” RMS reading but they still will not give a correct reading.  The reason is that the meters are AC coupled and ignore the DC component of the half-wave rectified waveform. I finally gave up and used a Weston wattmeter to measure the value of the rectified RMS voltage.  A variac with a load resistor was used.  It is then only necessary to measure the “power” (120 line volts) with a diode to the resistor, and then measure the “voltage” without the series diode to obtain that same power.  This voltage without the diode is then the equivalent to the RMS of the rectified waveform. For 120 volts RMS, the value of the diode output is 85 volts.  An Internet search often yields a value of Vrms/2, or 60 volts.  The correct value is Vpeak/2, or 85 volts.

It is quite unusual, but I find that the best way to measure the RMS value of this rectified waveform is to measure the actual DC voltage at the rectifier output with a DC multi-meter and multiply by a constant.  This constant is found by calculation of the ratio of the RMS to average DC values of the rectified waveform. The RMS value of the rectified waveform is Vpeak divided by two. The average DC value is Vpeak divided by Pi (3.14).  Vpeak is the RMS line voltage multiplied by the square root of two (1.414).  The ratio (RMS/DC) is then 1.57.  The calculations are obtained by using the algebraic definitions of the “root-mean-square” (RMS) and the “average” value of the rectified waveform.  It is interesting that a difficult to measure RMS value can be obtained by a simple DC measurement.

  

 The diode recommended for voltage dropping use is the 1N4005 for 300 mA (or 150 mA) tubes.  This diode is capable of 1 Ampere continuous and good for a peak repetitive voltage of 600volts.   Remember that when a diode is over-stressed, it has a bad habit of shorting rather than opening.  

  If your tube voltage filaments add up to less than 85 volts (120 line voltage assumed), then you need some resistor in series with the diode.  In the Monarch case the tube voltages added up to 49 volts, thus 85 minus 49 was the required drop.  Then 36 volts divided by the current of 0.3 Amperes gives a resistance of 120 ohms. The power requirement is then current squared times this resistance or about 10 watts in this case.   

This Monarch does not have a pilot light. If you have a pilot light it is usually necessary to protect it from the surge current at turn on, because the cold-tube filament resistance is quite low. The resistive line cord will often protect the pilot light.  Any method other than using a large series resistor for voltage drop may cause a surge current that will burn out the pilot light at turn on. A zener across the pilot light is recommended and the value would be nine volts.  Just check that the pilot light has a current rating the same as the tubes.

  

This note was written to encourage restoration of the resistive line cord type of radio and to make note of an unusual means for measurement of the rectified AC waveform voltage. The actual correction factor for this DC to RMS measurement is Pi (3.14) divided by two. This yields a value of approximately 1.57.  To find the RMS value of a half-wave rectified waveform, it is only necessary to measure the rectified DC value, and multiply it by a correction factor of 1.57

I wrote this article after restoration a Monarch resistive line cord radio. It is a TRF and has very good sensitivity. A picture of the Monarch is shown after it had been restored.  I believe this set was manufactured by Champion Radio labs.  I have seen similar variations of this circuit on a number of radios. The radio has an unusual finish. The first coat on the radio is a white wash that is then lightly stained with walnut.  It was then covered by a clear lacquer.  

A picture of the set is shown below. The set has a new resistive line core which is a new old-stock replacement cord.  While the cord is new it was not good.  I put it on the radio just to make the radio appear original.

The radio is AC-DC or can be used on batteries. If you want to use it on batteries it requires changing a switch position. The switch that is on the back side of the chassis.  The batteries must be connected to a five prong plug that is beside the switch.  It is doubtful that the radio was ever been used with batteries.

Questions or Comments? Please e-mail meat whnj@att.net  Thanks, Bill.