If you have a Nems Clarke 120 series Field Strength Meter or any instrument that uses the 67.5V “B” batteries Bafaro Engineering has a replacement module that will allow you to run your vacuum tube instrument from eight “AA” batteries. The eight “AA” batteries will deliver 67.5V regulated at up to 17mA – this is better than the original “B” battery. The module is the same size as the original battery except for the output terminal strip on the top of the unit. The module has been tested in a Nems Clarke 120B and it does not affect the meter reading. The boost supply is isolated from the case of the unit and will not disturb the bias network used to set the gain of the tubes used in the Nems Clarke 120 series meters.
ONLY INTENDED FOR THOSE EXPERIENCED IN HIGH VOLTAGE CIRCUITRY!
The BE5080-1718A lamp replacement module is designed to replace the lamp module E1 in the HP4800A Vector Impedance Meter. The BE5080-1718A should also work in the HP8640A/B option 1. Although it has not been tested in the HP8640A/B option 1. The E1 module is in the negative feedback path of the Wein Bridge oscillator. The purpose of E1 is to regulate the negative feedback in order to keep the overall loop gain of the in the range of linear operation; This is the range of signal output in which the sine wave is pure and not clipped by running the output stage of the amplifier into saturation or cutoff. The lamps do this by changing resistance in response to the signal level coming from the output of the oscillator.
This change in resistance is highly dependent on the temperature of the tungsten filament of the lamp. This temperature coefficient of resistance is nonlinear so this makes it a good control device. This lamp in the negative feedback path was used in the first HP product, the HP200A Wide Range Oscillator. This was the subject of William Hewlett’s master’s thesis.
Since the lamps originally used are no longer available, a newer control device was sought. The Junction Field Effect Transistor (FET) has variable resistance characteristics that can be made to control the lower half of a voltage divider network inserted into the negative feedback path.