Mullard GM4140 CR Bridge


Mullard GM4140 CR Bridge s/n1662 Housed in black crackle finish steel box with magic eye null detector, mains operated (100 -250volts, 40 -1000Hz)
I have replaced most of the capacitors, and where possible have inserted these inside the original casings. The bridge is in working condition and regularly used. My instrument is equipped with EF37,EM4 and EBZ34 although other Mullard GM4140 bridges are equipped with EM1 magic eyes.

This is one owned by Stephen Scott (model GM4140/1 ser no.RT877):


The GM 4140 bridge can be used for the measurement of resistances from 0.1 ohm to 10 megohm and of capacities from 1 uuF to 10 uF. However larger resistances and capacities can be measured by comparing their values with standard resistors and standard capacitors. Inductances can also be determined and compared and also short circuits between windings can be determined. The capacity and resistance of electrolytic capacitors (and consequently the loss factor) with help of a known capacity and a known resistance can be measured. The bridge can be feed with an external frequency source up to 10,000 Hz to measure the resistance of electrolytic capacitors. This voltage source has to supply only 7 W at 100 V to and 11 W at 220 V.

Here is a view of the inside of mine showing the mains adjustment link and the two screw contacts which connect the bridge to a winding on the mains transformer. When partially unscrewed the bridge may be energised from an external source.

Similar instruments were made by Philips which have EF6, EM1 and AB2 and others AF7, AM2 and AB2.


This one belonged to the husband of Rama Bhikhabhai in Sweden On this site you will find documentation in German and Dutch on this instrument. I have translated the instruction manual with Frank's help and with his permission include it here click for instruction manual. Incidentally, Frank has an excellent resource for tube data Tube Data

I have traced out the circuit diagram, this was not very easy as the unit is very compact and all the wires are covered with black insulation which over the years has become sticky. (Note that there may be slight differences between the various Philips and Mullard models). Click here for circuit diagram
Bob Worsley has sent me scanned pages of an instruction manual in English. You can download it here (3.6MB) gm4140.pdf