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'LIVEWIRE?' · 11th Oct 2014, 9:03 am

The '96' range of valves filament current is 25mA, whilst that of the earlier '91/92' series is 50mA, so, because the two filaments in a DL91/92 96 are wired in parallel, the total filament current for a set of '96' series valves is 125mA @ 1.5v, whilst that for the '91/92' series is 250mA. I was reminded of that recently, when trying to power an Ever Ready model 'C', which uses DK91, etc., from a Murphy PSU designed to power a radio using the DK96, etc. (or equivalents). The LT winding on the little transformer was being overloaded by the older valves!
To measure the filament current, insert your meter, on a low DC current range, say 500mA FSD or less, between the +ve LT supply and the connection to the valve filaments, which should be pin 7 on each valve. A reading of just under 250mA or 125mA, dependant upon which series of valves is fitted, should be obtained. Although it's important not to exceed 1.5v, you may find that no series resistor or choke is neeeded, but fitting a couple of shunt diodes ( 2 x 1n4007 or similar) across the LT supply to ensure that it cannot exceed about 1.4v may be desirable.

11th Oct 2014, 9:03 am	#74
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Roberts RMB Problems The '96' range of valves filament current is 25mA, whilst that of the earlier '91/92' series is 50mA, so, because the two filaments in a DL91/92 96 are wired in parallel, the total filament current for a set of '96' series valves is 125mA @ 1.5v, whilst that for the '91/92' series is 250mA. I was reminded of that recently, when trying to power an Ever Ready model 'C', which uses DK91, etc., from a Murphy PSU designed to power a radio using the DK96, etc. (or equivalents). The LT winding on the little transformer was being overloaded by the older valves! To measure the filament current, insert your meter, on a low DC current range, say 500mA FSD or less, between the +ve LT supply and the connection to the valve filaments, which should be pin 7 on each valve. A reading of just under 250mA or 125mA, dependant upon which series of valves is fitted, should be obtained. Although it's important not to exceed 1.5v, you may find that no series resistor or choke is neeeded, but fitting a couple of shunt diodes ( 2 x 1n4007 or similar) across the LT supply to ensure that it cannot exceed about 1.4v may be desirable. Last edited by 'LIVEWIRE?'; 11th Oct 2014 at 9:09 am.