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Chris55000 · 16th May 2019, 6:55 pm

Hi!

Have you checked the anode/grid phasing?

The simpler bias oscillators use a simple single winding with a tap at an intermediate point for the h.t. supply connection, which normally ensures the correct phasing if the outer end of the winding is capacitor–coupled back to the grid, as is usually the case, but I think Brenell may use separate windings, in which there's always a possibility of mis–phasing, which results in failure to oscillate.

Is the coil you exchanged identical to the original one? If it's not, you might have to compare the two oscillator circuits carefully in case there's differences in the reaction–feedback coupling.

Another possibility is that the coil you fitted has a lower 'Q' than your old one, so it might be worth trying about 20V increase in the EL84's G2 voltage by adjusting the h.t. feed resistor! The negative G.B might also have been different between the two circuits, in which case you should use the.component values/connections corresponding to the machine your replacement coil came from!

Chris Williams

Chris Williams

16th May 2019, 6:55 pm	#2
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,874	Re: Brenell Mk5 Oscillator Hi! Have you checked the anode/grid phasing? The simpler bias oscillators use a simple single winding with a tap at an intermediate point for the h.t. supply connection, which normally ensures the correct phasing if the outer end of the winding is capacitor–coupled back to the grid, as is usually the case, but I think Brenell may use separate windings, in which there's always a possibility of mis–phasing, which results in failure to oscillate. Is the coil you exchanged identical to the original one? If it's not, you might have to compare the two oscillator circuits carefully in case there's differences in the reaction–feedback coupling. Another possibility is that the coil you fitted has a lower 'Q' than your old one, so it might be worth trying about 20V increase in the EL84's G2 voltage by adjusting the h.t. feed resistor! The negative G.B might also have been different between the two circuits, in which case you should use the.component values/connections corresponding to the machine your replacement coil came from! Chris Williams Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 16th May 2019 at 7:08 pm.