MagnetoSound
Well-known member
I have been pondering the fact that the U47 traditionally runs just a single unregulated power rail to both the filament and the plate of the VF14 tube ...
I am used to the idea of regulated filaments being the way to keep both a steady tube gain and optimum life expectancy for the filament - and yet this most hallowed of microphones gets off completely scot free!
So this got me wondering why the filament supply for an alternative tube (I've chosen an EF14) couldn't be just a duplicate of the HT rail, using a voltage doubler to allow similar filtering to that provided in the NG PSU, but from just a regular 6.3v winding?
I have taken as a reference Mr. Archut's recommended filament voltage of 5.05v and calculated filament current of around 380mA. Dissipation across the total R is 2.9w, total drop is around 7.55v, not too much more (and sometimes less) than you would find in a traditional discrete current regulator.
Filter cap values give cutoffs at around 7Hz, will this be good enough at three octaves below line freq (this supply will be providing cathode bias too, remember)? Ramping takes around 26s.
The big gap in my knowledge is in how voltage doublers behave with regard to ripple and switching noise. Also how much current the filament winding will need to supply at power up. Would a 1A heater winding cut it? I haven't seen doublers applied in microphone supplies as far as I am aware and I would be grateful for any insight into anything I may have overlooked. I know this must have been discussed before. I did search but didn't manage to find any definitive answer.
I am used to the idea of regulated filaments being the way to keep both a steady tube gain and optimum life expectancy for the filament - and yet this most hallowed of microphones gets off completely scot free!
So this got me wondering why the filament supply for an alternative tube (I've chosen an EF14) couldn't be just a duplicate of the HT rail, using a voltage doubler to allow similar filtering to that provided in the NG PSU, but from just a regular 6.3v winding?
I have taken as a reference Mr. Archut's recommended filament voltage of 5.05v and calculated filament current of around 380mA. Dissipation across the total R is 2.9w, total drop is around 7.55v, not too much more (and sometimes less) than you would find in a traditional discrete current regulator.
Filter cap values give cutoffs at around 7Hz, will this be good enough at three octaves below line freq (this supply will be providing cathode bias too, remember)? Ramping takes around 26s.
The big gap in my knowledge is in how voltage doublers behave with regard to ripple and switching noise. Also how much current the filament winding will need to supply at power up. Would a 1A heater winding cut it? I haven't seen doublers applied in microphone supplies as far as I am aware and I would be grateful for any insight into anything I may have overlooked. I know this must have been discussed before. I did search but didn't manage to find any definitive answer.