Positive grid bias and reducing heater voltage when operating tubes for low noise performance

Neutralising the self induced negative bias on the control grid by a typical 4.7MΩ resistor from anode to the control grid makes it possible for an electron tube to operate at a very small anode voltage. This is done by freeing the electrons. Plate voltage can now be even less than 6 volts. So the electron tube can be operated with a simple battery like a transistor or JFET. The best example so far is the EF183 triode connected, second best so far the ECC82. (More information and schematics on other euroelectron posts). In fact it is not that the potential of the grid becomes positive. It is just that at such low plate voltage and high heater emission the grid self assumes a heavy negative potential without the anode to grid biasing resistor.

With the anode to grid biasing resistor, not only can the anode voltage be now so low for a useful electron flow to plate, anode current. It also allows the heater voltage to be reduced even more than half with usually a further increase in performance, more gain and even less noise. So electron tubes can be operated at low cathode temperature and less power consumption. This goes hand in hand when we want to amplify the tiniest signal as for example from a moving coil or ribbon microphone. The same battery supplying both anode and heater not only simplifies the design but reduces to zero, hum, rf, 50Hz power transformer electromagnetic noise induced to the mic input transformer etc. It is a joy to hear the smooth silence.

Underheating the heaters is done on the Neumann U47 microphone by underheating the V14 tube by giving it a lower heater voltage that specified by the manufacturer.

An explanation may be at lower cathode emission, the induced grid current further decreases. This further improves noise performance. Also less electron traffic around cathode and grid allows more information extraction from the few electrons that have the job of carring the tiniest music signal from grid to Anode. An analogy is:


Καθώς μειώνεται η θερμοκραςια καθόδου αυξάνει το σήμα και μικραίνει ο θόρυβος σε σωλήνα ηλεκτρονίων.  Αυτο εξηγείται μάλλον με το οτι η περιοχή γύρω απο την κάθοδο με το νέφος ηλεκτρονίων γινεται μια πιο ήρεμη παραλία. (As cathode temperature goes down, the sea of space charge electrons around cathode becomes like a more calm beach).


This technology is used on the Pleiades V4, V5, V6 pre preamplifiers.


On these preamplifiers, short circuiting from ground to the coupling capacitor of the grid (not to grid as this will upset its self induced potential) brings almost complete silence. The same happens if one short circuits the primary of the input transformer.  The amount of silence is possibly much more than any other preamplifier tested. On other preamplifiers if a male XLR with a zero ohm connection from pin 2 to 3 is inserted, they do not do this so dramatically. If an XLR with an internal wire wound 200ω resistor simulating a microphone in complete silence is connected, only the slightest hiss can be heard (distinguished from 0 ohms) which is the thermal agitation motion of the electrons inside the resistor at the room temperature. The Pleiades preamplifier is so sensitive and quite that it can hear this.


It would be nice if the noise figure is measured.


It would be nice too if the Pleiades preamplifier is used in space as a radio frequency amplifier front end to a radio telescope.

A great acknowledgement to Hliana for insisting that the grid should not be negatively biased.

Reference: Operating features of the Audion - Edwin H. Armostrong