http://www.ozvalveamps.org/hum.htm | Created: 9/11/10 | Last update:
The causes of hum within valves
by Tim Robbins
The original use of directly heated cathode valves forced an early understanding of mains related signals generated from a valve's heater. Terman (1932, Radio Engineering - http://dalmura.com.au/projects/Hum - Terman 1932.pdf) described four mechanisms that generate hum related signals for directly heated cathodes:
#1. Non-symmetric heater-to-grid capacitance between each end of the heater causing a difference current to return to the cathode via the grid leak resistor. (See Radiotronics 119 - http://greygum.net/files/radiotronics/hum.pdf). This mechanism is proportional to heater voltage, and grid leak resistance, and frequency, and capacitances. Hum signal is at line frequency (f).
#2. Magnetic field of heater current deflecting electron flow away from anode, due to the left hand rule. This mechanism is proportional to heater current. Hum signal is at twice line frequency (2f).
#3. Voltage difference to ends of heater changing the amount of emission from the cathode at that end. This mechanism is inversely proportional to anode-heater voltage, and proportional to heater voltage. Hum signal is at twice line frequency (2f).
#4. Intermodulation of any input signal by the hum signal.
The change to indirectly heated cathodes in the 1930-40's has had the following influences on hum generating mechanisms #1-3:
#1. The cathode minimises the difference in capacitance between the heater ends and the grid, as the majority of the heater length now has effectively the same voltage with respect to the grid due to the cathode acting as a shield. Each unshielded end of the heater will have a stray capacitance to the grid. Dual heater valves (ie 12AX7) typically have a physically large 'centre' tab (pin 9), and the significant assymetry may be noticeable when comparing 6V versus 12V heater configurations.
#2. The loop area of the heater that causes a magnetic field to extend into the main electron path between cathode and anode is minimised. The tightly twisted section of the heater wire within the cathode will generate some stray field due to assymetry of the winding, and whether the winding is single or double helix, and any such stray field is very close to the electron path, and only partly shielded by the cathode. The base section of the heater has a much more open loop, and the loop depends on the pinout arrangement of the valve.
#3. The ends of the heater (external to the cathode shield) that are capable of emission to the anode are quite short and often isolated from the anode by a mica spacer.
However an indirectly heated cathode introduces a new hum generating mechanism.
The heater and the inside of the cathode shield tube are not made of highly emissive material, but a hum current still flows between them due to emission and the AC voltage variation between portions of the heater and cathode.
This hum current flows in a typical cathode biased circuit to the ground return for both heater and cathode power supply circuits. The voltage difference between heater and cathode has a significant impact on the emission flow. Terman (http://dalmura.com.au/projects/Terman 1932 emission hum.pdf) plots the current-voltage curve characteristic for two emitting surfaces in a vacuum (ie. a diode valve).
When the heater-cathode voltage differential is low, the dynamic (AC) resistance is low and the hum current highest, but as the voltage difference exceeds 5-10V then the dynamic AC resistance increases as voltage saturation sets in and the hum current is minimised.
This characteristic is somewhat symetric around 0V, where the role of 'cathode' and 'anode' swap for the heater and cathode tube - as shown by Klemperer in 1936 (http://dalmura.com.au/projects/Heater cathode insulation performance.pdf) for a number of different valves.
The advantage of elevating the heater to a DC voltage, typically +20 to +60VDC, can now be appreciated, as the AC resistance to the fluctuating voltage between heater and cathode (due to AC heater voltage) is at its highest value.
Some circuit configurations inherently elevate the cathode voltage by a substantial level, such as cathode biased output push-pull stages, and cathodyne/split load phase inverter stages. A negative elevated DC voltage is likely to be just as effective, but the issue of heater-cathode voltage breakdown within an amplifier usually dictates the need for a positive elevation.
Baxandall describes this effect in a 1947 Wireless World article (http://mike.wepoco.com/Home/docs). The circuit loop includes the cathode bias resistor and bypass cap (if used), and returns via the heater circuit ground, which is typically a humdinger or heater winding centre-tap arrangement.
The hum voltage developed across the cathode bias components then couples into the output anode signal. The hum from this mechanism is proportional to the cathode bias circuit impedance. Hum signal can be at line frequency, or at twice line frequency, depending on bias conditions. The elevated DC voltage source should be AC bypassed (ie. capacitor filtered) although Baxandall doesn't include this in his preamplifier circuit.