Need a More Capable Bias Supply?
Often, when implementing mods or improvements, you’ll need a raw bias supply that is higher in magnitude or capable of more current delivery than your existing bias supply.
Low Impedance and High Impedance
“Low-impedance” supplies are either a tap or dedicated winding, with a rectifier and filter cap. Minimal or zero resistance is present between the winding and the cap. This type of supply can usually provide many tens of milliamps.
“High-impedance” supplies are known for high resistances between the winding and the filter cap. An example is the use of the plate supply with grounded centre-tap, with a 100-220k resistor in series with a diode feeding the bias filter cap. To keep the filter cap voltage from rising to the same magnitude as the B+, but negative, a place a resistor or zener across the cap. The zener is preferred as it gives a constant voltage-clamping action at a specified value.
A second form of high-impedance supply is the type that is capacitively coupled from the plate winding. In this case, the winding uses a full bridge to generate B+. A cap, resistor and diode feed the bias filter cap. The coupling cap “scoops” charge into the bias filter cap on a half-cycle basis. The cap impedance is very high, so maximum current from this supply is limited, as is maximum output voltage.
Raw Bias Voltage and Applied Bias Voltage
We should also make a distinction between the “raw bias voltage” and the “applied bias voltage”.
“Raw” bias is the value of the supply at the first filter cap and prior to regulation. This voltage should be fairly constant regardless of the loading, or not, of a bias-set network.
“Applied” bias voltage is the voltage actually applied to the tube control grid to set its plate current. In most resistive and capacitively coupled bias supplies, the “raw” and “applied” voltages are the same. This is an economic choice made by the manufacturer. Marshall, Fender, Peavey, Traynor, Hiwatt, Mesa, Trace Elliott, and all the other major builders have done this and still do this. But, for our own builds, we do not have to pinch pennies in such an important part of the amp.
If your amp has a capacitively coupled bias supply and you are thinking about adding Power Scaling – or merely wish to add a tracking bias regulator for other purposes – add an auxiliary transformer to generate the bias supply. This will provide a low impedance that can support multiple bias pots, etc., and have sufficiently high voltage overall to let the regulator function properly.
The resistively coupled high-impedance supply can be modified to provide more voltage and more current. We simply parallel a few resistors to decrease the impedance between the winding and the final filter cap. Typically, three 100k-1Ws are paralleled to make a 33k-3W. We add 80-100V worth of zeners across the cap to clamp the voltage and protect the cap. Now, we have a -80V to -100V “moderate-impedance” supply.