Raw Bias Supply Issues & Mods

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.

Solutions

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.

RBX Raw Bias Auxiliary Supply – When Do You Need It?

London Power Raw Bias Auxiliary Supply Kit
London Power’s RBX Raw Bias Auxiliary Supply Kit

Here is why some amps will need London Power’s RBX Raw Bias Auxiliary Supply along with a Power Scaling kit.

Most guitar amps that are fixed biased have very poorly designed bias supplies, barely adequate to support the tube grids connected to them. Few have enough control range to actually turn the tubes ‘off’.

Two Problems With Stock Bias Supplies

First, they are usually “high impedance”, which means they are derived from the plate winding through very high value resistances (100-220k) or through capacitors. A high-impedance bias supply cannot support proper bias-set networks nor will it support a bias regulator.

The second issue is that they lack enough voltage range to properly control all samples of tube that may be plugged into the amp. Resistively derived bias supplies can have this range, but will lack the current needed for a bias regulator. Decreasing the series resistance creates a high amount of waste heat, and the bias regulator could be damaged by excess voltage to its input. Capacitively coupled bias supplies are inherently limited in both their voltage range and current output.

A separate bias winding has the potential to be “low impedance” and also of high enough voltage to properly support a bias regulator. Marshall’s 1959 and 1992 models use bias windings of sufficient voltage to properly support a bias regulator, provided the stock series resistances in the supply are reduced to 470R each. Although many Fender amps have separate bias windings, these are all too low in voltage to support a Power Scale circuit. Hiwatt’s bias winding will work if the bias supply is rewired as a voltage doubler, which requires lifting the grounded end of the winding. See our book, The Ultimate Tone, Vol. 2 (TUT2) for details.

So, most fixed-biased amps will require RBX Raw Bias Auxiliary Supply to achieve proper performance from the SV1 or SV-TT Power Scaling kits.

Note that the very earliest Power Scaling kits included RBX along with a BMK Bias Mod Kit, but these add-ons are not required to achieve Power Scaling, nor are they universally missing or required with newer amps, so the kits were separated so that installers could buy just the parts they needed. This also made the basic Power Scaling kits much less expensive.