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02-17-2026, 04:16 PM
Hi Guys
The bias regulator in SV1 and TBS Tracking Bias Supply have a feedback resistor that controls the proportion of bias voltage to screen voltage. In absolute numerical terms, we describe this as a percentage, where the bias voltage available to the bias pot is expressed as a percentage of the screen voltage at the tube.
Most large bottle power tubes (and 6V6) used in musical instrument amplifiers have about the same voltage gain and end up with very similar grid-voltage to screen voltage proportion, where |-Vb| is around 10% of Vs. This is the "target" control voltage. Some amplifiers are designed to only have this much bias voltage, for example Hiwatt, and there is no leeway to run the tubes cooler or to accommodate tubes with higher transconductance - these will red plate. As TUTs recommend, the bias pot should have closer to 15% at its 'cold' end to be able to turn off most tube samples.
In our bias regulator, the feedback works against 330k, so we get these percentages for different Rfb values:
56k2 provides 15%
47k5 provides 13%
36k0 provides 10%
30k1 provides 8.5%
The lower values are suitable for EL-84 and 8417.
RBX Raw Bias Auxiliary Supply has a limited output, which is still higher than most stock bias supplies, but its applicability is reduced with higher percentage bias range combined with higher Vs, as follows:
56k2 limits RBX to amps with Vs=560V
47k5 limits RBX to amps with Vs=640V
36k0 limits RBX to amps with Vs=840V
30k1 limits RBX to amps with Vs=1kV
The bias regulator in SV1 and TBS Tracking Bias Supply have a feedback resistor that controls the proportion of bias voltage to screen voltage. In absolute numerical terms, we describe this as a percentage, where the bias voltage available to the bias pot is expressed as a percentage of the screen voltage at the tube.
Most large bottle power tubes (and 6V6) used in musical instrument amplifiers have about the same voltage gain and end up with very similar grid-voltage to screen voltage proportion, where |-Vb| is around 10% of Vs. This is the "target" control voltage. Some amplifiers are designed to only have this much bias voltage, for example Hiwatt, and there is no leeway to run the tubes cooler or to accommodate tubes with higher transconductance - these will red plate. As TUTs recommend, the bias pot should have closer to 15% at its 'cold' end to be able to turn off most tube samples.
In our bias regulator, the feedback works against 330k, so we get these percentages for different Rfb values:
56k2 provides 15%
47k5 provides 13%
36k0 provides 10%
30k1 provides 8.5%
The lower values are suitable for EL-84 and 8417.
RBX Raw Bias Auxiliary Supply has a limited output, which is still higher than most stock bias supplies, but its applicability is reduced with higher percentage bias range combined with higher Vs, as follows:
56k2 limits RBX to amps with Vs=560V
47k5 limits RBX to amps with Vs=640V
36k0 limits RBX to amps with Vs=840V
30k1 limits RBX to amps with Vs=1kV
