RBX Question

[Champ81]

I'm planning on adding seperate power tube bias for a fixed bias amp. I have the BMK already. Would it be recommend I get a seperate bias supply (BMX) for a jcm800 50W type amp? In the notes if I read it correctly the power is supplied from Va which is doable. But I'd guess it is much better to have a seperate supply?

[K O'Connor]

Hi Guys

It is always preferable to have a separate PT or PT winding for bias supplies AND to use a full bridge rectifier and proper filtering, all of which provide a reliable source for the most important voltage in the tube amp WHILE allowing for proper grounding.

RBX covers this application and is powered from the 6V heater already in the amp.

BMK uses low-value pots and to power it from the plate winding would require a low-ish dropping resistor value of significant power rating. R would be 100k, say, made up from a couple of 220k in parallel, or better, three 330k in parallel, each 1W. The filters should be protected by a 100V zener, as the TUT3 projects show.

The RBX approach is used in TUT5's The Standard project, but using out-dated filter cap values (0ne-tenth what they should be).

[makinrose]

Hi Guys

It is always preferable to have a separate PT or PT winding for bias supplies AND to use a full bridge rectifier and proper filtering, all of which provide a reliable source for the most important voltage in the tube amp WHILE allowing for proper grounding.

RBX covers this application and is powered from the 6V heater already in the amp.

BMK uses low-value pots and to power it from the plate winding would require a low-ish dropping resistor value of significant power rating. R would be 100k, say, made up from a couple of 220k in parallel, or better, three 330k in parallel, each 1W. The filters should be protected by a 100V zener, as the TUT3 projects show.

The RBX approach is used in TUT5's The Standard project, but using out-dated filter cap values (0ne-tenth what they should be).

Was there a lot of ripple or other problems with the older 10 uf value?   I'm not sure if it is applicable but I have noticed a tightening of the sound in older Fenders when I go from the sometimes very small value of the cap in bias supply to 100 uf.

[K O'Connor]

Hi Guys

The 10uF bias filter cap values work okay in the resistively-derived plate-winding-sourced bias supplies where the 100k to 220k series R helps create a long time constant for the first RC section, and typically a 15k works against the second 10uF. In this classic design the bias-set network parallels the second cap. These light values allow the bias supply to vary as the screen and plate voltages vary, which helps maintain the q-point for the output stage over the signal cycle and output loading.

To me, those values are archaic and the high-impedance bias supply is irksome considering the importance of this supply. My preference is always to use full bridge rectification and have proper filtering. In the case of a Power Scaled amplifier, a well-filtered and stiff supply is preferred, as Power Scaling maintains the output stage transfer function and will do so better under these conditions.

The first RBXs issued used the archaic values but for a while now it is shipped with better filter values and lower-loss RC sections. Most hobbyists use it along with a Power Scale kit, but it can also be used simply as a bias supply for passively tracked circuits.

Any stiffness added or dynamic quality that you feel is lost by using more ideal circuit values that reduce noise, can be accommodated by other means. Best to design out the noise and then tailor the circuit to the tone you want.

Be truthful to tone NOT to circuits.

[Champ81]

I have drawn up a layout before installing.  I had a copy of the installation notes from a previous build but that had an SV1.  

Have I gotten this correct?

[K O'Connor]

Hi Guys

That is not how you connect RBX. How is it you have an ancient version of it?

Assuming the old version: when used as a raw bias supply with BMK, the negative output goes to the bias pot-0 via 10k or so, otherwise the negative sweep is too extended and of no use for typical amps. If the amp uses low-mu triode output tubes,, or is something else with more usual tubes and Vs is c600V, then a direct connection to the bias pot-0 is okay.

RBX positive output ties to ground at the screen filter cap negative.

Bias pot-X ties to ground either individually through 6k8 or collectively through the same or lower value.

With the current RBX PCBs there are onboard positions for both range resistors, selected and supplied by the user. Follow the kit notes.

[Champ81]

I got the RBX from London Power many years ago for my other builds with power scaling.  This was many years ago around 2016 or 2017.    This time I am only incorporating RBX and BMK.  I ordered the RBX yesterday so I will be getting the newer version of it.  In that case I will see the instructions on that.

[K O'Connor]

Hi Champ81

As noted, if you have a new RBX on order then you will see the correct way to attach it to BMK once it is in your hands. No need for guessing.