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London Power TBS Tracking Bias Supply - Printable Version

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London Power TBS Tracking Bias Supply - K O'Connor - 07-08-2021

Hi Guys

TBS Tracking Bias Supply is a slight misnomer inasmuch as it is not the supply itself; rather, TBS is a regulator for the bias supply that tracks the screen supply (Vs)  or plate supply (Va,) as appropriate.

TBS requires a high-voltage input equal to about 16-20% of absolute Vs. For example, if there is 500Vs, then -Vb should be 0.16 x 500V = 80V, up to 0.2 x 500V = 100V.

The raw bias voltage should be generated to present a low- to medium-ompedance voltage source. We do this by using a separate winding with full bridge rectification. Filtering should be high-value capacitors with low-value resistors feeding them. A typical supply would use 100R feeding 47u to 100u, feeding a second identical RC section.

Remember to tie a 100k across the bridge output to protect it, and secondarily to drain the raw bias filter caps at power turn-off.

If the winding voltage is too low to generate the required voltage, a voltage doubler can be used. This simply places two cas across two of the diodes in the bridge. These extra caps perform the doubling function and the extra RCs are still needed.

Any kind of capacitively-coupled raw voltage source should be avoided. Same goes for deriving the raw bias supply from the end of a centre-tapped plate winding.

Once TBS is assembled and installed TEST WITHOUT TUBES.

Make sure that negative voltage of an appropriate range is present at the control grid pin of the power tube sockets.

TBS has a trimpot that can be used to set the maximum nagative output suitable to control the tubes to be used. This adjustment should turn the tubes off and is a set-and-forget adjustment. The trimpot is a convenience only. Ideally, R5 is increased to the next standard value as needed and the three holes for the trimpot are jumpered.

A panel-style pot is provided to make day-to-day bias adjustments, or when you change tubes. This pot should be mounted on the tube plane of the chassis near the tube it controls. This may be tricky to install but once in place is quite convenient. The bias pot is then as protected and as accessible as the tubes are, as TUT3 The Ultimate Tone vol.3 illustrates.