The LR8 in tube circuits

The high voltages required for many tubes rule out or complicate integrating many otherwise useful solid state parts. The LM317 and TL431 are ubiquitous regulator solutions, but they’re limited to 36-37V. Too low in most cases for a simple one-chip B+ supply.

The LR8 (datasheet here) is a lesser-known TO92 high-voltage regulator. The maximum input voltage is 450V and the minimum dropout voltage is 12V. Output voltage is set with a simple resistor divider. With just a handful of passive parts, you can use the LR8 to create a regulator for tube B+:

LR8 simple

As a little TO92 device, dissipation and current are limited of course. The circuit above might work for something like El Matemático (one per channel), but higher current applications require the addition of a pass device. In this case, a MOSFET uses the LR8 as the voltage reference on the gate, in turn setting the source voltage just a few volts lower: LR8 compound

While zeners and VR tubes also make a good gate reference in similar series regulator applications, they come in fixed values. The great thing about the LR8 is that we can set the output to any value we like, alleviating the need to keep a bunch of zeners or VR tubes on hand.

I have PCBs of the series circuit made up and will be testing in an upcoming build. In the meantime, this isn’t so complicated that it couldn’t be done on a proto board.

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