Solid State Voltage Regulator for Valve Receivers etc.
All photos copyright John Mills, last updated 30 Dec 2009
Having put the LR8N3 voltage regulator IC to good use in my valve PSU project, it seemed logical to develop a smaller unit
to replace the older voltage stabilisers found in valve Rx / Tx’s. Whilst these regulators such as the OA2, OB2, VR150 etc
are still available, they do take power from the circuitry which is wasted in heat, and a modern solid state version will cut
down on this wasted power along with providing a better regulated and smoothed supply. A slight trade-off is that the
reduced current flowing through the original limiting resistor will decrease the voltage dropped from the main HT supply. This
will increase the voltage to be dropped by the LR8, and will increase the dissipation of the pass transistor. It is prudent
therefore to add an additional resistor to reduce the difference to a lower value - or you will have to use a larger heat sink !!
The LR8 only requires that the input voltage is at least 12V greater than the output voltage. Limiting the difference pays off in
terms of heat - 260v input, 100v output @ 20mA, Wattage = 260v-100v x 0.020A = 3.2W, reducing the input to say 180v,
Wattage = 180v-100v x 0.02A = 1.6W. The dropping resistor will obviously need to dissipate the difference instead, so it’s a
trade off for the size of heat sink you want to use.
The unit will also lend itself to powering battery valve units, replacing perhaps the 90v or 120v batteries.
Made on small Stripboard
Fitted in place of the OA2 150v Reg in my 9R59 Rx
Experimental unit incorporating a bridge rectifier
(hidden behind the Nichicon 180uF 450v cap) and
variable pot for adjusting the voltage manually from
45v up to approx 350v (depending on ac input)
The value of Rs is best found by first substituting a
variable pot of approx 250k ohms in series with a
75kohm resistor. adjust the pot to achieve the
desired output, then simply measure the combined
resistance of the two. A rough guide to the value is
1.64k ohms / volt. So a 150v output would require
approx 244k ohms, you would probably choose to
use a 200k ohm and 47k ohm resistor in series. Or
use a preset pot instead !! REMEMBER - These
voltages can kill - be very careful when finding the
value required and discharge all capacitors before
measuring or fitting Rs.