High Voltage Shunt Regulator Kit
300-600VDC
Introduction and Theory
This combined current source shunt regulator kit has been designed to provide a high quality,
simple-to-implement regulator to isolate amplifier input stages from the output stage and raw power
supply or to provide high quality power supply isolation for line stages or phono stages.
The current source circuit is identical to that used for over 30 years incorporating JFETs, except
that to accommodate the high voltages present in tube circuits we use an N-channel MOSFET.
Normal enhancement mode MOSFETs won’t work in this circuit without a source of bias (now it’s
not simple anymore), but special depletion mode MOSFETs are available that work just like a
JFET. With the addition of two resistors, a current source is born. One of these resistors, a
trimpot, is provided to allow easy user adjustment of the current flow.
An improvement in the performance of the simple current source is had by adding another
MOSFET in a cascode arrangement with the original MOSFET. This results in an increase in the
impedance of the current source by more than a factor of 10, and, many say, a more neutral sonic
character.
The shunt regulator circuit is similar to that originally developed by Salas
(http://www.diyaudio.com/forums/power-supplies/134801-simplistic-mosfet-hv-shunt-regs.html).
It’s a simple circuit that performs very well, like the cascode CCS, and is stable under a variety of
operating conditions. The shunt regulated voltage is set by a simple JFET current source, which
controls the shunt transistor by means of a driver transistor. An RC network across the output
manages the transient response to provide quick recovery from transient demands without
significant overshoot. The regulated voltage is adjusted using a trimpot that sets the current
through the reference current source.
The combination of the current source and the shunt regulator provides all that’s necessary for a
simple first-class high voltage regulator.
Application to Vacuum Tube Circuits
The primary purpose of this shunt regulator is to provide a stabilized power supply voltage source
for vacuum tube audio circuits. A short summary of its application will help illustrate the ways in
which it can be used.
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The most common use is to provide a low noise fixed voltage supply for single-ended circuits. In
this case the shunt regulator supplies active stabilization of the power supply voltage presented to
a circuit as an alternative to the passive stabilization that is usually implemented with capacitors,
resistors, and chokes. While most power supplies will still require a capacitor or choke input filter
following rectification, the use of a shunt regulator makes additional stages of passive filtration
unnecessary and provides a low impedance voltage source across the audio frequency band for
the following amplifier circuit. The generalized circuit looks like:
In the case of a single ended circuit as illustrated below the shunt regulator has to act in opposition
to the current flow in the audio circuit load to maintain the voltage at fixed value. So, for example,
when the plate of the audio circuit tube swings toward ground and the load draws more current, the
shunt regulator acts to draw less current, thereby maintaining the power supply voltage constant.
You can see, if you think about it, that to keep the voltage at the audio circuit load constant under
extreme load current swings, you must adjust the current source so that it supplies enough current
for the no-signal audio circuit load and an equal amount to the shunt regulator, so that the shunt
regulator can react properly to all changes in the audio signal current load.
Practically speaking, if your single-ended tube stage requires 20mA to operate at the bias point(s)
you’ve selected, you will need to adjust the current source to supply 40mA. Some users go farther
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than that, because they hear sonic improvements when the current is advanced beyond this
recommended level. So long as the heat sinks are large enough for the heat resulting from the
extra current (see heat sink discussion below), no harm will be done.
For push-pull and/or differential circuits the applications story is different. Since the signal current
in the two triodes flows in opposite directions there are no big current swings that result in power
supply voltage fluctuations. In fact, one oft-repeated advantage of push-pull circuits is that power
supply design is simpler for this reason.
However, there is still a some variation due to audio signal, because in push-pull circuits that are
not properly differential in action the consequences of imperfect tube matching show up in power
supply voltage modulation. A shunt regulator can be applied to reduce this effect by a large factor.
In fact, I became interested in shunt regulators when I found that the application of a shunt
regulator to the center tap of a line output transformer in a transformer loaded differential stage
improved the sound to a pretty significant degree. Even in practical tube differential amplifiers with
very well-matched triodes and very good quality common cathode current sinking the positive sonic
effect is readily heard. In this case, I found that relatively little shunt regulator current (just enough
to keep the shunt regulator operating nicely plus a small engineering margin) sounded the best
with a well-matched pair of triodes in the stage. Increasing the current shunted through the
regulator beyond just enough to sustain regulation actually decreased sound quality.
Heat Management
Current sources and shunt regulators are inherently wasteful, in the sense that they generate heat
in order to successfully go about doing what you ask them to do. Both the current source and the
shunt transistors are provided with PC board mounting heat sinks. These heat sinks are available
in heights (above the PC board) of 1” (2.5cm) to 2.5” (6.3cm), providing some space flexibility
depending on your voltage and current requirements. You will need to carefully consider your
application to ensure that you have enough heat sink area to deal with the heat generated.
The approximate power in the form of heat that can be dissipated safely under “average
conditions” by heat sinks of the available lengths are listed below:
1” - 2 watts, 1.5” - 3 watts, 2” - 4 watts, 2.5” - 5 watts or roughly 1 watt per half inch of heat sink
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This can be increased with forced air flow, but should also be reduced when the board is mounted
in an enclosed chassis with limited ventilation. Be conservative and you will receive better
reliability in return…
If either of these heat sink requirements are calculated at significant;y more than 5 watts, then this
kit is not the appropriate one for your application. You should use the kit that’s configured so the
heat producing devices can mount either on the chassis (if it’s constructed from aluminum or
copper) or on a larger chassis mount heat sink.
A couple of examples below should help you with the calculations necessary.
Example 1
Starting with the current source heat sink, the voltage to be dropped across the current source is
300VDC-250VDC=50VDC and the current through the current source is 2 x 20mA (20mA for the
circuit and 20mA for the shunt regulator as discussed earlier). So the heat dissipated in the heat
sink will be 50 x 0.04 = 1.2 watts. So from the heat sink info, it’s clear that a 1” heat sink will easily
be adequate.
For the shunt regulator the steady state current is 20mA (equal to the audio circuit current) and the
voltage dropped is the regulated voltage, 250VDC. So the heat dissipated by the heat sink will be
250 x 0.02 = 5 watts. It is similarly clear that nothing less than a 2.5 inch heat sink will be
adequate.
Example 2
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This time we will examine a push pull circuit. Again, starting with the current source heat sink, the
voltage to be dropped across the current source is 300VDC-250VDC=50VDC and the current
through the current source is 26mA (20mA for the circuit and 6mA for the shunt regulator, as
discussed earlier). So the heat dissipated in the heat sink will be 50 x 0.026 = 0.8 watts. So from
the heat sink info, it’s clear that a 1” heat sink will again be more than adequate.
For the shunt regulator the steady state current is only 6mA (minimum shunt regulator operating
current plus a little margin for mischief) and the voltage dropped is the regulated voltage, 250VDC.
So the heat dissipated by the heat sink will be 250 x 0.006 = 1.5 watts. So once more a 1 inch
heat sink will be adequate.
General Information and Cautions
This kit is relatively straightforward to build, but assumes some prior skill with a soldering iron and
some knowledge of electronic components and assembly. Take care in handling electronic solder
as most of it still contains a significant quantity of lead. Wash your hands thoroughly after
construction sessions and keep all kit materials well out of the reach of small children.
High voltages are present during the adjustment of these regulators. These voltages can be
lethal. If you are not familiar with safe techniques for completing the installation of this kit with a
high voltage power supply, then you should seek someone who is qualified to help you construct
this kit safely. If you wish to review safe practice, consult the ARRL Handbook for Radio Amateurs
(www.arrl.org or often at used book stores), which has a section on high voltage safety. Although
they are usually referring to 1000v+, the techniques are still applicable. As the purchaser, you
assume responsibility for safe assembly, testing, and application of this kit.
Read each assembly instruction completely before executing any part.
Assembly
1. The component side of the regulator PC board is the side with the white alphanumeric
labeling. Install the components only from this side.
2. Install and solder the resistors R1-R4, R7, R8, R10, R12 and R13, and trimpots R5 and R6
onto the board. Space R9 and R11 off of the PC board about 0.1” (2.5mm). Trim leads.
3. Install and solder the test points, TP1-TP4.
4. Remember that 3 of the active devices here are MOSFETs and, as such, are static sensitive.
Although special precautions are not normally required, on especially dry days or in the
winter there may be potential for static damage. Keep the MOSFETs in the conductive bag
until time to assemble each one to a heat sink.
5. Install Q4 and Q5 to the small heat sink. Use the silicone insulating wafers between the
bipolar transistors and the heat sink and insulate the screw head and nut from the transistor
tab using the shoulder washers. Finger tighten the hardware prior to installing the assembly
to the PC board. Solder and trim leads. Tighten hardware until very snug.
6. Note that the three MOSFETs supplied are not interchangeable, so take care to get them
installed in the correct positions on the PC board. Install the MOSFETs Q1 and Q2 to one of
the supplied heat sinks. Use the silicone insulating wafers between the MOSFETs and the
heatsink and insulate the screw head and nut from the MOSFET tab using the shoulder
washers. Place a serrated lock washer over the end of the screw before threading on the
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nut. Finger tighten the hardware prior to installing the assembly to the PC board. Solder and
trim leads. Tighten hardware until very snug.
7. Install the MOSFET Q6 to the other heat sink. Use the silicone insulating wafer between the
MOSFET and the heat sink and insulate the screw head from the MOSFET tab using the
shoulder washer. Place a serrated lock washer over the end of the screw before threading
on the nut. Finger tighten the hardware prior to installing the assembly to the PC board.
Solder and trim leads. Tighten hardware until very snug.
8. Install Q3, observing the outline on the PC board to ensure that it is the right way around.
Push Q3 down until the bottom of the black case is about 0.25” (6mm) from the top of the PC
board and solder.
9. Install and solder C1 and C2. They should be installed so that the capacitor information
printed on the labels is read normally with the heat sink attached to Q6 closest to you. Trim
leads.
10. Install and solder D1 from the underside of the PC board as shown to pins 1 and 3 of Q6.
The polarity of the diode must be as shown for the circuit to function correctly.
11. Inspect all solder joints for a uniform shiny surface that wets both the board pad and
the component lead, which indicates a good joint. Don’t be ashamed to use a
magnifying glass to see then clearly — I do!
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Start-Up and Adjustment
The circuit components have been selected so that the regulators will power up to moderate
currents and voltages, however you should preset the current source before powering up your tube
circuit. Test points on the regulator boards are there to facilitate checking and adjusting. To do
this you need a low voltage bench power supply or some other source of 9-12VDC and a voltmeter
that will measure mVDC. Connect the meter leads to TP3 and TP4. Connect the positive lead
from the power supply to TP3 and the negative to TP2. The voltmeter display in mV equals the
current source setting in mA. Adjust R5 to get the desired current level for your circuit. You will
probably need to readjust this current later after your circuit has arrived at its steady state
operating temperature in your audio circuit, but this preadjustment will do fine to begin with.
Install the Shunt Regulator into your project circuit and power it up. The regulated voltage (TP2
relative to TP1) will vary with the individual components supplied, but we have chosen components
to have it be (unadjusted) on start-up at about 400VDC. After the voltage has stabilized, adjust R6
to your target regulated voltage. The regulated voltage is temperature sensitive, so the voltage will
increase as the shunt regulator warms up. As a result, it’s preferable to allow the circuit to warmup fully prior to doing the final adjustments.
Component Side View of PC Board
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Schematic
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List of Kit Materials for each PC board
Quantity Part Description
1
1
2
1
2
3
2
5
5
2
1
2
1
1
PC board
Heat sink, small
Heat sink, large
4-40 x 3/8” screw
4-40 x ½” screw
4-40 nut
#4 lock washer
#4 shoulder washer
Thermal pad
MOSFET, IXTP08N100D2
JFET, 2SK170BL
Bipolar transistor, MJE5731AG
MOSFET, FQP8N80C
Diode, zener
3
1
1
1
1
1
3
2
1
1
1
1
1
1
1
1
Resistor, 1K 0.25w (brn-blk-red-gld)
Resistor, 49R9 0.25w
Resistor, 1R 0.25w (brn-blk-blk-slvr-brn)
Trim potentiometer, 200R
Trim potentiometer, 200R
Resistor, 100R 0.25w “1000F”
Resistor, 100R 0.25w (brn-blk-brn-gld)
Resistor, 150K 2w (brn-grn-yel-gld)
Resistor, 5K6 0.5w
Resistor, 3R3 0.25w (org-org-blk-slvr-brn)
Capacitor, 0.47uF 600V
Capacitor, 1uF 600V
Test point, black
Test point, yellow
Test point, red
Test point, orange
Parts Designation Part Function
Q1, Q2
Q3
Q4, Q5
Q6
D1 (see
instructions)
R1, R2, R15
R3
R4
R5
R6
R7
R8, R9, R13
R10, R11
R12
R14
C1
C2
TP1
TP2
TP3
TP4
CCS cascode and control devices
Reference current JFET
Driver transformer
Shunt transistor
Shunt transistor protection
Gate stopper
Maximum current limit resistor
Current measurement resistor
Current adjust potentiometer
Voltage adjust potentiometer
Maximum voltage limit resistor
Gate/base stopper
Reference voltage resistors
Driver stage bias resistor
Output response damping resistor
Reference V bypass capacitor
Output Bypass capacitor
Ground test point
Regulated voltage test point
Input B+ test point
CCS current test point
Specifications
Current range: (with R5=200R and R3=49R9): approximately 10mA to 50mA,
as built, prior to adjustment, set point is ~15mA
Voltage range: approximately 300VDC to 600VDC,
as built, prior to adjustment, set point is ~400VDC
Minimum recommended overhead voltage: (B+ voltage minus Reg V): 25VDC
Voltage rating maximum: Current Source  1000VDC
Shunt regulator  600VDC
Current rating maximum: 100mA
Maximum power dissipation: 6.5 watts CCS with 2.5” heat sink
6.5 watts shunt regulator with 2.5” heat sink
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