Friday, July 24, 2020

Thermionic Valve Power Supply

Tinker, Tailor, Soldier, Sailor

I like to tinker with old fashioned thermionic valve circuitry - tubes, for the 'Merrykins.  It is strangely crude and simple and they make a friendly orange glow in the evening.  However, powering the things is hard, due to the high voltages that are required to overcome the vacuum.

The below picture shows how I build these toys.  It happens on the fly.  This is a hobby, so I don't kill myself with design calculations.  The tag strips enable experimentation to adjust things till it works properly.  The VU Meter even has a little transistor in there - sheer desperation!


Transformerless Valve Power Supply

One can use transformers, but they are big, heavy and expensive and shipping hunks of metal around the world make it even more so.  It makes sense to use transformers if you build a high power circuit with multiple valves, such as a guitar amplifier, but a small circuit with only one or two valves presents a problem.  

For a small fun display project using Nixie, Magic Eye or VFD tubes, you certainly don't want a huge box with a heavy transformer.  However the project should be self contained, with nothing else connected to it.  Rather don't put a headset on a valve amplifier!

VU Meter Top Panel

This article describes how to build a reactive heater supply and a rectified mains high tension supply, that will not break the bank, or your back, or risk burning your house down.

The problem is that the mains supply is 'dirty'.  The Earth wire sometimes isn't earthed.  The Neutral and Live wires can be swapped.  There can be high voltage spikes caused by air conditioners, industrial machinery and lightning.  So if you want your circuitry to last longer than a few months, then you need to protect it carefully.  You can either use a big hunk of transformer iron and copper to provide the protection, or you can use a few carefully selected specialized components, to do that with a little more finesse.

Circuit Protection

You should combine the two circuits below after the choke.  I drew them separately to make it more clear.

Safety Capacitors
A Safety Capacitor is self healing and self extinguishing.  If it gets zapped by a high voltage spike, then it will carry on working despite the puncture.  Eventually, the capacitor may fail, but while it may smoke, it will not burst into flames and burn your house down.  They are expensive, but rather cheaper than a new house.  It is the big blue block in the picture.

Polyfuse
A polymer fuse typically contains little spheres of metal inside a rubber compound.  Normally the spheres make contact and the fuse conducts.  When it heats up, the rubber relaxes and the spheres lose contact, interrupting the current.  When the device cooled down, it will again conduct.  In case of a fault, it will cycle on/off.  In an extreme failure, it will melt permanently and open the circuit.  You absolutely must use a fuse.  You will get shorts, arcs, or blown parts from time to time, when working with high voltages.

Common Mode Choke
A common mode choke will block current spikes that are the same phase on both the live and neutral wires - for example lightning induced spikes.  There are two ways to wind a common mode choke: 
1. Double up some thin hookup wire and feed it through a toroid ten to twenty times, but then it is prone to arc between the two windings - it is easy to damage the wire coating while winding on a toroid with sharp edges (some toroids are really dreadful).
2. Make two separate windings on the 'left' and 'right' sides of the toroid for beter isolation. Start on the outside and dive into the middle - keep going.  It seems to be opposite, but it is not, the two windings rotate the same way. 

To hold the windings, drop the toroid onto a sharpened pencil held upright in a vice, then glue it with epoxy or a drop of varnish.

Gas Arrestors
Place a gas arrestor between both Live and Neutral to Ground.  A gas arrestor is a non-linear device (it is a special neon bulb).  A high voltage spike will cause the gas to form a plasma and conduct. It will continue to conduct, until the voltage subsided.  In the extreme, it will arc over between two sharp points.  This will absorb both common and differential mode spikes on the mains wires.  You can get centre tapped ones and single ones - your choice.  

You can also use a couple of zinc oxide MOVs, but since we are talking about vacuum tubes, gas arrestors are more cool.

Turret Boards and Terminal Strips
I build olde fashioned circuits on olde fashioned Turret Boards and Terminal Strips.  These are authentic early 20th century strips of Bakelite with solder pins or eyelets https://www.digikey.com/product-detail/en/tubedepot/534-831/2197-534-831-ND/10488281

Building on these strips is error prone.  This time I managed to solder a resistor directly accross live and neutral - it made a nice flash.  So it is good to have a self resetting Polyfuse.  If you prefer glass tube fuses, be sure to have a dozen available, since they only give you one shot at a mistake!

Heater Current Supply

The heater supply uses a ballast capacitor to drop the excess voltage, without generating heat as you would with a series resistor. This is a nifty trick which is rather smaller than the smallest available 6V3 transformer, but your need to get a capacitor that is designed for the purpose and it is therefore much bigger and more expensive than a garden variety capacitor.  They are known as Safety Capacitors, or Motor Run capacitors and can continuously source AC current.

Heater voltage for two valves in series = 12.6 V
Heater Current = 300 mA



(I used an ancient manual CAD program, known as a pencil)


Mains: 220 VAC at 50 Hz
Vdrop = 220 V - 12.6 V = 207.4 V RMS 
(Yankees need to recalculate using 115 V and 60 Hz)

Capacitor impedance = 1 / 2 x Pi x f x C 

4.7 uF:
Z = 1 / 2 x 22 / 7 x 50 x 4.7 x 10^-6 = 677 Ohm imaginary

Assuming that the mains voltage is much larger than the heater voltage, we can ignore the 90 degree phase shift for the following calculations and simplify, just because I'm lazy to punch more buttons on the calculator:
Vin^2 = Vc^2 + Vh^2
Vin ~= Vc + Vh

230 V Supply:
217.4 V / 677 Ohm = 321 mA

220 V Supply:
207.4 V / 677 Ohm = 306 mA

Capacitor: 871-B32926A4475K
Safety Capacitors, 4.7uF, 10%, 350Vac, LS, 37.5mm
https://eu.mouser.com/ProductDetail/EPCOS-TDK/B32926A4475K?qs=ZxCuU5VshqBvujh9kykPlw%3D%3D
$10.13 

Initially, I did not have a 4.7 uF safety capacitor available, so at first I used five caps in parallel for the prototype, which is of course 5 times more expensive and bulky, but it took a while for the next Mouser delivery to get here.

Also put a 1 Megohm 2 Watt bleed resistor in parallel with the capacitor and a 22 Ohm, 5 W resistor in series with the capacitor, to limit the start-up current until the heaters warmed up, because the heaters are non-linear and have a much lower resistance when cold (~6.5 Ohm vs ~22 Ohm).

Test the heater supply with a 22 Ohm (one valve heater) to 47 Ohm (two valve heaters), 10 Watt resistor.  Once wired to the valves, measure the current and voltage and adjust the series resistor if necessary. The green thing next to the blue block in the picture is a 22 Ohm 5 Watt resistor (The schematic above still says 10 Ohm, but 22 Ohm, 5 Watt works better for me!).

Be sure to wire the two tube heaters in series.  When you turn the system on, the current will slowly go down and the voltage will slowly go up, as the heaters heat up and their resistance increase.  Eventually, the tubes should be glowing a nice and friendly orange and the voltage should be around 12V6 AC.  I measured up to 11.9 V AC, which is gud enuff.  If the voltage and current is way out, then you need to adjust the series resistor (don't touch it - it gets hot!).

High Tension Supply

The high tension for a typical valve circuit needs to be in the order of 100 to 300 VDC.  Valves are very forgiving - when they work, they work - and contrary to popular belief, they last for many years.  Lots of the parts on the market were 'lightly used' in a military installation for 30 or 40 years, half a century ago and they still work!

You can get the HT voltage by rectifying the European mains supply and then drop the excess with a series resistor on the valve Anode.  Use high voltage diodes with a rating higher than the protection circuitry and another safety capacitor to stabilize the voltage.  (If you live in the US of A or Canada with 115 VAC, then you need to make a voltage doubler instead.)

Note that most European plugs can be inserted any which way.  If you live in Europe, then it is probably a good idea to install two fuses, in both the L and N leads and also use a DPST toggle switch.

Mains: 220 VAC @ 50 Hz
Bridge rectifier Vht = 1.414 x 220 V = 311 VDC




Mains: 230 VAC @ 50 Hz
Bridge rectifier Vht = 1.414 x 230 V = 325 VDC

Mains: 240 VAC @ 50 Hz
Bridge rectifier Vht = 1.414 x 240 V = 339 VDC

Diode, 1000V, 1A: 1N4007 (1 off or 4 off)
863-1N4007G
https://eu.mouser.com/ProductDetail/ON-Semiconductor/1N4007G?qs=y2kkmE52mdOJ200gEKhp%2FQ%3D%3D
$0.21

Capacitor: Electrolytic, 100 uF 450 VDC
$3.44

Capacitor: Ceramic, 10 nF 1000 VDC
$0.59


I used a single diode, half wave rectifier and a 10 uF, 350 V DC capacitor for the prototype, till my Mouser order arrived.  That is the advantage of terminal boards - easy to change and fix later.

However, the 10 uF, 350 VDC electrolytic capacitor was a very bad idea.  It self oscillated, thus generating a very high voltage >1000V, with various weird and wonderful things happening as a result, but since the rest of the circuit wasn't built yet, it did not cause further damage.  

Eventually I replaced it with a Ceramic 10 nF 1000 V, paralelled with a Ceramic 1 uF 1000 V,  paralelled with an Electrolitic 100 uF 350 VDC capacitor, which calmed the HT supply down and since it now finally works, I'll just leave it like that.  How this works, is that the X rated ceramic capacitor has the lowest impedance and handles the brunt of the ripple, which is then further smoothed by the 100 uF Electrolytic and the little 10 nF in parallel with it, damps any self oscillations in the Electrolytic capacitor.

Note that you must use 2 Watt resistors to discharge the capacitor bank and drive the Neon indicator bulb, not because of the power dissipation, but because of the high voltage.  A 1/4 Watt resistor will arc and burn.

Protection Circuitry

Resettable fuse: 576-600R150-RAR
Voltage: 350 V maximum
Current 300 mA
https://eu.mouser.com/ProductDetail/Littelfuse/600R150-RAZR?qs=sGAEpiMZZMsgjL4JkW1EEUfdUrimm9NEBXyO2non48E%3D
$1.08

Gas arrestors: From each mains wire to ground
Gas arrestor: 652-2045-40-BT1LF
Spark over: 400 V
https://eu.mouser.com/ProductDetail/Bourns/2045-40-BT1LF?qs=qcv5MX6YzaKx5e3H%2FmTy%2Fg%3D%3D
$0.819

Common mode choke: 10 to 20 turns, double wound
Toroidal core: 80-ESD-R-10E
Size: 10 mm OD, 5 mm ID, 5mm height
https://eu.mouser.com/ProductDetail/KEMET/ESD-R-10E?qs=BenOyfdfAroZzp6LE7YowQ%3D%3D
$0.18

Also put a 1 Megohm 2 Watt bleed resistor in parallel with the capacitor banks.  You won't be sorry if the capacitor is safely discharged when you touch the wires...

Test the HT supply with a 33 kilohm, 5 Watt resistor.


Indicator Lamp
What is a power supply without an indicator lamp?  A little NE2 neon bulb with a 1M to 3M3 resistor in series over the 325 VDC supply output will work, but it will be boring.  If you put a 470 nF or 1 uF capacitor in parallel with the bulb then it will flash - a relaxation oscillator.  

The capacitor will never see a voltage higher than about 90 V, but if the neon would ever pop, then the cap will pop too, so it is best to use a 1000 V ceramic capacitor.  

Please don't use a blue LED with valves.  A blue LED in a valve circuit should be illegal - A crime against humanity.

Personal Safety

When you work with thermionic valves, be sure to put a rubber carpet on the floor and house your project in a well insulated ABS, bakelite or wood enclosure, away from curious little prying fingers!

Power on: Put a piece of transparent plastic over the project and stand some distance away when turning it on.  A capacitor may explode, or something may arc and maybe you can see what happened through the plastic shield and smoke...


La Voila!

Herman

2 comments:

  1. What do you use the Thermionic Valve Power Supply with?

    ReplyDelete
  2. I use it to drive two tubes for a new VU Meter https://www.aeronetworks.ca/2016/08/audio-vu-meter.html Building a VU meter that actually works, is complicated and the circuitry is bulky, so reducing the size of the PSU helps a lot.

    ReplyDelete

On topic comments are welcome. Junk will be deleted.