Radiometer Copenhagen OSG 42b Oscilloscope Repair, From The 1950’ish

The previous owner of this Danish vintage oscilloscope had tried to plug it in and the only result was smoke! I was asked if I wanted it for repair or it would be thrown out. The Radiometer Copenhagen OSG 42b …

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Power Vacuum, Radio and Electron Tubes – Isolation, Day 7

A talk about various power vacuum, radio or electron tubes, depending on what you like to call them. Triodes, Tetrodes and Pentodes in a range from 10 to 800 Watt plate dissipation. PL36, EL34, 6P45S, PB2/200, PB2/250, QB3/300, 250TH, QB5/2000, …

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100 Watt 6P45S amplifier update #2

The latest update to the 100 Watt 6P45S tube amplifier is about a lot of testing and fault finding that have gone into the tube amplifier. This is to correct noise and hum issues. It is now in a state where the …

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100 Watt output from 6P45S tube amplifier

The latest update to the 100 Watt 6P45S tube amplifier is the measurements of frequency response from square waves and bandwidth measurements. The prototype is now considered done as it has proved itself capable of outputting 210 Watt peak before …

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50W 6P45S monoblock tube amplifier


The main reason for building a new set of amplifiers came with purchasing a set of old studio monitors, the legendary JBL 4333. These 75 Watt speakers with 15″ bass drivers needed a amplifier that could deliver some more punch than my 20 Watt EL34 amplifier.

For a long time I have had a large quantity of 6P45S (PL519 equivalent) sweep power tetrodes lying around and have therefore looked for a amplifier design using these tubes.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!


When searching for a amplifier design to follow, I came across a Hungarian article on rebuilding a old amplifier called the APX-100. It’s was based on the PL509 tubes. I added in some good ideas from a user on to use self balancing preamplifier and phase splitter. All modified in regard to the design of the EL34 amplifier by Claus Byrith.

The User Kruesi on gave a good explanation of his design ideas.

After considering many splitter topologies, I finally settled on using a Long Tailed Pair, since

a) it is able to swing to the full supply rail unlike the split-load “accordian” splitter”

b) Both outputs are equal and opposite, unlike the floating paraphase types in which one output has almost no second-order THD and the other output does – and the outputs also have different clipping behavior

The LTP avoids both these issues, but performs acceptably only when operating into a constant current sink. This is often approximated with a large cathode resistor, but that’s usually a long way from an active constant current source. Of course a tetrode or pentode could also be used for this at the expense of great complication. A constant voltage at the base of a bipolar device translates into a constant current at its collector, given a high beta. Seems like just the thing…

Rather than derive the base voltage of the bipolar current sink from a fixed (regulated) voltage source, it’s derived from the combined plate voltages of both halves of the 12AX7.

DC analysis:
The two 820k resistors are equivalent to a single 410k resistance fed ftom the plate voltage of either section of the 12AX7. the value 820k is chosen to be much higher than the 82k plate loads of the 12AX7 so they should have minimal effect on plate loading.

I’d like about 1mA Ib for each section, running the plates at about 200V (as we’ll see). The two 820k resistors combine to 410k, and in series with the 2200 ohm resistor form a divider to produce about 1.2V at the base of the NPN device. Subtracting 0.6V for Vbe we have 0.6 V across the 330 ohm Re. Thus the emitter current is 1.8 mA. For devices with high beta, the collector current is about this same value, so each half of the 12AX7 has a cathode current of 0.9 mA. Since Ip=Ik, the 82k plate load has 0.9 mA through it, dropping 75V from 300, leaving the plate voltage at 225. (It’s not exactly 200 V due to the fact that I’m using 0.6 V as the value for Vbe in this example -the actual value is slightly higher).

The fun starts when we look at the AC signal:
If the two halves of the pair are perfectly balanced, one plate will be swinging more positive while the other is swinging more negative, and the combined AC voltage at the junction of the two 820k will be zero, leaving only the 200 VDC component.

Let’s say the two halves don’t have identical mu, and the input side has a higher gain than the feedback side of the pair. In this case, the voltage at the junction of the 820k will be an AC signal, out of phase with the input signal. This causes an AC variation on the base voltage which in turn modulates the collector current in such a way as to place an AC signal on the cathodes in-phase with the input signal, of exactly the right amplitude to cancel out the excessive gain of the input side of the pair.

It can be seen that the AC balance of the differential pair is now primarily dependent on the match of the two 820k resistors, and is now much less dependent on the intrinsic mu of each triode section. Using standard 1% resistors with no special matching, I measured a 65 dB Common Mode Rejection Ratio (both halves of the splitter driven from the same source). Very good balance indeed!

So now we have a self-balancing circuit without the need to hand-select 12AX7s, also a very high impedance current sink in the cathode circuit, and also a form of local feedback within this stage to improve balance.

Since the 6SN7 driver also operates as a differential amplifier, we may as well employ this same technique there as well, to preserve good balance going into the KT88s.


Power consumption
Power output
50 Watt
Input tube
Phase splitter tube
Russian 6N8S (6SN7 equivalent)
Output tube
Russian 6P45S (PL519 equivalent)
Output transformer
Dagnall electronics
3K5 Ohm primary
4 and 8 Ohm secondary
Power transformer
Dagnall electronics
Primary: 230 VAC
Secondary 1 : 340 VAC at 600 mA
Secondary 2 : 40 VAC at 50 mA
Secondary 3 : 3,15 – 0 – 3,15 VAC at 3,5 A
Secondary 4 : 3,15 – 0 – 3,15 VAC at 3,5 A


Power supply for one mono block

Mono block amplifier


Phase splitter 6N8S



An estimation of the power output this amplifier is capable of is to look at the full output tube plate voltage swinging across the primary side of the output transformer.

470 Volt peak is 332 Volt RMS over half of the primary resistance of 3500 Ohm giving us around 190 mA. So the power through is around 63 Watt and taking losses and rounding into account, it is fair to say this is a 50 Watt amplifier at low distortion. The output transformers can however not handle this output power so the bias will be adjusted for lowest power but still in the linear range. The amplifier will just have to be driven in a sane manner and never played with maximum input voltage.


9th April 2013

I came across two 50 Watt output transformers and one power transformer to a very good price. As I wanted to build mono blocks I contacted the company that originally made the transformers and had a second identical power transformer constructed at a very reasonable price. The transformers is made by Dagnall electronics located in Britain and with production on Malta.

I decided to build a prototype without a PCB so changes was easier to make and there would be room for experiments and complete rebuilds.


21st August 2013

The test housing is all made from scrap metal and so will the final version be. I am planning to order nicely painted front covers to have a professional finish to it.


17th October 2013

Tube sockets and transformers are placed to minimize influence between components and the possibility for lots of airflow around the tubes.


24th October 2013

The first version of the firmware for the ATMega16 micro controller is written, it is basically a 4 page menu system on a 16×2 LCD display that can be flipped through by the push of a button. Code examples will be made public later when the software is thoroughly tested.


9th November 2013

The amplifier circuit itself is soldered directly on to the sockets and a ground bar runs through the middle of the amplifier. The heater wiring is done in stiff, thick and twisted wire with good clearance and 90 degree angle to the signal wires.

The filter capacitors on the power supply board are mounted on the normal side and all diodes and resistors are mounted on the backside. With the PCB facing downwards the capacitors are shielded from all intense heat sources and will only experience the ambient temperature.


12th November 2013

The power supply resistor values are chosen to give the right voltage under load, the load is represented by large power resistors, voltages will have to be double checked with the tubes as load instead.

Before turning on the amplifier for the first time the bias balance potentiometer is adjusted to the middle position and bias voltage adjusted for most negative voltage possible. This first adjustment can be done with amplifier on at full voltage but with the output tubes taken out.

Very low voltage testing of the amplifier, only 115 VAC through a variac, showed that it worked fine and could amplify a sine wave from the signal generator. As soon as input voltage came above 180 VAC, the speaker would suddenly click and the fuse for the high voltage would blow.

A sure sign of high frequency parasitic oscillations. What comes next is a long journey to locate the source of these oscillations. As I only had old used tubes, I tried to change the output tubes but without any improvement, not even after four different.

Grid resistors on the 6P45S tubes was changed from 2K2 Ohm to 10K Ohm to follow the more conservative high frequency stopper design of the APX-100 amplifier. No noticeable change.

The 175 VDC supply for the screen grid was in my first layout tapped through a resistor from one of the capacitors in series for the high voltage, this unbalanced the power supply greatly and I made a 175 VDC linear MOSFET regulation directly off of the high voltage. Parasitic oscillations still occur.

The feedback signal from the secondary side of the output transformer had a long signal path in a single wire, I changed it to a screened cable with screen connected to ground. Parasitic oscillations still occur.

I had used wire wound resistors for the screen grid, exchanged them for carbon resistors without any noticeable improvement.

High frequency bypass capacitors, value 4.7 nF, was installed from filament supply legs to ground on the output tubes. Parasitic oscillations still occur.

Pulling the phase splitter tube out when the parasitic oscillations are running showed that the oscillation kept on going and therefore is located in the circuit of the output tubes and not in the preamplifier, phase splitter or negative feedback.

10 Ohm 11 Watt wire wound power resistors was installed as plate stoppers between the output tubes and the output transformer. This damped the signal by a great magnitude but the parasitic oscillations would still occur.

Now being very close to rebuilding the whole amplifier, as I had been unable to locate a faulty component, I brought the whole box of 6P45S tubes and tried one after another. I tried another five tubes before having a couple that actually worked.

So the problem all along was old used, some broken, some gassy, some very worn and some almost new together, this was also the point where I at once started construction of the tube tracer kit I had bought, next time I test the tubes in advance and not just think they are working just because I have the same fault with 7 different tubes 🙂

Here is a video of the first time the amplifier is working at full input voltage and negative bias adjusted for 1000 mV over the cathode resistors. This is almost double of what it will be running with, as these high values would exceed maximum plate dissipation if it was running at maximum input signal amplitude.

19th December 2013

The first measurements on the output power and quality of the amplifier have been done.

The first test is looking at 1 kHz square wave and by looking at it and comparing with charts of square wave forms from old radio books, it can be determined what kind of short comings or faults that are present in the system.

The slight sloping of the square waves shows that the low frequency response is good and that the response of the amplifier is pretty flat.

As frequency rises it can be seen that rounding occurs, rounding of the square wave is a sign of bad high frequency response.

As square waves are a sine wave with all its harmonic frequencies, looking at 400 Hz and 1 kHz square waves is enough, as the harmonic frequencies passed by the transfer is in the order of 10 times the frequency. So massive rounding is expected at 10 and 20 kHz.

The next test is done with a sine wave to find the -3dB points. First the clipping point is found at a 1 kHz sine wave and the output voltage noted down. To measure the bandwidth of the amplifier, this is done at half output power of the clipping power. That corresponds to 0.7 * clipping voltage. That voltage will be out reference voltage. To find the lower -3db point, the frequency is turned down until the output voltage is 0.7 * reference voltage. Upper -3dB point is found by turning the frequency up until the output voltage is 0.7 * reference voltage.

Clipping here occurs at 32.8V across a 7R3 resistor load with a sine wave, this is 147 Watt peak power.

Lower -3dB point is at 9.45 Hz and upper at 45.45 kHz.


4th January 2014

To improve the high frequency response, C7 in the negative feedback network was changed from 1 nF to 0.47 nF, moving the cut-off frequency from 41 kHz to 87 kHz.

A slight kink on the 10 kHz and 20 kHz square waves show that the high frequency response have improved.

Clipping here occurs at 39.2 V across a 7R3 resistor load with a sine wave, this is 210 Watt peak power. The resulting -3dB point, half the power, is just above the design goal of 100 Watt output power.

Lower -3dB point is at 11 Hz and upper at 72 kHz.


23rd June 2014

I made new printed circuit boards, both for the power supply and amplifier. There was some changes to the power supply from the prototype. I added 150 V stabiliser tubes for the 300 V supply and the screen supply is also on the board.

Everything is installed in a enclosure from the Italian company HIFI2000.


25th November 2014

The first power up and test with signal generator as the amplifier is installed in its enclosure. There is some problems with hum that will have to be investigated.

8th June 2015

Further investigation of hum issues was conducted by waving a isolated 1000 VDC rated screw driver around in proximity of different components while watching the secondary side of the output transformer on my oscilloscope.

I identified two vulnerable places where a great deal of noise could be induced through capacitive coupling and there is also sensitive to noise through induction from magnetic fields.

The first issue was a small part of the signal line in coaxial cable that was not shielded. Explanations are written on each screenshot from the oscilloscope. The first pictures show the output without any interference with the circuit. The second shows the effect of touching the isolation on the part of the signal line in cable that was not shielded.

The second issue was the coupling capacitor in the input circuit before the pre-amplifier. The yellow wave form with the highest amplitude show the induced noise by touching it as it was installed.

The two blue wave form screenshots show the test to locate the pin connected to the outer foil layer in the capacitor, the capacitor is simply connected to the signal and ground of the oscilloscope probe and squeezed around with your fingers. Switch the connections around to perform it at reverse polarity.

The wave form with the lowest amplitude tells us that the pin currently connected to the ground clip is the pin connected internally to the outer foil layer in the capacitor. This outer layer will also function as a shield in high impedance circuits and that pin should be connected to ground or the path with lowest impedance towards ground.

This shows that film capacitor can have a sort of polarity when it comes to very sensitive circuits. A film capacitor in a audio amplifier can actually be mounted backwards.


10th June 2015

All wave forms are from the secondary side of the output transformer.

The first oscilloscope screenshot shows a Fast Fourier Transform (FFT) analysis of the noise generated by the normal diodes for the 340VAC high voltage supply 1N5408 and 40VAC bias supply 1N4007.

The second oscilloscope screenshot shows the difference between normal diodes like 1N5408/1N4007 that have reverse recovery times around 2uS and fast diodes like MUR480/MUR420 that have reverse recovery times around 50nS is shown in the oscilloscope screenshot with yellow and green wave forms. The spike amplitude is around 15% less but the overall 50Hz hum at the positive half cycle is a little more prominent. Changing the diodes gave a difference in the sound from the switching spikes.

The third oscilloscope screenshot shows the much reduced noise levels after a ground loop formed from star ground point to signal input plug was removed and along with the much shorter switching spikes from the new fast diodes.

Audible it appeared like 90% of the hum disappeared. The greatest performance gain was however from removing a ground loop, the faster diodes did not have such a dramatic effect, it was hear able, but not on the magnitude of removing the ground loop.


11th June 2015

Short demonstration of the amplifier playing music.


26th September 2015

I ran measurements on my HP 8903A audio analyzer. Dummy load was a 8.6 Ohm 200 Watt resistor and thus the output power from the output level test gives some 70 Watt out at 0.5 V in. The other tests are done at 0.5 V input too.

I had the amplifier hooked up to my JBL 4333s for the first time and it is now obvious that there is a reason for the high thd+n measurements, there is a great deal of noise, still not sure which kind, but sounds like white and harmonic. Next step is to analyse the noise in a spectrum analyzer.

Suspects of the noise could be the 50 Watt output transformers running at 70 Watt, so maybe bias is set too high or AC balance is not good enough.

A sad side note to this testing is that I had the audio analyzer looped to itself for testing and output voltage was set to the maximum 5V. I forgot about this setting and hooked the amplifier up to the audio analyzer and just as test began there was sparks flying from the output transformer and some smoke. The output transformer is damaged from internal arcing and I will have to buy a new one.

I changed the output transformer with the one I had for the 2nd amplifier. The output transformer was also shifted 90 degrees on two axis’s in order to cancel any possible magnetic coupling to the power transformer. It did however not show any difference in measurements on the audio analyzer.



The amplifier is still under construction and testing.


The amplifier is still under construction and testing.





2x30W EL34 Tube Amplifier


Ever since building the 2W single ended tube amplifier, I wanted to build something better, something that is considered a good amplifier among tube amplifier enthusiast.

It is in no way cheap to build a tube amplifier, so this project got a good kick start a day where I got a old broken 120W bass amplifier for free, the power transformer was burned out and it had been left in a barn for about 15 years.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!



When searching for a amplifier design to follow, I came across a good paper written by Claus Byrith where he took a well known Mullard design and gave it a proper discussion. Through this discussion  he came out with a improved design with valid arguments to his choices. You find the instructions and original schematics here:

His design was recommended by many on and I chose to go ahead with his paper as it was well documented compared to other older designs that mostly consist of a schematic and what forum threads you can find throughout the internet.


Power consumption
80 Watt, 200VAC at 0.4A
Power output
Adjusted for 20 Watt
Input tube
Phase splitter tube
Output tube
Phillips EL34
Output transformer
V78A01F, 25W
5K6 Ohm primary
4, 8, 16 Ohm secondary
Power transformer
Custom made from




Power supply

EM800 VU meter

Construction in 2009

4th January 2009
Obtained and disassembled a old Sound City 120 guitar amplfier, parts scavenged 6x El34 tubes, 5x ECC83 tubes, sockets, 120W mono output transformer, various jacks and potentiometers.

27th March 2009
Traded the 120W output transformer for 2x 30W ultra liniar output transformers.

5th May 2009
Ordered a custom power transformer.

11th June 2009
Received the custom made power transformer along with covers, and 2 EM800 ( ) magiceye tubes for free! These will look neat used as VU meters!

16th June 2009
Designed power supply PCB.

17th June 2009
Designed EM800 VU meter PCB.

22nd June 2009
Etched, assembled and tested EM800 VU meter.

24th June 2009
Etched and assembled power supply.

1st July 2009
Designed mono stage PCB.

7th July 2009
Tested power supply, bias voltage is -150VDC, tweaking is needed.
Etched and assembled first mono stage PCB.

11th October 2009
Plan for the baseplate is made

12th October 2009
Construction of the baseplate begins

30th October 2009
Construction of baseplate is done

Construction in 2010

4th January 2010
Polishing and varnishing of baseplate

5th January 2010
Assembly of baseplate with transformers and tube sockets.

14th January 2010
Etched second mono stage PCB

20th January 2010
assembled second mono stage PCB

7th March 2010
Assembly on baseplate begins

16th March 2010
Assembly progress

20th April 2010
Assembly progress

25th April 2010
Assembly progress. the amount of different colour wires and insuring a proper twisting made the process take much longer than expected. But in order to get a good result, one needs to invest the necessary work in making it so.

26th April 2010
First power on, test and adjusting. There are problems with the negative feedback.

Construction in 2011

12th August 2011
I found a wooden box for the amplifier to be installed in.

10th August 2011
It is a problem having two mono stages share a power supply, negative feedback voltage needs to be adjusted for each channel. Matching of some grid resistors necessary.

17th August 2011
The amplifier runs and can play audio, but there is noise problems from ground loops.

I got the ground loops sorted out in such a manner that you have to put your ear close to the speaker in order to hear the mains hum. All ground wires goes to a star point at the power supply and all ground wires that could, is twisted or braided together. When I install the amplifier in its final enclosure, I will take additional measures to wire the ground better.

24th August 2011
The amplifier is complete on the baseplate, everything and in the right sizes are soldered on and only the enclosure and mounting of jacks remain the last to do. Been listening to the amplifier all day on the Isophon BS35 speaker set. wonderful.

Construction in 2012

13th February 2012
The best piece of wood for roof constructions that I could find at the home improvement store was dragged home, I needed some good thick and tall wood in order to make a cut with a router for the amplifier base plate to slide into.

9th February 2013
Ever since I built the amplifier, there was an issue with a small amount of 100 Hz hum from the power supply. Realizing that I did not double the capacitance in the power supply, as described if it was to be used for a stereo amplifier, I wanted to correct that mistake. With only two 220 uF capacitance on the 450 VDC rail I would see 2,3% ripple. With the recommended four times 220 uF there would be 1,05% ripple. As I had spare 1000 uF capacitors I used those to bring it further down to a mere 0,4% ripple.

While I already had the amplifier on its back I also decided to add some other modifications that I felt was necessary to give it more years to live in. A soft start circuit for the filaments and a delay on the high voltage. These steps are taken to avoid that the cold filaments with their very low cold resistance would take damage from the magnetic forces when power is applied without soft start, this will eventually break the filament as it also glows up bright yellow at first.

Delaying the high voltage will prevent the tube trying to conduct from a filament that is not properly heated, this could lead to the thoriated tungsten filament losing its ability of giving off electrons as the contamination is ruined. The soft start of the filaments is done through a 1 Ohm 100 Watt resistor and after one minute the filaments get full voltage and the high voltage is also applied. All done with just one relay and one timer. A NTC resistor was also added to the primary side of the power transformer in order to soft start the five times bigger capacitance in the power supply, this should remove a great deal of stress on the rectifier diodes.

The EM800 indicator tubes are also installed and their circuits connected to the amplifier. To obtain galvanic isolation between the EM800 circuit and the amplifier I used a 1:30 current transformer on the output lead to the speakers. With a proper ferrite ring core for high frequencies the indicator tubes now show the level of the treble output.

Construction in 2013

21st December 2013

Through some time I had noticed lower sound level in the right channel and found some time to turn the amplifier around and measure what was going on.

The left channel was almost in balance at 205mV and 195mV over the cathode 10R resistors. The right channel was however at 235mV and 145mV. The bias level was also on the low side from when I first adjusted it very conservatively and was for both channels set to 350mV with balance between push-pull tubes measured to 0,005V.

After adjusting the weak tube in the right channel began red plating and the plate took enough damage/discolouring for me to change it for another from the big box of tubes. It was most likely just burned out as they are all old used tubes.


Mono stage
Pros: Close to the speaker, easy adjustment of power supply, no critical matching with the other channel.
Cons: Need separate power supply which will increase total cost.

Stereo stage
Pros: Everything in one box, lower total cost, everything is equally coupled regarding heat and noise.
Cons: Critical matching of stages and power supply, ground loops appear easier.

It might seem that there are most cons that pros, but the fact is that the cons are much harder to deal with. For the future I would try to build mono stages to gain more experience in choosing between the two. Looking back at the process, matching the two stages to each other and the power supply took much of my time.

I have tried my Isophon BS35 speaker set on my regular amplifier for my stereo speaker set for my computer, they are miles ahead in sound quality over the stock speakers that came with this semi Hi-Fi set, Edifier S2000. The largest difference from the transistor amplifier to the EL34 tube amplifier is the bass. Its deep as the internal hell, clear like tuned piano and makes a world of difference to enjoying music. It have to be experienced as a description is far from enough compared to feeling it in your chest!

Cathode current settings are a bit on the low side, but for now they perform good enough for playing up my apartment. Setting AC balance with a distortion meter is also on the list of future improvements.

I still need a proper pre amplifier that will have the ability to deal with input impedance matching, ground potentials and noise. I will properly build this myself, but for now I just use my laptop as source and pre amplifier.


Published on: Oct 11, 2011. Last updated: November 4, 2021.

Nixie tube clock


Like many other people that have seen nixie tubes I got drawn towards wanting to have my own nixie tube clock. Prices on tubes, sockets, transformers and everything else is pretty steep due to their rarity on the exotic tubes and demand.

I found a home made Volt meter on a HAM flea market containing four Phillips ZM1020 tubes, five sockets, driver ICs and a transformer. This was the basis for building my own nixie clock. I had to buy two additional Mullard ZM1020 off of Ebay and got a sixth socket for free from a user on a Danish HAM site.



WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!


There are many kits available, but prices are high enough for me to make my own circuit design and PCB layout.

Clock frequency can be derived in a few different ways.

  • Internal crystal oscillator with dividers
  • Micro controller
  • Live wire 50/60Hz frequency with dividers
  • Radio controlled

I chose to get my 1Hz signal from the live wire powering my clock, this was simply made with 7490 ICs to divide the 50Hz sine wave to a 1Hz square wave.

For powering the tubes it is needed to have around 200 – 300 Volt, easiest way is to just rectify the input voltage but this leaves us with no isolation from the wall socket.Using a transformer is safe and reliable, but it takes up a lot of space. Switch more power supplies can be a lot of work and prove to be a bigger project that the clock itself. I went with the transformer as I already had it salvaged from the Volt meter.



Transformer input: 230VAC

Transformer output: 10VAC and 200VAC

Nixie tubes: Phillips and Mullard ZM1020



D2 half wave rectifies the 50Hz 10VAC for driving Q1, this gives a 50Hz square wave for the input on U2 that divides by five, the new 10Hz signal is fed to U3 that divides by 10 and a 1Hz clock frequency is obtained.

U4 to U9 are counters and depending on which digit it will pass on a signal to the next counter and resets are set backwards.



16th September 2010

Bought a home made Volt meter with tubes, sockets, ICs and transformer

10th December 2010

Circuit design and PCB layout is complete, PCB is etched and drilled.

16th December 2010

Assembling of the PCB with components and sockets is complete. Two minor faults was made in the PCB tracing, was fixable by drilling new holes for a couple of jumpers to make the right connections.

7th January 2011

The enclosure is entirely made from 8 mm acrylic salvaged from LCD monitors. The white squares on the surface originally made for diffusing the background light adds a nice finish.

All pieces for the enclosure is finished and final assembly can begin

12th January 2011

The nixie tube clock is put together



The enclosure turned out much bigger than I first imagined when I set out on building this clock, it is mostly due to the large transformer and the fact that fitting 6 digits that each are 33 mm wide will take up some space.

Setting the time can be a little tricky as the input signal from the push switches have some noise that can make the digit jump several places, this is due to button-bouncing and adding a capacitor across the switch will remove the problem.

The enclosure turned out real good, edges are aligned perfectly on the front and have some minor flaws on the back, but considering this is only made with a hand held saw and file I am satisfied.



2W UCL82 SE tube amplifier


I got a very old TV from a colleague, it had been stored in his parents attic for almost 30 years. It was huge and ruined from the time it was stored, so he only brought me the electronics which mainly was produced in Denmark. It did not contain PCBs, only bird nest wiring between tube sockets and plugs.

I scavenged the different parts for among other things: AC flyback transformer, tubes, sockets, plugs and resistors. All the capacitors was worthless due to age.

I decided to reconstruct a tube amplifier from the power transformer, output transformer, tubes, sockets and resistors from the TV set.



WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!



The output transformer is a single end transformer and as I only had one, it was going to be a mono amplifier with only one output tube.

WARNING: The power transformer is more of a autotransformer as the primary and secondary are not isolated, so grounding the secondary neutral is not possible, this can result in dangerous situations and the mains neutral is also the neutral for audio input!



I found a old schematic that had a good resemblance with the components I had from the TV set, the original schematic can be seen here.

Here is a redrawn schematic with a few changes, tone and volume control is left out since they did not work as intended and only brought a lot of noise and squeaking into the amplifier. I used a UCL82 tube instead of the ECL82, the difference is the heater voltage where I use 50V instead of 6,3V.

I first made a test setup to tinker with the amplifier, not very pretty or for that matter, safe.

The amplifier was built into a stainless steel box that was thrown out due to a damaged lid, so I used the lid as a bottom plate and installed the transformers, capacitors and circuits on this, the box comes on as the entire enclosure. Only the single tube is left outside the box along with the different jacks on the backside.

The original Telefunken UCL82 tube made in Germany.

The complete amplifier as it stands today.



I reached my goal that was to build a tube amplifier from the parts I had already at hand. I only spent money on some proper speaker and input connectors.

This small 2 Watt amplifier is capable of playing real loud and the many different types of the triode/pentode xCL82 tubes like PCL82, ECL82 etc. makes for good small amplifiers or for headphone amplifiers.



Demonstrating a audio amplifier is something that can only be done experiencing the amplifier in person, but you will have to do with this video of my amplifier playing.

Kaizer VTTC I

Introduction to the Kaizer VTTC I

A VTTC is a Vacuum Tube Tesla Coil and it uses vacuum tubes / valves for the oscillator that is self biasing from a grid leak circuit.

I chose to build a VTTC because the operation and components were simple and few. It did turn out to be hard to find all the required components at a reasonable price in Denmark.


High Voltage Safety

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After spending many evenings reading about vacuum tube Tesla coils and comparing tubes, I chose to build a Tesla coil utilizing two 811A transmitter tubes. These tubes have been used by others with great success, they are fairly cheap and have a moderate output power for a tabletop size Tesla coil.

Having studied and read about all the VTTCs I could find on the internet, it became clear to me that there are no pre written text book example on how to design your own VTTC from the bottom, as there do for other types of Tesla coils. Its about finding the old radio amateur books and manuals and combine some of the theory from those about oscillators and the knowledge gained by those who have build VTTCs.

The schematics for this dual 811A VTTC are by most people recognized as Steve Wards, but they do in fact go way back to a article from the magazine “Science Experimenter” fall 1963 issue, which are the drawings most dual 811A VTTCs are based on. I have not been able to find earlier material about this Tesla coil design.

I will build a Tesla coil that resembles the one Steve Ward build, in order to be able to compare the two coils. He succeeded in producing 180mm long sparks, due to it being his first VTTC he wrote there was room for improvement. I have tried to correct some of the issues he described and my goal was to achieve at least 200 mm long sparks. A major limitation to this project is money, so its being designed around components I have already available.

Here are some of the things I want to improve.

The high voltage supply needs to be able to exceed the maximum ratings of the tubes. For very short run times the tubes can be driven at a higher voltage / current than they are rated at, in order to achieve longer sparks.

The secondary coil will be bigger, from the unconfirmed theory saying that larger secondary coils gives longer sparks at the same power input in a VTTC.

Adding a Staccato controller to be able to adjust the break rate of the sparks, this will make me able to apply a higher voltage without excessive heating of the tuber, longer sparks can be achieved this way.



Tubes2x RCA 811A transmitter tubes
High voltage supply230 / 2300VAC MOT with voltage doubler.
Supplied with 160VAC through a variac for 3200V.
Primary coil180 mm diameter, 1.78 mm diameter copper wire, 26 windings, tapped at 22 windings.
Capacitor tank4 nF Fribourg capacitor 20kV pulse rated.
Secondary coil75 mm diameter, 440 mm long, 1600 windings, 0.25 mm enamelled copper wire.
Topload45 mm small diameter, 152 mm large diameter, John Freud toroid.
Resonant frequencyAround 300kHz.
Grid leak circuit19 windings feedback coil, 1 nF capacitor and 500R to 2K5 grid leak resistor.
Input power300W at 150V.
900W at 180V.
50W for the tubes heating.
Spark lengthup to 330mm long sparks.

 Schematic for Kaizer VTTC I

Here is the schematic with all the component values. Here is a few notes on choosing some of the components.

Tesla Coil circuit schematic

All ground connections are made at the same ground rail. R2 is a wire wound resistor with L4 wound around it, one for each tube. C4 is a decoupling capacitor mounted directly under the socket, one for each tube. R1 needs a high wattage rating, bigger is better, or supply sufficient cooling.

Staccato controller schematic. There are slight changes from the original schematics. I do not use a center-tapped transformer and have removed the polarity switch.

Here is the layout for a veroboard, green traces are the traces of the veroboard, remember to cut them where they do not join. Red traces are copper wire jumpers on the component side of the board. Notice that the red jumper next to D2 is connected to 14VAC in N along its way, this is the only jumper that does.

The ExpressPCB files for the PCB layout, however made for veroboard, can be downloaded here:

Tube Load Calculations

Nearly all the following equations is originated from John Freau and is to be found in Steve Wards VTTC FAQ, there is a link at the end of this article. These calculations are not exact and will only work as a guide to building a system that is easier to tweak into good performance.

My goal with these calculations are to determine a Q value and a value for my tank capacitor.

First I will calculate the load impedance for my two 811A tubes. This equation is for class C amps supplied with AC or pulsed DC. If you want to calculate the tube impedance for a filtered DC supply use R = V / (2 * I) instead.

The maximum ICAS plate current rating for my RCA 811A tubes are 175 mA each.

R = V / (4 * I)

R = 3200 / (4 * 0.350) = 2285 Ohm

Next I want to find the primary reactance, f is my resonant frequency in Hertz and L is the primary inductance in Henry.

X = 2 * π * f * L

X = 2 * π * 300000 * (102 * 10^-6) = 192 Ohm

Now the Q value can be determined

Q = R / X

Q = 2285 / 192 = 11.72

Generally a Q of 10 to 20 is ideal. Lower Q gives less tube efficiency, more difficult coupling, but less tank losses. Higher Q gives higher tube efficiency, easier coupling, but more tank losses.

Now I can calculate the capacitance needed for my tank capacitor to work at the desired Q for the tubes I use. Result will be in nF.

C = (Q / (2 * π * f * R)) * 10^9

C = (11.72 / (2 * π * 300000 * 2285)) * 10^9 = 2.72 nF

The closest value capacitor suited for this high frequency and current was three 10 nF capacitors in series for 3.33 nF. The resulting Q for this tank capacitor value is as follows.

Q = 2 * π * f * R * C

Q = 2 * π * 300000 * 2285 * (3.33 * 10^-9) = 14.34

For my final tank capacitor I ended up using a 4 nF capacitor to only have one capacitor instead of a series connection of other types, this capacitor is intended for use in radio oscillators so it can handle the frequency and current, the losses are despite the higher Q value not enough to heat the tank capacitor. The resulting Q value for the 4 nF tank capacitor is as follows.

Q = 2 * π * 300000 * 2285 * (4 * 10^-9) = 17.23

 VTTC Construction

14th July 2008 to 6th  August 2008

It takes its time to find all the special and at times rare components and materials at a price where I feel its worth it. It would take 1½ month to have found all the things that I needed.

The two 811A vacuum tubes are imported from USA through Ebay. Sockets and terminals are imported from England through Ebay. Heater transformer is bought from a private person in Denmark. The high voltage supply are from old microwave ovens with their capacitors and rectifiers. The rest of the components and materials are mostly thrown out stuff from various industry and private persons.

9th August 2008

The staccato controller is assembled on a breadboard and tested.

12th August 2008

The  staccato controller is build on a vero board.

30 – 31st August 2008

The secondary coil is wound and enamelled with 3 layers of varnish.

75 mm drainpipe with 1600 windings of 0.25 mm enamelled copper wire. End terminations are made from brass bolt and brass nut.

5th September 2008

A test setup have been build to experiment with the optimal number of windings on the primary and feedback coils, adjustable from 18 to 26 windings. With these options I can adjust the systems efficiency, frequency and coupling. In the first nights of testing 80 mm long sparks are achieved.

6th September 2008

The Tesla coil is tuned and adjusted

2300V high voltage supply, 1K/1nF grid leak circuit, 24 primary coil windings, capacitor tank 3.33nF.

160mm long sparks are achieved.

This is where I discover that my vacuum tubes are not conducting evenly, which they should as they were bought as a matched pair. I solved this problem by driving the weaker tube with 1 winding less on the grid leak coil, that was enough to adjust the screen voltage to a level where the tubes would conduct very close to matched conditions.

20th September 2008

A voltage doubler is added to the high voltage supply.

3200V high voltage supply, 3K/1 nF grid leak circuit, 24 primary coil windings, capacitor tank 3.33 nF.

260 mm long sparks are achieved.

3rd October 2008

3200V high voltage supply, 0.5K/1 nF grid leak circuit, 22 primary coil windings, capacitor tank 4 nF.

280 mm long sparks are achieved.

15th October 2008

The staccato controller is connected to the Tesla coil and it is now a completely different world, to modulate the sparks. It can vary the number of sparks per second from 1 to 50 outbreaks, the sound and shape of the sparks changes all the way through that scale.

Up to 330 mm long sparks can now be achieved through over driving the tubes further with a lower break rate.

27th October 2008

New clear PVC have been bought along with nylon supports for the primary, screws, shoes and fittings.

1st November 2008

Removing the isolation from 15 meters of 1.78 mm hard copper wire took a real long time as I need it to stay in its circle form. It did cost me a deep and violently bleeding cut to my thumb because I did not pay enough attention when cutting off the isolation. But I got the job done in all my bandages.

A platform was build from thrown out wood and clear PVC, that does not mean its not in good condition.

High voltage, filament and 12VDC transformers are placed on the platform, the capacitor is for the high voltage doubler.

All wiring is done, staccato controller added and the grid leak circuit is in place, everything is in its place.

 VTTC Sparks

The complete coil in all its beauty, the blue and purple sparks go very well with the warm soft glow of the tubes.

This series of close up pictures really show the thin sword like sparks that are characteristic for a VTTC.


Running in CW mode.

Running in interrupted mode


The most important goal was to achieve at least 200 mm sparks and I went through the roof with sparks leaping out to 330mm.

I had assumed that the grid leak voltage would be so low that the coil could be wound on top of its own layers, but eventually it burned through and I had to find a place for the new grid leak coil wound in one layer. The 3 positions I had made for the grid leak coil in the nylon supports were no longer of any use unless I took the whole primary support apart and remade them, which I decided not to do.

The new grid leak coil is still made from 0.5 mm diameter enamelled copper wire, this is not optimal as its isolation is not thick enough, ideally normal wire with PVC isolation would be to prefer.

A higher Q value seems to be preferred if you have a tank capacitor that is able to withstand the heavier load.

Too much of the construction was not drawn in hand before building, that is a sure way to run into problems, but I built most of it from my head and took the battles along its construction, something that I do not advice others to do and I will try to avoid this in the future. 2D/3D CAD drawings would have been a great help, so for the future this will be standard in bigger projects.

Further reading Steve Ward have documented 5 of his VTTCs, the staccato controller and written a good FAQ that is a must for anyone wanting to build a VTTC.

Published October 19, 2009. Updated October 29, 2021.