components, Power Supply

A bit of history

In the time that zener diodes didn’t exist, a gas filled voltage reference tube was used to get a stable voltage. Nowadays these are seldom used and that is why I dedicated this article to explain what different types there are and how they work.

Different types of the voltage reference tube

The voltage reference tube can be filled with different gases to meet the particular voltage level. For the stabilized power supply  in the flea-power, I used the 85A2 tube.

85A2 voltage reference tube in flea-power amp
The voltage reference tube 85A2 applied in the flea-power tube amplifier

I will briefly describe the principle of a neon tube: In a small gas filled glass tube, 2 electrodes are placed opposite.  Electrons and ions present in the tube get a certain speed on their way to the electrode. The speed is dependent on the applied tension and in case of the 85A2 the tension is 85V. As soon as the speed is reached, the electrons collide with other atoms and this results in more free electrons.  The snowball effect will eventually result in a glowing light.  The color of the light is dependent on the gas the tube is filled with. For neon, the color will be orange, for argon it will be blue.

In case of the  85A2  the tension upon the tube will remain constant. This is only applicable when the current remains between certain margins. Each tube has a certain kindling tension to start the snowball effect. When the amplifier is switched on, it will take 5 to 10 seconds (this can be type and brand dependent) before the tube flashes. Then it will shine a glowing light. This kindling tension is higher then the stabilized tension (125V for the 85A2).

For these kind of tubes, it is important that the current is held between the max. and min. working point with a resistor. These neon tubes are still easy obtainable and the lifetime is very long. A spin-off  is the magnificent visual effect when the little tube is glowing on top of your amp.


Connecting a stabilization tube is simple. Even though it has 7 or more pens, internal it has only 2 connections: the anode and the cathode.


Foil capacitor

When you buy a better quality foil capacitor, it could be that the manufacturer added a warning Not because you should watch what you spend  but a warning how it should be polarized. I received it with the Auricap-R capacitors that have a long and a shorter wire. The long lead is the inside foil and should be treated the same as the red lead on the standard axial auricaps.

flat plate and rolled foil capacitor
flat plate and rolled foil capacitor

The outside foil is a useful noise shield when input and output impedances are considerably different values. This is true when the outside foil is connected to the lower impedance (sometimes earth) .

Coupling / X-over caps

Circuit outputs are always of lower impedance then inputs and should be connected to the outside foil.

Test with scope

In the little movie I made, you will see several  capacitors going through my hands. Wima capacitors, the auricap capacitor and at the end 2 Philips (Mullard) mustard caps. You will see the difference in amplitude on the scope. I was surprised there was a lot of difference in sensitivity between the different brands and form factors. The big foil caps used in the stabilized power supply did not show any differences.

The oscilloscope was measuring at 5mV/dev most of the time so it was hard to keep a steady waveform. Still I hope you get the message: Even though foil capacitors are non polarized, it can matter how you install them.

In week_47 issue: negative feedback, cure worse then the disease?

components, Power Supply


Turrets are mainly used in guitar amps, less in audio amplifiers

I worked with terminal-strips for a long time and actually without any problems. Every solder joint can be good if you apply the right solder technic.
I will write an article on solder technic shortly.

Turret’s have this name because they look like a castle’s turret or a chessboard turret. I think it is best to call it turret in dutch too so everyone knows it is not the place where politics takes place (het torentje)

When I saw the massive turrents for the first time on a fiberglass board, immediately I saw the potential to use it in an audio amplifier as well. From e-Bay I ordered some turrets and fiberglass board (3mm). I had to read some articles first, and on the site I found most of the things I wanted to know.

When all material was on my working-desk, I made some small turret-boards that could be used in the   flea-power gestabiliseerde voeding.

To drill the holes at an even distance, I used my dremel workstation (drill-press). I had to make some tools to be able to drill as precise as possible.  You can make the distance between the holes exactly even by using small tiles of wood or plastic. By adding them one by one between a stable point and the fiber board, you shift the board in even mm for every hole. I made a movie to show how I did the drilling and the pressing of the turrets. You will see I drilled a second time, this was to countersink the hole as my fiber board was too thick to get the turret flanged.

The Dremel workstation is not the most rigid when you drill in hard surface. I have the experience that the drill starts drifting when I apply more force. So sharp drills and pre-drilling with smaller sizes will help you to avoid that. I don’t know if Proxxon is better in this respect? Fortunately I have a large drill-press for the heavy work.

To drill the holes evenly spaced, I made some small tools. Proxxon has a small carpenter’s square on it’s base, for the Dremel I made one myself. And of course it has to be attached to the base so I made some clamps for that as well. In the next video I show both tools in short. Use your fantasy to make more tools that help to make life easier.

Which turret

Using turret’s needs more preparation then the “normal” solder lugs.  The choice which length is determined by the thickness of the board. You can decide yourself where the turrets will be on the board and also which size. In the next picture you can see some of the different sizes.

turrets with different size
On top a turret with small diameter and hight. the lower 2 are 2.7mm diameter and bottom hight of 3 en 4 mm