SBM20 The quintessential russian geiger tube.

The SBM20 is the most produced of the russian geiger tubes. Probably it’s also the most produced tube in all world. It’s characteristics and cheapness makes it as a first choice of every DIY geiger project.

Two SBM20 tubes.

There are different versions of this tube:

  • SBM20 This is the standard one. 400V operation 105mm lenght clip-style connectors
  • SBM20U This is shorter 95mm and positive pole have no end cap. You can solder a wire to it but pay attention that the glass seal around the rod could crack and your tube became unusable. The better way to use it is wrapping around various turns of a thin wire.
How to wrap a wire at the positive rod of the SBM20-U
  • CTC5 This is the older version of the SBM20. They usually work well and with the same characteristics of new SBM20 but pay attention! This tube stocks are older than SBM20 and tubes could do not last as long as new’s in service.

A comparison chart between this tubes. Thanks to for the image.

Mightyohm comparison chart
Original datasheet


Gas Filling: Ne + Br2 + Ar
Cathode Material: Stainless Steel, 50 mkm
Maximum Length (mm): 108 / 101
Effective Length (mm): 91.0 / 83.5
Maximum Diameter (mm): 11
Effective Diameter (mm): 10
Connector: Pin
Operating Temperature Range: -60 to +70 C
Areal Density (mg/cm2): 40
Thickness (mm): 0.05

Minimum Anode Resistor (meg ohm): 1.0
Recommended Anode Resistor (meg ohm): 5.1
Recommended Operating Voltage (volts): 400
Operating Voltage Range (volts): 350 - 475
Initial voltage (volts): 260 - 320
Plateau length (volts): at least 100
Maximum Plateau Slope (%/100 volts): 10
Minimum Dead Time (at U=400V, micro sec): 190
Working range (mkR/s): 0.004 - 40
Working range (mR/h): 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr): 29
Gamma Sensitivity Co60 (cps/mR/hr): 22
Inherent counter background (cps): 1
Tube Capacitance (pf): 4.2
Life (pulses): at least 2*1010
Weight (grams): 10 / 9

Simple Geiger clicker

This is a simple schematic of a Geiger clicker. It power a Geiger tube with 350-400V and extract/amplify it’s “clicky radiation sound” making it earable.

The first transistor on the left is an NPN BC546. It regulate the second transistor, it’s complementary PNP BC557 working point. This second transistor is an oscillator drived by the first one. The third is a MPSA44 and is a fast switching high-voltage NPN. This part is critical. You must use the MPSA44 other transistor probably will not work. It drives a current tru a 20mH coil. This coil must be capable to resist to 2-3A impulsive. Select one with low series resistance. The lowest the best. This inductance could be hard to find so a good idea is to wind one by yourself. The 1N4007 could be replaced by UF4007 diodes or better shottky high voltage diodes.

The feedback network that regulates the output voltage is 30Mohm series 400V zener diode. The 30Mohm could be made with 3x10Mohm 1/4w carbon resistors and the 400V zener could be a series of 4x 100V zeners. The last transistor is the led/speaker driver. A 2n2222 or a bc547 will work well.

FEU84-5 ФЭУ-84-5 PMT photomultiplier tube for gamma spectrometry

I’ve bought a couple of FEU84-5 PMT’s from an eBay seller. I’ve bought because of an occasional low price and because I heard that this tube is very good for gamma spectrometry.

Photocathode -  antimony-potassium-sodium-cesium
Optical input - front
The diameter of the working area of the cathode - 25mm 
The number of amplification stages - 12
Wavelengths of maximum sensitivity  -   420-550 nm green light
Dark current  -   < 50 nA
Sensitivity of the photocathode   -   < 80 mkA/lm
Anode sensitivity, A/lm - <80
Life time   -   more than 1000 hours
Maximum Ratings :
Max Voltage -  1900 V
Max anode current - 5 micro A
FEU84-5 and it’s datasheet

It comes into a cardboard envelope with some sponge that starts to disintegrate after 30 years of storage. Nothing to complain… some paper and alcool and I’ve cleaned the tube.

FEU’s box
FEU’s sponge :3








This version of the tube doesn’t have a socket, it have just some teflon insulated wires. This is not bad…there is no need to buy a proper ceramic socket.

End wires
FEU front window

Click to download


MS-13 MC-13 glass tube

This tube is very hard to find. It’s compact size and it’s connection socket, similar to the car brake light bulb makes it the stangest tube I’ve ever seen.


There is very few information about it and it’s usage… it’s from the M-series of professional tubes so I could guess it was used in some kind of portable precision survey equipment.

MS-13 light bulb type base

It’s base looks very similar if not equal (I must try) as a car light bulb.

The tube and it’s datasheet

I don’t know russian so I just know how to recognize the most important parameters to make working the tube. I’ll appreciate more info’s about this tube, feel free to contact me at “l.i.papadopol at”

-Working voltage 720-780V
-Plateau lenght 200V
-Plateau slope 0.15%
-Load resistance 8-15MOhm
-Working temperature -40° +50°
-Lenght 100mm diameter 21.5mm

MS-6 MC-6 γ-selective geiger counter

This is another glass russian Geiger tube made for professional use, not for military like all the M-series tubes. The MS-6 is gamma-selective and have a copper cathode. The internal wire is the anode and is made of platinum.

MS-6 tube

I’ve found two different but similar specs for this tube from two different eBay sellers

First eBay seller spec:

 ·         Radiation determine:   (γ) gamma soft/hard
 ·         Voltage of start of counting: 720 - 780V ;
 ·         Operating voltage (recommended): 820 - 880V;
 ·         Length of flat part of counting curve (plateau):  200 V
 ·         Slope of the counting curve:   < 0.1 %/V;
 ·         Sens. background:   < 120 counts/min;
 ·         Maximum radiation: 5*10e4 counts/min
 ·         Load resistance: 8 - 15  MOhm
 ·         Counter resistance: 4 - 8  MOhm
 ·         Allowable stray input capacitance: < 25 pF
 ·         Working temperature range: -40…+50 C
 ·         Anode material: Platinum (Pt)
 ·         Cathode material: Copper (Cu)
 ·         Dimensions: counter length 260mm, diameter 21.5mm

Second eBay seller spec:

Beginning of the counting voltage - 720V-780V.
The working voltage interval - 820V-880V.
The length of the plateau of the counting characteristics - not less than 200V.
The slope of the plateau of the counting characteristic - 0.1%/V.
The maximum count rate - 25000imp/min.
The biggest overload irradiation - 50000imp/min.
Sensitivity (when irradiated with 0.1 μR/s) - 4500imp/min-5500imp/min.
The largest background - 120imp/min.
Service life - 3∙108imp.
Temperature - from -40ºС to +50ºС.
Dimensions: diameter - 23mm  length - 266mm Weight - 65g


CNC 3020 USB

Finally, after so much time spent in thinking, I’ve bought a Chinese CNC. I’ve choosed the model “3020T USB” from CNCEST manufacturer because it have a USB port; other cnc’s need a pc with parallel port and I have no pc with it! I’ve just a MacBook PRO with Windoze7 running on Parallels…it must work!  I’ve bought it from ebay seller “top5zoom” that shipped it from Germany via DHL for free.

The machine arrived into two big boxes. The packaging was made very well with 5cm soft plastic foam that protect and stabilize the machine into the box. The total weight of the two parcels was 35kg!!!

The machine is very noisy during operation. I’ve enclosed it in thick foam boards to reduce it.

The first thing to do after unboxing was assemble it. Just screw in 4 power supply connectors. Simple and easy. The second was check the ball screws,all 3 axes are moved on ball screw, and greas it with heavy aeronautical grease “Anderol” from US Army surplus. Probably standard grease will do the job but I’ve preferred my military grease.

Power on!

At first I’ve found that the VFD controllor had a corrupted configuration. Following the first part of this post that I’ve found on a forum, thanks to user “gmungas”, I could have it running well.

This is the VFD controller.

PRT-E1500W vfd settings

The post:

"I finally figured out the VFD and Mach3 software settings for my 1.5 kW, 110Vac, 400Hz Spindle with the PRT-E1500W VFD in order to run the spindle either in manual mode or with PWM control through the PC. Here's the procedure I used. (Note I didn't change the 220V setting to 110V in the VFD since there isn't a mechanism for generating higher voltage than the VFD supply and it doesn't appear to be necessary, but this should be vetted by another party (CHIMERA1960?) prior to long duration running).
 1) Reset the VFD to factory settings
 a) Set D001 to 1 (may be able to skip this step)
 b) Set D176 to 1 and press Func (this resets VFD to factory settings)
 2) Reset the VFD to 400Hz max operating frequency rather than the 50Hz default:
 a) Set D002 to 400 (need to set this before D000)
 b) Set D000 to 400
 c) Set D003 to 400
 d) Set D009 to 400 
 e) Set D021 to 400 
 f) Set D093 to 400
 3) Set VFD to receive input frequency setting for spindle throttle control from the AVI terminal:
 a) Set D031 to 1
 4) OPTIONAL but I would recommend for additional debug/safety reasons: Set VFD to allow manual spindle "stop" from terminal even when running from PC
 5) OPTIONAL: Set AVI terminal to readout spindle rpm instead of VFD frequency:
 a) Set D039 to 1
 6) To run the spindle manually with variable speed directly from the AVI terminal:
 a) Set D032 to 0
 b) Make sure switch next to AVI terminal is set to "Manually"
 b) Use AVI "Pot Dial" to set spindle rpm
 c) Use AVI "Run" to start spindle
 d) Use AVI "Stop" to stop spindle or turn down Pot Dial to 0
 7) To run the spindle from the PC (note the PWM signal from Mach3 adjusts the setting of the AVI terminal as if it were the VFD Pot. The PWM signal is NOT going directly to the spindle motor):
 a) Make sure you have the right Port/Pin settings in Mach3 (see example settings below)
 b) Set D032 to 1
 c) Make sure switch next to PRT is set to "PC"
 d) Very important: On Mach3 User Panel, make sure you select and input the max spindle speed (i.e. 24000).
 e) Select the actual spindle speed by adjusting up/down arrows or dragging the column up/down for relative speed setting changes.
 f) Use "Spindle CW F5" button or "F5" to toggle turning the spindle on/of
 Mach3 software settings that work for my spindle and wiring configuration (if you have an issue with the X,Y,Z position controller settings let me know, but I think most don't have this issue):
 A) Ports/Pins
 1) Port Setup and Axis Selection
 i) Port #1 Enabled
 ii) Kernel Speed Set to 35000 Hz (this worked for my 2.9 GHz desktop controller, but you may need to use the lower 25000 Hz setting for slower machines)
 2) Motor Outputs
 i) Spindle Enabled = Active
 ii) Spindle Step Pin# = 1
 iii) Spindle Dir Pin# = 0
 iv) Dir LowActive = Active
 v) Step Low Active = Active
 vi) Spindle Step Port = 1
 vii) Dir Port = 0
 3) Input Signals
 i) EStop Enabled = Active
 ii) EStop Port = 1
 iii) EStop Pin Number = 10
 iv) EStop Active Low = Active
 v) EStop Emulated = Inactive
 4) Output Signals
 i) Enable1 Enabled = Active
 ii) Enable1 Port# = 1
 iii) Enable1 Pin Number = 14
 iv) Enable1 Active Low = Active or Inactive
 iv) Output#1 Enabled = Active
 v) Output#1 Port# = 1
 vi) Output#1 Pin Number = 17 (this should NOT be the Motor Output Step Pin (e.g. Pin #1 in my configuration) as may be indicated in some of the user manuals)
 vii) Output#1 Active Low = Inactive (important to make sure Spindle is On when Mach3 Toggle Switch is on. May need to change this setting if switch appears to be operating backwards)
 5) Spindle Setup
 i) Select "Use Spindle Motor Output"
 ii) Select "PWM Control"
 iii) Input "PWMBase Freq." (I used 421)
 iv) Input "Minimum PWM" ( I used 0%)
 B) Pulley Selection
 i) Pulley Number 4
 ii) Min Speed = 0
 iii) Max Speed = 24000
 iv) Ratio = 1
 Good luck! Hope this helps!"

The machine comes with some accessories:

  • 4th axis with it’s accessories
  • USB cable – power chord
  • fixing screws – clamps
  • some wrenches
  • 4 flute mills
This is the 4th axis with it’s wrench

The 4th axis after an hevy greasing works well. It’s not precisely machined but it works.

Mill bits

The spindle is water cooled, included into the package there is a water pum. I’ve made this super-simple water cooling system. It’s work without any problem but is ugly!

Water cooling

I’ve tested this CNC with Mach3 software. The setup is very simple. Just download USB 4 axis 3020 and unzip it into your Mach3 main folder, typically “C:\Mach3”. Pay attention there is a licence file in it supplied by the CNC’s seller, probably is fake/illegal/cracked/cloned but is working. Buy a licence of Mach3 if you like it. The VFD uses “RnRmotion.dll plugin”. Select it if Mach3 ask you during startup wich plugin must be used for motion control. With the supplied package you obtain a correct motor/step contiguration so there is no need to change it.

More testing/video soon… stay tuned!