Lighthouse Electric Vacuum Tube Project

Vacuum Tube Tester for Your Bench

Tube Tester Jr's Features:

  • Test all low power tubes. Triodes, penthodes and tetrodes up to 25mA cathode current
  • any type of tube socket may be used
  • Filament (Heater) Test
  • Heater to Cathode Insulation Test
  • Tube Vacuum Test
  • Mutual Conductance Test
  • BIAS Test - Plate Current
  • Tube Burn-In

This project is under development and future upgrades and/or changes may be implemented.

Introduction

This is the basic version of the more advanced VT-Tester (under development). With this tube tester you will be able to test and match most low power vacuum tubes. This will be very handy if you have a stockpile of old tubes of unknown condition. VT Tester's circuit consist of a power supply, supplying 6.3 and 12.6 heater voltages, 150VDC plate and screen voltage and a negative grid voltage. Several toggle and push-button switches connect and disrupt current flowing to the tube under test, thus performing the featured tests listed above. Tester outputs are connected to the tube sockets with banana patch cords. In most instances, different types of tubes are connected differently.

TUBE TESTER Jr. SCHEMATIC

Building the Tube Tester:

Build a 150V plate supply source or buy the SMPS6 power supply. Gather all the parts you already have in your inventory. Make a list of what you need to order. First, buy or build an enclosure or frame for the tester. Any size or material may be used that you feel will fit your needs. For our prototype, we used a wooden frame and an aluminum top plate. Make a drawing of the top plate placing all the switches, banana jacks, tube sockets, pilot lights and the LED Panel Meter around it. You may use a picture of a commercial design as a guide. Decide which tube sockets you want to install. If you do not use 7 pin tubes or Compactron 10 pin tubes, then omit them. Wire the tube-pin to banana-jack connections as follows: wire in series jack #1 to pin #1 of all tube sockets. Next, connect jack #2 to pin #2 of all tube sockets and so forth up to pin #7 in a 7-pin socket (if used), pin #8 in a 8-pin socket. End with the 9-pin socket connection or the highest pin count socket. Next label all switches and In/Out jacks. Place labels next to each switch in center position:

S1-POWER
S2-PLATE HT
S3-HEATER SUPPLY (center)
S4-HEATER TEST
S5-GRID SUPPLY
S6-FIXED - ADJUSTABLE
S7-VACUUM TEST
S8-CATHODE/HEATER TEST
S9-A1, A2, PLATE SELECT (center)

In addition to above, make sure all on-off toggle switches have clear ON and OFF markings!

LED Panel Meter's range is 200mA. If that is not sensitive enough you may substitute a 2 Ohms 0.5% sensing resistor for R1 for a 100mA range. In that case you have to divide the readout value by two.

VT-Tester Jr. Parts List

Designators Qty. DescriptionSupplier Part No.
PL1  1  Pilot Light Neon RedJameco215803
PL2  1  Pilot Light Neon YellowJameco215812
S1, S2  2  Switch Panel Mnt. Toggle 15AJameco318002
S3,S5,S6,S9  4  Switch Min. SPDT 250V 3AJameco22833
D1  1  Diode Zener 22V 1WJameco782911
S4, S7, S8  3  Switch PB Mom. SPSTJameco164566
NA  1  Banana Jack 10 PK BlkJameco256779
NA  1  Banana Jack 10 PK RedJameco263864
NA  25  Banana PlugAll Electronics256779
P1  1  Potentiometer 10k Lin. 0.5WJameco29082
DVM1  1  LED Panel Meter 200mVJameco2144606
R1  1  Current Sensing Resistor 1 Ohm 1W 0.5%Mouser71-CPF-D-1.0-E3
TR1, TR2  2  Transformer 115V/12.6V CT Jameco104387
PS1  1  Switched Mode PSU KitLEVTPS6
Q1  1  Transistor PNP generic 100mANA2N3906
BR1  1  Diode Rectifier Bridge 200V 1.5AJameco103018
LD1  1  LED generic 5mm GreenNANA

Substitutions
S3, S5, S6, S9 Switch Full Size SPDT Jameco #318141
S4, S7, S8 Switch PB Mom. SPST Jameco #164639
TR1 any generic 12.6V CT transformer 1.5A and up
TR2 any generic 12-14V transformer 0.3A and up. No CT needed.
BR1 any generic diode rectifier bridge 50V 1A and up

Miscellaneous

  • panel mnt. tube sockets 7, 8 and 9 pin
  • F1 any generic 0.25A/125V time delay fuse and fuse holder
  • material for wooden or metal enclosure (frame), Aluminum top plate, hardware, etc
  • Knob for potentiometer, wire, crimp terminals, solder and assorted bench tools.
  • Digital Volt Meter for testing.

Testing Procedure:

1) Setup: Make sure the Main Power switch S1 and Plate Voltage switch S2 are off. All other switches in [ NORMAL ] position.
Select heater voltage 6.3 or 12.6V. Using banana jumpers, connect one heater tube pin to the heater [ H ] jack. Connect the other heater tube pin to [ COM ] jack. Connect plate pin to A1, grid pin to G1 and cathode pin to
[ C ] tester jacks. Connect the screen pin if testing Tetrode or Penthode tubes to S2 jack. Also make suppressor-grid connection if there is no internal connection. If testing a double triode connect the other plate to A2, grid to G2 and cathode to second [ C ] jack.
TIP: To make the testing faster and avoid errors, create a designated notebook page (or Excel sheet) and make a drawing with all connections and data for each type of tube you will be testing.
Insert the tube into the tube socket. Turn on Main Power switch S1. Pilot light PL1 comes on.

2) Heater Test: press momentary S4 PB switch. LED lamp LD1 should come on. If does not, the tube heater (filament) is broken.
3) Emmision (or BIAS) Test: After ca. 5 minute warm-up time, flip S2 toggle switch PLATE HT to [ ON ] position. Plate current meter M1 should indicate current flow. Compare indicated current value with data sheet. 70% or more of rated plate current indicates a good tube. 40 to 70% is a usable tube. Less than 40% indicates a weak tube. If plate current exceeds rated maximum value, turn on S5 GRID SUPPLY and flip S6 switch to FIXED. If plate current still exceeds max rate, flip S6 to ADJUSTABLE and set negative grid voltage shown in data sheets (connect an external DVM to TP1 and COM). If that doesn't help, the tube is bad.
4) Heater - Cathode Insulation Test: (Only indirect heated tubes) with the S2 switch still in [ ON ] position, press the momentary S8 PB switch. Plate current should drop to zero with good insulation.
5) Vacuum Test: Push momentary PB switch S7 and observe plate current. For a good vacuum, current flow should drop a bit. Increased current flow indicated weak vacuum.
6) Mutual Conductance Testing Via Grid-Shift Method: Connect an external DVM to TP1 and COM. Select 20VDC range. Flip S5 toggle switch to [ON] and S6 to ADJUSTABLE position. Adjust plate current with P1 potentiometer to 50%. Read and write down the grid voltage at TP1 and plate current. Next adjust the plate current to about 75% and note new grid voltage and plate current. Calculate gm from formula.
NOTICE: True gm testing requires same conditions (for grid & plate voltages) as shown in data sheets.
7) Tube Matching: For best results match BIAS (step 3) and gm (step 6) as close as possible.
TIP: new tubes should be matched after few hours burn-in time. Burn in tubes in gm (step 6) setup. Adjust plate current with P1 to 35-45% rated value.

Feedback Or Suggestions To Improve This Tester Are Welcome.

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