Lighthouse Electric Vacuum Tube Project

Vacuum Tube Tester for Your Bench - V1b

Tube Tester Jr's Features:

  • Test all Triodes, Penthodes and Tetrodes( Only indirect heated tubes).
  • continously adjustable Plate and Screen voltage
  • adustable Plate current limit
  • any type of tube socket may be used
  • Filament (Heater) Test
  • Heater to Cathode Insulation Test
  • Tube Vacuum Test
  • BIAS Test - Plate Current
  • imroved Mutual Conductance Test
  • Tube Burn-In

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


This is the imroved version of the basic VT-Tester introduced a couple of years ago. The Tester now features adjustable plate and screen voltage regulators. Easier Mutual Conductance test. With this tube tester you will be able to test and match most common, indirectly heated 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, 50-250VDC adjustable plate and screen voltages 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.


Building the Tube Tester:

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, voltage adjusting potentiometers P2, P3 and the LED Panel Meter around it. You may use a picture of a commercial design as a guide. Build the plate and screen voltage regulators on a separate PCB. Use large heat-sink to mount Q1 and Q2 on it. They will get hot. 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, potentiometers. Place labels next to each switch in center position:

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,S9  3  Switch Min. SPDT 250V 3AJameco22833
D1  1  Diode Zener 22V 1WNA1N4748A
D2, D3, D4, D5  4  Diode Schottky 1ANA1N5819
D6, D7  2  Diode Zener 68 VoltNA1N4760A
D8, D9  2  Diode Zener 16 VoltNA1N4745A
S4, S6, S7, S8  4  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 50k Lin. 0.5WgenericNA
P2, P3  2  Potentiometer 250k Lin. 0.5WgenericNA
P4  1  Trim Pot. 5kgenericNA
DVM1  1  LED Panel Meter 200mVJameco2144606
R1  1  Current Sensing Resistor 1 Ohm 1W 0.5%Mouser71-CPF-D-1.0-E3
TR1  1  Transformer 115V/12.6V CT Jameco104387
TR2  1  Transformer 115V/230 190mA Mouser553-VPS230-190
Q1, Q2  1   N Channel Power Mosfet 500V 4ANAIRF820
Q3  1  Transistor NPN genericgeneric2N3904
BR1  1  Diode Rectifier Bridge 400V 1AgenericNA
LD1  1  LED generic 5mm GreengenericNA

NOTE: The VPS230-190 TR2 transformer has 2 x 115V primary and secondary winding. Connect primary winding parallel and secondary winding in series.
HT jack is an auxilliary plate voltage direct output, bypasssing the current meter.

S3, S5, S9 Switch Full Size SPDT Jameco #318141
S4, S6, S7, S8 Switch PB Mom. SPST Jameco #164639
TR1 any generic 12.6V CT transformer 1.5A or higher
TR2 any power transformer with 230-240VAC 125mA secondary winding
BR1 any generic diode rectifier bridge 400V 1A or better


  • all diodes not labeled are generic silicon 1A ea. 1N4004
  • panel mnt. tube sockets 7, 8 and 9 pin, what needed
  • F1 0.5A/250V (MDL) time delay fuse and fuse holder
  • F2 75-80mA/250V AGC quick blow fuse and fuse holder
  • material for wooden or metal enclosure (frame), Aluminum top plate, hardware, etc
  • Knobs for potentiometers, wire, crimp terminals, solder and assorted bench tools.
  • Digital Volt Meter for testing.

Powering-up the tester for the first time

Inspect instrument carefully for wiring errors. Install fuses. Make sure the Main Power switch S1 and Plate Voltage switch S2 are off. Set voltage adjustment potentiometers for plate and screen voltages, P2 and P3, to fully CW position. All other switches in [ NORMAL ] position. Do not insert any tubes at this time. Connect your DVM (set to 500V DC) to HT jack (positive probe) and COM jack (negative probe). Turn on power switch S1 and plate switch S2. The DVM should indicate about 300 VDC. Turn off Main switch S1 if the voltage is missing or very low. Look for wiring errors, popped fuses etc. Otherwise turn the plate voltage adjustment pot. P3 CW and see if the voltage changes. Connect the probe (+) to S2 jack. Do the same with screen voltage regulator. Next, check the negative grid voltage. Connect DVM probes to G1 or G2 jack and COM. Turn on switch S1 and S5 only. Turn grid voltage pot P1 back and forth to see if the voltage changes from 0 to -20V. Set the pot fully CW and push switch S6. There negative voltage should jump 1 Volt. Switch your DVM to AC Voltage and check heater voltages (H and COM) 6.3V and 12.6V. Calibrating the the plate current limit. Power switches S1, S2 OFF. Make sure, filter capacitor C1 is discharged. Set the current limit trim pot P4 to position X (see schematic). Set the plate voltage pot P3 to the lowest voltage, CCW. Connect your DVM, set to 500mA or 1A DC current, to TP2 (+) Connect a load resistor 220-270 Ohms 15W between DVM (-) and COM. Turn on S1 + S2. DVM should indicate current flow. Increase slowly voltage with P3 until desired max. current (100ma-150mA). Now turn the trim pot until you see a slight drop in current. That is the point where current limit kicks-in. Turn off Main switch quickly as the series MOSFET Q2 will be dissipating lots of power in form of heat. This conludes the set-up.

Testing Tubes

Have datasheet ready for the tube to be tested. Make sure the Main Power switch S1 and Plate Voltage switch S2 are off. All other switches in [ NORMAL ] position.
Plate voltage and Screen voltage pots set CCW (lowest voltage)!
Select heater voltage 6.3 or 12.6V. Using banana jumpers, connect one tube-heater pin to the heater [ H ] banana jack. Connect the other heater tube pin to [ COM ] banana 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 (only). Pilot light PL1 comes on.

1) Heater Test: press momentary S4 PB switch. LED lamp LD1 should come on. If it does not, the tube heater (filament) is broken.
3) Emmision (or BIAS) Test: Set the grid G1 voltage according to datasheets. After ca. 5 minute warm-up time, flip S2 switch PLATE HT to [ ON ] position. Adjust the plate voltage. For Tetrodes or pentodes, then adjust the S2 screen voltage.
IMPORTANT: never set the screen voltage, before plate voltage is set first. 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.
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: While in BIAS mode, 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: Note plate current from BIAS test. Push Grid-Shift switch S6 and note new plate current. Calculate gm from formula.
NOTICE: Pushing the S6 switch increases the negative G1 voltage by 1 Volt.
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 BIAS setup. Adjust plate current with P1 to 35-45% rated value.

Feedback Or Suggestions To Improve This Tester Are Welcome.

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