A low cost, easy to build diy valve/tube tester

For quite some time I have been looking at developing a design for a diy valve/tube tester that is easy to build and uses readily available, low cost parts. A valve tester is an invaluable tool for those who wish to build and repair valve amplifiers.

Old vintage valve testers are available online, but most of them provide only a crude “go/no-go” result. They also fetch ridiculous prices for such simple tests. What is really needed is a tester that will measure a valve at true operating conditions.

In looking at a new valve tester design, I wanted it to provide these features:

1. Provide emission readings at the recommended databook operating conditions (Plate, Screen and Grid voltages)
2. Provide Gm or mutual conductance readings, directly or indirectly
3. Test valves for internal shorts
4. Test for “gassy” valves – valves whose internal vacuum has been compromised
5. Test a wide range of valves, from 12AX7s to KT88s and any other “receiving” class of valve
6. Be expandable with options to include heater/cathode leakage testing, other valve bases, different heater voltages, “life test” etc

I have recently completed the following tester design which does a great job of testing any valve/tube in the “receiving” class. Unlike the old “emission” testers of the past, this low cost tester provides a true test of a valve at valve databook conditions. In addition, it tests for shorts and “gassy” valves with the ability to measure the Gm or transconductance of a valve. It uses low cost, readily available parts and can be built for well under A$100.

Full design and constructional details can be downloaded by clicking on this link:

An inexpensive, easy to build diy valve tester

Here is the completed unit testing a KT88 valve:

OLYMPUS DIGITAL CAMERA

60 thoughts on “A low cost, easy to build diy valve/tube tester”

  1. Hi Grant I am looking to build this project and after looking at your PDF instructions seem to be a bit confused with the back of the power board wiring.. There seems to be 3 resistors that are located on the board but not listed on the schematic or have I miss read something here. Thanks for the post of this tester and hope to complete soon…

    1. Thanks Wayne for your interest in my valve tester design. Yes, the photo of the prototype power board shows 3 bleeder resistors, but the single 150K resistor as shown in the schematic will do the job fine!

      1. Hi Grant I have started the build on the valve tester. Just trying to clarify the use of the DPDT TOGGLE switch listed in your build shopping list. From the photos of the build it looks as though this switch has been used to tap the two HI/LO taps from the transformer and also an additional supply to the bias supply via D2. Would this be correct? Adding further to this, is the DPDT switch be bridged across each pole to effectively use as a bigger SPST switch. Just trying to work out what looks to be solder on those lugs from your supplied photos. Thanks once again.

        1. Hi Wayne,
          Great to hear you are progressing with your build. Yes, I wired both sections of the DPDT switch in parallel to increase its current rating.
          The bias supply is permanently wired across the full 30V winding.
          Cheers!

  2. HI there 🙂 I am interested in building this handy tube tester of yours but I am a little confuse with the switch in the picture and in the schematic,In the full version of your schematic it looks like the Screen voltage and Plate voltage are using DPDT switch and the HI /LO switch is a SPDT correct?. In the picture it looks like you have either 2 HI/ LO SPDT or an independent Screen and Plate voltage control. Would you kindly guide me through? 😀 I’m 16 and I have a passion for tube amps 😀

    1. Hi Ben,
      Thanks for your enquiry.
      The plate and screen switches are SPDT centre off switches. The hi/lo switch is a SPDT switch. In the prototype I used a DPDT switch wired with both sections in parallel.
      Hope this helps and all the best with your interest in valves!

  3. Hey Grant.

    Very awesome project. Ordered my parts. Had to change a few things here and there, only because South African Electronics stores don’t always carry the stuff I need – only separate transformers for supply and heaters. What I actually wanted to ask is I see there is a resistor next to the Neon lamp on the schematic with no designator or value. Is it needed?

    Regards

    1. Hi Nicholas, great to hear you are building my tester. The neon is shown with a series resistor as a complete 240V neon assembly. When you purchase a 240V neon “lamp” it has the resistor built in. Of course if you are using just a neon bulb, you need to add a resistor of approx 220K in series. Hope that helps!

  4. Interesting project.

    In my ´transformer bin´ I have an old PT110. Primary 230V secondary 6V3 and some 265V. Please help me how to convert the secondary 265VAC to the required voltages 90/125/180/250VDC. A simple transistor in a resistance divider would not suffice I think because of the high CE voltages involved. But maybe you have a simple solution?

    1. Hi Arjen,
      Thanks for your interest in my valve tester project. Unfortunately a standard high voltage transformer does not allow for the different voltages required for a tester. My tester which uses a low cost low voltage transformer and voltage multipliers provides the voltages required with easily sourced parts. Cheers.

  5. Hi Grant. First thanks for making available such a great design. Will build this probably on a PCB as I will use a smaller enclosure. You say other tube sockets can be used of the receiver type. Could you explain a bit more. I never use 7 pin but lots of the other two and 300B, 2A3. Can this be done? With probably more grunt in the PSU particularly for the 2.5V 2A3’s.

    regards Dave

    1. Hi Dave,
      Great to hear from you. My tester was developed for indirectly heated valves and could be expanded with say UX4, UX5, UX7, loctal, etc sockets depending on the type of valves being tested. As it stands, it was not developed with directly heated valves like 2A3 and 300B in mind, but in theory the tester could be adapted by using different taps on the M6674 to provide the different heater voltage for these valves. If you have success modding the design for directly heated valves, let us know!

  6. Thanks Grant,
    1. Had a look and a big think. Simplest way to add most 4 pin DHT tubes to your design is to just suck it in and buy another transformer. The Hammond 266M5 will do but VT4C has one at USD15 +post much cheaper.
    2. For the displays I understand cost was the design mantra. I will use 2 DC DC isolated converters and LCD displays from China – 20ma each no big design deal. The DC converters are about the price of two good batteries. And must admit I hate batteries as they seem to die just when you need them.
    3. Just another point of view I guess which I hope helps.
    4. As would getting this Project to the PCB stage. Used to make my own PCB’s but now firms in China deliver a quality product cheaper than I can do. And they deliver 5 boards as the minimum order for less cost to build my one..
    5. So Grant gets a freebie of course. I already have 2 more interested. With Grant’s OK anyone else interested ?

    regards Dave

    1. Hi Dave,
      If you still have the PCB available, I would be very interested. Happy to pay postage etc. Regards John.

  7. Hello, very nice your tube tester.-
    I want to, but I have to ask you a question:

    Banana sockets : 7

    P : Plate (anode)
    S: Screen (g2)
    G: Grid (g1)
    H: heater
    H: heater
    K: cathode (k)
    E: ??????? —– For what electrode is?
    I await your response
    Thanks
    Best regards

    1. Hi Gus,
      Great to hear you are planning to build my tester.
      To explain, the E socket is wired to ground and is useful for testing certain valves. For example, for connecting grid 3 of an EL34 to ground, or for supplying heater power to a 12AX7 by connecting pin 4 of a 12AX7 to “E” ground so that pin 5 can be grounded to heater ground and pin 9 connected to heater 6.3V.
      Hope this helps!
      Grant

      1. Grant, thank´s for answer.-
        Another question:
        In the part list : D1-D4, D11 are 1N4007, but in the circuit diagram D11 is 1N5404, so wich is right ?
        I await your response
        Best Regards
        Gus

  8. Hi Grant.

    Share your love of valves so as said earlier here now working on a version to include the valves I use. Yup I know very selfish but let’s face it. Why build something you don’t use. Your great design as the basis pretty much intact. Would have taken a while.
    Don’t know how to post layouts here. If there is a way will share.

    regards
    Dave

  9. Nice project… I’ve managed to source most of the parts, but I’m in the US which makes the transformer a bit of a puzzle. I could conceivably use a clothes dryer outlet to power the 240v transformer, but the M6674 doesn’t seem to be available in the states and shipping one here drives the cost up quite a bit. Far better would be a transformer with a 120v primary, but I’m just not having any luck finding a suitable substitute… just curious if anyone else has had any luck finding the right transformer.

    1. Hi, great to hear of your plans. Re a power transformer for the USA, you can substitute the M6674 with two transformers – one 120V to 6V 2A and the other 120V to 30V 2A centre-tapped (ie 15V-0-15V). Both are available from ebay or Mouser etc. Happy building!

      1. Hi Grant,

        I’d love to build this tester. I’m in the USA. Can you go into a little more detail on how you would change the schematic for the two transformers (120V to 6V, and 120V to 30V) you suggested?

  10. Yeah, that was what I was thinking, but I wasn’t sure what amp capacity I would need.

    I think I will dispense with the opticoupler part of the circuit. Instead I think I’ll build the tester inside a suitcase type of box with room to store a pair of bias probes and a pair of multi-meters that do double duty. I can just grab the kit to match tubes and bias an amp.

  11. Hi Grant,

    I’m looking to build the tester and looking at a solution for the heater supplies. Will it be possible to draw the heater supply from the top half of the 15V AC winding? (assuming the bottom end of the winding is grounded). I am looking at using at 30VAC center tap transformer of 80VA rating? Am planning to use a regulated DC for the heaters either via 78xx or LM317. Appreciate your input. TIA..

    1. Hi David,
      Great to hear of your plans for the valve tester. Re using a 15-0-15 transformer and supplying the heater from the 15V winding, it could be done, but is very wasteful of power and would result in large dissipation (read heat!) in any dropping regulator etc. For example, if you were testing an EL34 with its 1.5A heater requirement, the regulator would be dissipating around 18W of power with attendant heat problems. I’d suggest that you chase up a separate 6V 2A transformer – simpler and easier to wire up and no heat issues.

      1. Hi Grant,

        Thank you for your response. In other words what you are suggesting is just going for an AC heater supply instead of DC regulated supply. In order for me to test valves having 12.6V filaments, i would need to get a dual secondary 6V transformer. The problem i am facing where i am is the AC supply is about 240-245VAC. Transformers i am able to purchase are 230VAC. Usually there’s a 20% increase on the supply on the secondary windings. I would need to get the supply adjusted down to either 6.3 or 12.6. Using a series resistor to drop the voltage down is one way, but wouldn’t this vary for different valve filaments? Do you have a better way of implementing this?

        thanks,
        David

        1. Hi David,
          Re your concern about mains voltage variation, I wouldn’t be too concerned about it. As long as the heater voltage is within say 15%, you should be fine.
          Cheers.

  12. Hi Grant,

    Great DIY project just what I need. I am having difficultly in locating a transformer M6674 some searches come back with M6674 L I live in the UK and this transformer does no show up here. Tried a search for a 240v/30v 2a center tapped transformer but cannot find this type in the UK either any thoughts would be great as I would really like to build the tester.

    Thanks

    Dave

    1. Hi Dave,
      There are several ways to provide the AC voltages required for the tester. I suggest looking for two 240V to 15V 2A transformers if you can’t find a 30V 2A. Ebay is your friend here 🙂

  13. Hi Grant,

    I located 240v/30v 2a center tapped transformer. You mentioned getting a 6V 2A transformer for the heater supply. How is this used as in the schematic this taken from the 24v tap of the M6674. Not sure how that comes out at 6.3 volts.

    Thanks

    Dave

    1. Hi David, just connect one side of the 6V winding to the bottom of the 30V winding as per my schematic and connect the other side of the 6V winding to the heaters. Cheers

  14. Hi Grant,
    a couple of quick questions: The M6674 transformer seems to be discontinued at Altronics. Jaycar have the MM2005 30V 2A multi tap transformer, with 15v as the centre tap. Can you suggest if this is a suitable replacement? And secondly, having trouble sourcing 470uf 350v caps…any ideas here? Thanks.

    1. Hi John,
      Yes, the MM2005 is an equivalent of the M6674 so is interchangeable. 470uf 350V capas are available from Altronics in Australia. Cheers!

  15. Hi Grant,

    Planning to build the tester but can’t find the 470uf 350v caps anywhere. Jaycar haven’t got them and I can’t find them online.

    Any pointers?

    Cheers

    Rohan

  16. Hi Grant,
    I’m also planning to build this valve tester, have gathered most of the parts except the capacitors. I did have some questions but reading through the previous posts they have been answered!
    Thanks for your work.
    Cheers Keith.

  17. Hi Grant, first off I would like to say that your plans have been very helpful in helping me understand how to test tubes.

    I have been working through your schematic to understand how it works before I just start throwing parts together. I plan on building the MKII with the banana plugs and such. So far I understand why your are using the seven banana sockets and understand that P = plate, S = Screen, G = Grid, etc but the only one I don’t get is the E = ?. Could you please explain to me a little further as to where and/or how that would be used? I was comparing both schematics for the MKI and MKII but I’m lost as to what the E would be used for in the second design. If it’s one of those things where it would take to much time to explain, could you please point me in the direction of what I should study? I am just beginning to learn about tubes but I have a strong background in audio and electronics. Any help will be greatly appreciated!!! Thanks.

  18. I probably should have read through all the comments first because that is where you already answered my question…..duh. Anyways you have a GREAT site here, very informative and helpful.

  19. Hi again Grant, this maybe another noob question but here goes. You said in your write up that you could measure Gm indirectly by varying the grid voltage by 1 volt and measure the change in plate current. Could you please explain to me the formula you use to get a Gm measurement in umhos?

    1. Hi Richard, Gm is a measure of the change in plate current for a change in grid voltage. Typically so many mA of change in plate current for a 1V change in grid voltage – mA per volt also referred to as milli-mhos. To convert to umhos (micromhos) just multiply this figure by 1000.
      Hope this helps! Grant

      1. Thanks Grant!!! That helps quite a bit…I was wondering how everybody was getting such large figures. Thanks to you and several other online resources, I believe that now I’m finally getting a grasp on how it all works: tubes, amps, testers, etc.

        I think I am going to use your basic design and try to incorporate that into an Arduino with a 16×2 Lcd. I know I will be able to get it to measure the grid volts properly but getting the proper resolution for the mA might prove to be beyond the abilities of the standard Arduino (I’m trying to be cheap and not buy anymore extra parts 🙂

        Anyway thanks again for you help and all the info!!!!

  20. Hi Grant, thanks for publishing your design for such a fantastic basic tube tester. My audio system uses tubes in all amplifiers, and the DAC output, and my Monos are OTL’s which have 16 output tubes each,…so being able to test and match tubes is paramount for me. A simple query if I may, have looked at the readings for the 3 tubes you tested and published, 6V6, KT88 and a 12AX7. I whipped out my performance sheets for the 12AX7 (RCA) and your result matchs fairly well with RCA’s data using a Plate V of 250V, and indicates that triode half is good as new. But the KT88,….well I then pulled a performance sheet from Svetlana and their sheet has data for “Vscreen of 300V”, “Vscreen of 140V”, “Triode Connection”, and finally “UltraLinear Connection”. OK, so I’m guessing that I should be referring to one of the first 2 graphs, but the tester cannot supply Vscreen voltages for either. Could you please help me apply your results to a know KT88 graph so I can then interpret my own results against other tube graphs when I start testing tubes with this tester, I’m about half way thru my build. Cheers, Mark.

    1. Hi Mark,
      Thanks for your kind comments and background to your build.
      Re the KT88 or any other valve, I would suggest you do some web searching for several datasheets. I found this Genalex datasheet at National Valve Museum http://www.r-type.org/pdfs/kt88.pdf and it refers to tetrode data of 250V anode and screen which suit the voltages available from my tester.
      Cheers, Grant

  21. Grant, really nice concept and design, it’s not easy making something that works AND simple!!!

    That being said, I’m a Yank, and that tranny won’t work easily for us… What do you suggest, a 30V CT and a 6.3v heater tranny’s?

    A quick look through ebay, and I saw some dual display panel meters, DC Volt/Amp… Not sure of teh range, but that might be teh ticket…
    CR

    1. Hi Chris,
      Good to hear from you.
      Yes, numbers of my tester have been built in the US with substituted power transformers. You can find them on ebay or Mouser – you need a 120V to 6V 2A and a 120V to 30V centretapped at 2A.
      The ebay panel meters could be used, but be aware of how they reference the ground – ie the common input will be typically tied to the power leads. If they are used, you will need to power them from an isolated power source. That is why I used isolated DMMs for the meters in the prototype.
      Have fun!

  22. Hi Grant,

    Love your project so much that I have already printed a dozen copies of the pfd. Still, I ask myself, how does your baby compares to the sussex tube tester?

    Cheers,
    David

    1. Hi David, thanks for your comments. Re my tester vs the Sussex tester, my design is quite a lot simpler. The Sussex tester allows you to select a wide range of plate and screen voltages, while mine simplifies this to a lower number of test voltages that are those commonly used in valve data sheets. This simplifies the power supply considerably, while still providing usable measurements at data sheet settings. Cheers!

  23. I greatly appreciate your circuit since it is most necessary for the tube guru on a budget.
    However, I have a question that I hope you can resolve.
    Viewing the basic schematic, four toggle type switches are present as noted below:
    Hi/Lo on the sec of the xfmr (SPDT)
    Screen Volts (SPDT)
    Plate Volts (SPDT)
    12AX7 sec1 & sec2 (SPDT)

    Why does your pictures and parts list show a DPDT switch, is only 1 section used?
    In the event the latter is the case, can I use another SPDT to replace the DPDT.
    Please identify/clarify where the switches are regarding the schematic in reference to the pics.

    The above is not written out of any disrespect, I am puzzled…

    Thank you!

  24. Sorry, for my previous question – I noted the info within the thread.
    Thank you for the excellent circuit, it is very thoughtful of you to present this circuit to us.
    I have noticed a few testers that have an audio IN/OUT RCA phono jack for actually listening to the amplification through the preamp tube test; would it be possible to place a circuit in your project to provide the latter?

    1. Hi Frank,
      Thank you for your encouragement and good to hear you found your answer in the previous comments.
      Re using the tester to check for noise and sound, I haven’t done this but in theory it would be possible. You would need to inject audio into the grid and put a transformer in the plate lead to couple audio out.
      Cheers!

  25. Hi Grant.
    Beautiful project especially for the availability of the components used.
    I’m trying to make it happen and in the meantime I have collected almost all the parts except for the transformer I’m doing … and built specifically to test other valves 2a3 and 300B kind I added another processor only for these tensions ….
    tThe My question is this ….. starting to assemble pending the transformer, I did not understand at best, even reading it tread, connecting 2 spdt and dpdt, it would be possible for her to post a more detailed picture of the various connections or mark with “s1”, “s2” etc .. the location of the switches in the wiring diagram ??
    Thank you
    Tony

    1. Hi Tony,
      Great to hear you are building my valve tester. Re the switches, I used a double pole switch for the HI/LO function with both sections in parallel. The other two switches are single pole.

  26. I’m interested to buil this tester thats seems very simple but i’m in doubt about DVM’s conectiion.
    I didn’t understand the connection of batteries Could you be a little bit more clear about? If possible send a photo showing the conncetions.

    Tks

    1. Hi Paulo, good to hear you are planning to build the tester.
      Re battery connections, just disconnect the positive battery lead and wire it to the opto isolator as shown in the schematic. The other side of the opto connects to the lead you have removed from the positive terminal.

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