VTA  front view. Title: What if HP Had Made a Tube Tester?

This project began as a simple but good quality vacuum tube tester. But it soon morphed into a quest for a highly accurate instrument, such as the legendary instrument maker, Hewlett Packard (now Keysight), might have built...

Now, HP (Keysight) never made a tube tester and I’m not an alumnus of that hallowed company, but I love the way they designed their gear: It was built to last and always seems to outperform expectations. Thus, as the design process went on and questions of accuracy and stability goals came up, it was natural to try to do the best I reasonably could. But that’s a slippery slope and it took seven years to finish this project. It was a cliffhanger to the end but the result is pretty much everything I’ve always wanted in a tube tester! You can see the results produced by the unit in the on-going series of 7591 tube evaluations here.

Could You Build It?

All of the documentation and PCB design files for the Vacuum Tube Analyzer (VTA) are available for download at the end of this article. [It’s called “analyzer” to set it apart from tube testers which don’t use constant DC supplies. Unlike those, you can use it to plot DC and AC characteristics.] I had always intended it to be something that others could build. For that reason, all of the parts (except for a kludge) are through-hole types. I even have extra PCBs that I can make available. But truthfully, the answer to the question is that it would be difficult and expensive to re­pro­duce. Nevertheless, I’m putting all this out there in the hopes that parts of the system could be useful to folks or that some might find it to be a useful or interesting example. Hence, this will be an overview, rather than the detailed type of construction article you typically find elsewhere on this website. See “Advice for Builders” near the end of page 2.

VTA front panel layoutA Simple Tube Analyzer

Simple to use, that is. The front panel layout at right shows the general organization. After setting the pin connections in gold at the bottom, you use the controls in red to set heater, grid, plate and screen volt­ages. Meter-1 in blue shows the voltage values for that. Turning on the Power and TEST switches in green starts the test. Results like currents, Gm and Gp are read from Meter-2 in magenta. Also, Ip is duplicated on Meter-1 for con­ve­nience. Test parameters can come straight from the original tube data books or from the given table.

Basic Specs

Measurements performed: DC Voltage and current for heater, grid, screen and plate. Transconductance (Gm), plate con­duct­ance (Gp = 1/Rp)

Overall accuracy goal:  0.1% in primary ranges. 20,000-count meters.

Gm and Gp AC test levels: Grid ±50mVpk, Plate ±1Vpk.

Physical: Weight 23.1-lbs, Overall dimensions 15”W 6”H 18”D.

Voltage Metering Burden: 10Mohms

Current Metering Burdens: (None affect readings or regulation.)
Plate, Screen 10.2ohms. Grid Hi 10.2ohms, Lo 1020ohms. Heater 0.03 ohms

Supply voltage and current limits:



Current Spec

Current Limit


1.25 to 13V

3A < 8.4V,  2A > 8.4V

3.3A < 8.4V,  2.2A > 8.4V*


+3 to -100V

-10mA, +3mA

-20mA, +6mA


0 to 400V




0 to 400V



*Heater current “limit” shown is a warning LED. Actual limiting is at about 5-6A.

Measurement Ranges and Resolutions:  (Two, 4.5-digit meters provided.)




Heater Voltage



Heater Current



Grid Voltage



Grid Current



Plate Voltage



Plate Current



Screen Voltage



Screen Current






Plate Conductance (1/Rp)



Plate Cond. in terms of Rp

500ohms min

1% at 1Mohm

Notable Features

  • Dual 4.5-digit meters make it easy to plot things like Gm vs Ip, Ip vs Vp, Ip vs Vg, Is vs Vp, Gp vs Vp and other common pairs.
  • Tube voltages are precision regulated to high-stability voltage ref­er­ences (as are the Gm/Gp AC levels), so there’s no need to readjust voltages.
  • Voltage-setting pots are 3 turns, including the grid-fine control, making settings precise yet efficient.
  • Virtual ground at the plate eliminates Gm error from low Rp and improves Gp resolution by 3.2X over typical techniques. (justification)
  • Current limiters with indicators on all four supplies for protection.
  • Built-in 0.01% calibration standards at Gm=10000 and Gp=1000umho.
  • Square-wave, AC test signals and synchronous detector improve ac­cu­ra­cy, reject hum and noise, and avoid AC detection flaws.
  • Low AC signal levels (for Gm and Gp) reduce error from nonlinear tubes. Grid test level is just 100mVpp and plate is only 2Vpp.
  • Separate drive and sense lines eliminate error from wiring voltage drops.
  • Separate heater and cathode grounds prevent heater current from af­fecting other readings.
  • Separate ground sense lines eliminate errors due to ground current.
  • Closed-case, internal fan and baffle cools the system while keeping circuitry dust free. Uses the entire chassis as a radiator.
  • Precision, switching, heater power supply minimizes heat while de­liv­er­ing up to 3.3A (5A briefly).
  • Any pin can be switched to the external connection binding post for use with lab supplies and meters.
  • Ferrite beads on all socket pins insure stability for high-Gm tubes.
  • Additional auto-range feature for the Gm high range extends max trans­con­duct­ance reading to 40000umho.

Meter1 function switch

The VTA front panel was designed using “natural” sym­bols rather than the ones from the old days. Example: using Vp instead of Va for plate volt­age. Hence, we’re using the nat­u­ral symbols in this article, as defined below.












g1 or c1



g2 or c2



a or b




Meter2 function switch

Test results are mostly read on Meter2, so its function switch shows what’s available. The meter formats below indicate ranges and resolutions:







Ig lo



Ig hi









Gm lo



Gm hi



Gm over*


X10 umho

Gp lo



Gp hi



*Gm hi auto-ranges by 10X above 19999, valid up to 4000 x 10 (40000).

The 100X step from Ig hi to Ig lo takes advantage of the 4.5-digit meter to provide 1% res­o­lution for a 1uA value.

Physical Layout

Construction is based on a 15 x 15 x 4.5-inch, sloped, aluminum chassis made by LMB-Heeger, which you can see a little better in the photo below right.

All electrical components and modules are mounted to the top cover and the bottom is mainly just a cover. This way, there are no connectors or dangling leads between the top and bottom. Below left is the inside of the unit. (Click for a larger pic). Labeled parts are listed below.

VTA inside
  1. Main board with DC regulators, measurement system and dither generators.
  2. Internal fan mounted on baffle for controlling air circulation. Air flow is upward here, across the PCB. Since the baffle is sealed to the top and bottom of the chassis, air must pass through a triangular area near (K). There is no vent, so the whole aluminum chassis becomes a heatsink.
  3. Power Supply Board includes unregulated plate-screen, grid and heater supplies. Also includes un­reg­u­lated auxiliary supplies for plate, screen and heater regulators, plus unregulated supplies for each meter.
  4. Main transformer.
  5. Meter-1 includes stock PM-328 4.5-digit meter plus the visible add-on board with precision reference, split-supply regulators, high-stability gain adjustment and input protection.
VTA side view
  1. Meter-1 function switch with mount-on board for decimal point and units indicator logic.
  2. Auto-ranging addition for Meter-2 extends Gm limit to 40000umho.
  3. Meter-2 with similar qualities to Meter-1 (E).
  4. Meter-2 function switch with features of (F) plus a relay-controlled, precision, 10X divider.
  5. Control switches: (top to bottom) Power, TEST, Pentode/Triode, Gm/Gp Calibrate.
  6. Pin switches connect each pin to one of eight functions: NC, Ground, Heater, Heater-ground, Grid, Screen, Plate, External-connection.
  7. Voltage controls: (top to bottom) Screen, Plate, Grid-fine, Grid, Heater.
  8. Main heatsink. Power devices are mounted directly to the heatsink, through holes in the chassis.

Block Diagram

Vacuum Tube Analyzer Block Diagram


Please click the block diagram above for a high-resolution version. The basic job of the VTA is to provide plate, grid, screen and heater supplies and measurement facilities for all DC voltages and currents, plus Gm and Gp. (Rp = 1/Gp) The tube under test is inserted in one of the sockets represented in the small block at the top. Pin connections are set in the matrix of switches in the large block to the left. Lines from the switch matrix to the rest of the VTA represent the selected tube pins as plate, screen, grid, heater and ground (cathode/suppressor grid). Two, separate lines are switched to each tube pin: drive and sense. Supply current uses the drive line and the sense line is used for metering and remote sensing for the plate and heater supplies. This scheme virtually eliminates errors due to voltage drop in the VTA wiring. It’s as if you put a meter right at the tube socket.

Switching and Power Supplies
Power SuppliesThe lines from the switch matrix (except heater) pass through the TEST switch to disconnect the tube when the test is not in progress. Heater remains connected in Standby mode and Meter-1 looks at the supply voltages instead of the voltages at the tube pins. This supports warmup and power supply setup. We’ll discuss the Gm/Gp cal switch below. Along the bottom of the block diagram we see the plate, grid, screen and heater supplies, as well as the auxiliary ±15V supplies which are used internally in each of the precision tube supplies. (The heater supply develops its own, internal operating voltage.) Each tube supply is provided with current limiting and a front panel overload indicator to let you know when it cannot meet the current demand.

DC and Gm Measurements
DC & Gm MeasurementsMeter-1 can show any of the tube supply voltages, so it’s used for setting up voltages. It also can display plate current. Meter-2 is used to display results and can show any tube supply current, as well as Gm and Gp. When Gm is selected, a precision, 100mVpp, 1kHz signal is imposed on the grid voltage. The Gm/Gp Detector block above the plate supply measures the amount of AC current generated by the tube and thus reads Gm. There are two selectable ranges (2000umho, 20000umho) and also an auto-range feature which extends readings to 40000umho.

Measuring Gp instead of Rp
When Gp is selected, a precision, 2Vpp, 1kHz, dither signal is imposed on the plate voltage. Now the Detector in the plate circuit shows the AC current caused by the dither and this is read as Gp (plate conductance). Plate resistance, Rp, is 1/Gp. Why read Gp instead of Rp? It’s because Gp is what you get if you want to test with a fixed AC voltage. To read Rp directly, we would have to provide an AC current source into the plate and be able to read a wide range of AC voltage that results. This is more difficult and prone to error. By reading Gp, we can keep the plate swing at a very low ±1Vpp, yet accurately measure plate resistances from 500ohms to 1Mohm or higher.

0.01% Gm and Gp Calibrator
Gm and Gp CalibratorTo help in achieving the overall accuracy goal of 0.1%, I decided to include a calibrator for Gm and Gp. While DC measurements can be checked easily enough with lab equipment, insuring AC accuracy with the measurement approach used, would be difficult without the calibrator. When the Cal switch is turned on, VTA circuits are switched away from the tube, to the calibration circuit. For Gm, it uses a precision opamp and a high-voltage MOSFET to create a precise value of Gm, which is set by a 0.01%, low-tempco resistor. The Gm value is 10000umho, half of full-scale. Checking it just now, after months of use, it shows 9997umho, showing just 0.03% drift. The Gp cal circuit is similar, except the input signal comes from the dither voltage on the plate. It provides a Gp reading of 1000.0umho, half of full-scale.

Photo Tour of the Development

Click the image for a captioned photo gallery spanning six years of development: the good, the bad and the ugly! Expand the window to full screen to enjoy the high-res pics. You can navigate with the keyboard: Home, Rarrow16x13, Larrow16x13, End.

VTA Photo Odyssey 850

Coming up next: Three interesting schematic topics, complete docs and more!

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Reader Comments

Posted by Steve L. April 22, 2023 - 10:07 am
Hi Herb, Thank you for the details on what you're seeing in EAGLE FUSION 360 V9.6.2. Well, to state the obvious, I guess that Eagle (under Autodesk ownership) has not maintained full backward compatibility with earlier versions, though it's encouraging that the Meter Mod Board loads okay. The issue with pin name="" not allowed seems to suggest that a pin name is blank. If I knew what part on the schematic or what library component is involved, I could look for something amiss and correct it. Searching the Web, I found this thread which might be somewhat instructive: https://forums.autodesk.com/t5/eagle-forum/quot-invalid-missing-attribute-quot-error/td-p/10086893

If I knew more about what you'd like to do with this, perhaps I could help more. Since this is getting "into the weeds" for some readers, I would invite you to contact me at the email address given on the About page: http://www.tronola.com/html/about___.html

I really would like find a solution for this.

Posted by Herb Diaz April 22, 2023 - 12:16 am
it appears that the cut & paste XML did not post
line 6545: invalid value '' for attribute 'name' in tag -pin-

line 6545, column 115: invalid/missing attribute 'name' in tag -pin-

line in question -- pin name="" x="0" y="2.54" visible="off" length="short" direction="sup" function="none" swaplevel="0" rot="R270"/-

Posted by Herb Diaz April 22, 2023 - 12:09 am
EAGLE update report

Date: 4/21/2023 8:54 PM
File: C:/Users/Herb/Documents/elec projects/VTA_Documents_Package/VTA Main Board CAD files closest to PCB/Vacuum Tube Analyzer main board.sch

An error occurred while parsing the intermediate XML file.
The XML file has been loaded into a text editor window as
C:/Users/Herb/Documents/elec projects/VTA_Documents_Package/VTA Main Board CAD files closest to PCB/Vacuum Tube Analyzer main board.rpt.sch


line 6545: invalid value '' for attribute 'name' in tag


line 6545, column 115: invalid/missing attribute 'name' in tag

line in question --

pin name="" (this is not allowed in later versions of eagleCAD) also will not import into Kicad.
I believe this has to with changes in the part library custom definitions --
part of

the meter board schematic loads fine

Posted by Steve L. April 20, 2023 - 08:11 pm
Hi Herb, Thank you for posting and I'm sorry you're having a problem opening the Eagle files. First, if you just want to examine the schematic, the easiest way is to click the link "Main Board Schematic," which is the 4th item in the Docs list. If you just want to examine the PCB, the link "Image of Main Board PCB layout (For casual viewing.)" offers a high-resolution gif composite image.

Two sets of CAD files are there because updating the schematic causes Eagle to tear up tracks from the layout and put changed parts out of the layout area, so the latest Eagle files are mainly good for the schematic. If you want to work with the PCB in Eagle, you will want to use the CAD files, VTA_Main_Board_CAD_files_closest_to_PCB.zip.

From your question, I imagine you're trying to open the files in VTA_Main_Board_CAD_files_latest_version.zip. If you want to work with the project in Eagle, I'll need to know what version of Eagle, and exactly what error messages you're seeing. I use Eagle Professional 5.10.0. I'm happy to help and I'd like to make sure folks can open the CAD files so please let me know.

PS: I apologize for the unattractive state of the schematic--it was drawn very compactly in an attempt to fit it onto a smaller sheet of paper (bad idea). In partial compensation, I later redrew the plate regulator and you can find that in the link "Plate Regulator concept and simplified schematics" which is the third item in the Docs.

Posted by Herb Diaz April 20, 2023 - 06:49 pm
Is their an updated schematic file, the one in the download zip of latest CAD files will not load in eagle ( errors -- pin definitions?)

Posted by Steve L. August 27, 2019 - 02:43 pm
Hi Francesca, Yes, there are still PCBs for the VTA but I doubt that this project would be a good solution for you. Please refer to the "Could You Build it?" section above and click the “Advice for Builders” link there for details. However, at that same link is coverage of Ronald Dekker's µTracer which is a very affordable and doable project that could fill your needs. You can find a complete review and construction article covering that on the home page. Thank you for your inquiry.

Posted by Francesca Smith August 27, 2019 - 08:05 am
So do you still have boards for this ??

I have a CA-1630 but its getting hard to move around and the prices for the RCA WT-100 are through the roof now.

In any case this is inspiring to say the least.


Posted by Steve L. June 05, 2019 - 07:32 am
Hi Roberto, It's nice to hear from Argentina! Thank you for your kind post and I hope that your uTracer construction goes well!

Posted by Roberto Lopez Santoro June 05, 2019 - 07:11 am
Congratulations Steve! I have not yet started the construction of my uTracer, and all these fantastic ideas and suggestions will be very useful for me and all the lucky ones who will have their uTracer. I live in Buenos Aires and I am attentive to all the communications that Ronald Dekker makes.

Posted by Steve L. November 24, 2018 - 07:20 am
Hi Brian, It's kind of you to say that. What an amazing collection of tube analyzers you have! Any one of them would be worthy of being a hobbyist's pride and joy. Thank you for posting.

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