‘Tube Testers’ Category

CBS Hytron 12BH7 letter about Hickok test

Here is a letter from the Technical Service department of CBS Hytron tube company, dated June 1954, that discusses a common problem with tube testers.

In this letter [ PDF ], which I scanned and restored via Photoshop for better readability, a person from the Max Fischman Co of Pittsburgh wrote to CBS Hytron Co. asking them why so many CBS Hytron 12BH7 tubes were testing weak on their Hickok 533A tube tester.

CBS Hytron investigated the matter by testing 50 CBS Hytron 12BH7 that were known to be top quality and passed factory testing.  They also tested 12BH7 from other manufacturers.  Their analysis revealed that the test configuration — the operating point — for 12BH7 as provided by Hickok was incorrect to target the listed micromhos value of 2380 µmhos.

Learning points from this letter:

  1. Again, as I have tried to instruct in previous articles, there is no such thing as a “correct” (or single) mutual conductance score.  Mutual conductance is a result of the operating point of the tube (plate voltage, signal voltage, grid bias, etc.).
  2. Factory setup data OFTEN provides a substandard operating point for the tube in question.  This is sometimes because of mistake or carelessness in creating the setup data, and sometimes due to design limitations of the test circuit (one fixed signal voltage that is substandard for a particular tube, or a fixed plate/screen voltage that is substandard for that tube).
  3. Learning to KNOW YOUR TUBE TESTER is of utmost importance, not blindly relying on the results that you see on the meter.  A seasoned tech who worked with 12BH7 tubes on a regular basis would have discovered this issue and learned to work around this problem — either by creating a new Bias setting that more appropriately would target 2380 for a typical new 12BH7, or he would have noted what Gm score was more accurate at the bias point given in the setup chart.


NOTES: (1) the letter refers to mutual conductance readings as “Sm”, which I am not aware of that abbreviation.  I believe that “Gm” is the standard abbreviation.  (2) the secretary who typed the letter misspelled Hickok as “Hickock”.


TV-7 micromhos conversion calculator

TV-7 tube tester micromhos calculator
Meter: µmhos


  1. Enter your meter reading (0 – 120) into the “Meter” box.
  2. Choose your Range Switch position in the drop-down box.
  3. Press the “Calculate Gm” button and read your micromhos score.
  4. Press “Reset” button to clear your entries or to start another calculation.
  5. Invalid entry ( meter > 120 ) will automatically reset the calculator.


  • calculator works for all TV-7 models: TV-7/U, TV-7A/U, TV-7B/U, and TV-7D/U
  • calculator requires javascript
  • Range A on TV-7 is only used for emission testing of diodes and rectifiers — no mutual conductance reading exists. Therefore, I have omitted Range A from the calculator. The Range A meter reading is an arbitrary emission score that is evaluated in relation to the “Minimum Value” notation in the setup book.
  • Range B signal voltage is 5.0v ac with bias 0 to -40 vdc
  • Range C signal voltage is 5.0v ac with bias 0 to -40 vdc
  • Range D signal voltage is 1.0v ac with bias 0 to -40 vdc
  • Range E signal voltage is 0.5v ac (500 mV) with bias 0 to -40 vdc
  • Range F signal voltage is 0.5v ac (500 mV) with bias 0 to -4 vdc.  Range F is 0 to 30,000 micromhos, not 60,000 as reported elsewhere.  Click to “Read the rest of this entry” below for details.
  • ©2014 TubeSound


TubeSound TTM-1 Tube Tester & Matcher

Meet the TubeSound TTM-1:

  • Testing of almost all amplifying tubes (triode, tetrode, pentode, beam power) from antique 4-pin (such as a #10, #45, or #50) through 9-pin novar (such as a 7868).   Socket configuration — 4-pin, 5-pin, 6-pin, 7-pin medium (aka 1625), 7-pin miniature, octal, loctal, 9-pin-miniature, and 9-pin novar.
  • All tests use exact tube operating parameters found in any “Receiving Tube Manual”
  • 5 digital meters (each better than 1% accuracy, as verified with two Fluke DMM’s) continuously monitor the tube operating parameters.  1 meter for each plate voltage, screen voltage, grid voltage.  1 meter for plate current, 1 meter for heater voltage.
  • VR (voltage regulator) tube testing throughout its entire operating range.  VR tube voltage drop continuously monitored, and starting voltage is easily observed
  • Mutual Conductance testing via grid-shift method
  • testing of tube Amplification Factor
  • Plate current matching at any single operating point, or you could plot a set of curves.

Design specifications:

  • regulated plate voltage, variable 0 to 500 VDC (0 to 410 continuous)
  • regulated screen voltage, variable 0 to 500 VDC (0 to 410 continuous)
  • regulated grid/bias voltage, variable 0 to -100 VDC
  • plate current up to 200 ma
  • heater voltage accurate within 0.1v.

My intention was not to replace any vintage tube tester, but instead, to supplement functionality that does not exist in traditional tube testers.  For example, transconductance testing is certainly much easier using the dynamic test of a B&K or Hickok.  Likewise, grid leakage sensitivity is best tested in a Mighty Mite or similar machine.  But none of those machines recreate the static operating parameters that a tube will see in an amplifier, therefore they do not meet the needs of some tube buyers who want their output tubes matched for idle plate current at the operating parameters of a real amp.  Moreover, no standard tube tester will properly test a VR tube and allow you to monitor its performance over its entire operating range.

Photos below show testing of a new Sovtek 5881/6L6WGC using two different receiving tube manual examples from 6L6GC “Typical Operating Conditions, Class A1 Amplifier – Pentode”.  The third photo demonstrates testing a new 0A2 regulator tube.

I have a few cosmetic issues to finish, but otherwise the first model is complete (for now). I have ideas for other features that I may add in the future. The TubeSound TTM-1, in combination with our classic tube testers, covers a wide range of tube analysis that will meet the needs of sophisticated customers.

Tube Matching with a Tube Tester

Copyrighted by Bob Putnak, all rights reserved.

“Sometimes output tubes must be selected which will provide a satisfactory balance adjustment.  A tube tester usually will indicate whether a pair of tubes have reasonably similar characteristics.” – Robert Middleton, 101 Ways to Use Your Audio Test Equipment, Howard Sams Inc. [1]

“Tube matching” is a controversial and complex topic, and there is no consensus regarding “what is best.”  In fact, some people even feel the whole topic is a waste of time, arguing that matched tubes “kill the mojo” of what makes a tube amp sound special.

People ask me about tube matching using a tube tester.  In most cases, they just want to do a reasonable job at matching tubes for themselves, and they have reasonable expectations.  Others are not satisfied unless some guru tells them they need to spend big bucks buying “matching” gizmos that will magically take care of it.

I will try to provide an simple overview of the tube matching topic vis-a-vis a tube tester.  Since the topic has no absolute answer, no conclusion can be offered…only opinions.  It is important to remember that this discussion has nothing to do with the importance of a tube tester as a diagnostic tool.  As a diagnostic tool, tube testers excel.  The question is whether they also do a good job at tube matching.


Testing a balanced rectifier on a tube tester

Most tube testers were not designed to evaluate balanced plate sections when testing a filament-type rectifier tube.

©2010, Bob Putnak, TubeSound.

When testing rectifiers such as 5U4, 5Y3, 5Z3 …have you ever wondered why one plate section tests stronger on your tube tester? Are all of your rectifier tubes really unbalanced?

I was recently sent an email asking for help to understand this topic.  The answer is that most tube testers were not designed to offset the difference in potential between each plate section.  For purposes of this discussion, assume that you are testing a rectifier tube with balanced plate sections.


1.  This discussion is only relevant to filament-type rectifier tubes. While most people use the words “filament” and “heater” interchangeably, they are not identical.

A filament is a directly-heated cathode; the filament is the cathode and emits the electrons. A heater is an indirectly heated cathode; it heats a separate cathode element.  Common examples of filament-type rectifier tubes would be 5W4, 5Y3, 5U4, 5R4, 80, 83, etc.  Examples of heater-type rectifiers would be 5AR4, 6CA4, 6X4, 6X5, etc.  Therefore, a 5AR4 would not be relevant to this discussion.  Most tube testers can evaluate balanced plate sections in a heater-type rectifier tube.

2.  The purpose of the article is to explain to end-users the problems they will encounter when trying to analyze a filament-type rectifier tube for balanced plate sections using their tube tester.

Why would anyone want to do that? Historically, old-school technicians did not put much thought into needing a “balanced” rectifier tube.  Similarly, I am not aware of any vintage tube tester manufacturer that instructed the user to test a rectifier tube for balanced sections and to reject those that were unbalanced.  Nonetheless, some modern tube buyers have been told that they need to buy rectifier tubes with “balanced plate sections”.


Most vintage tube testers were not designed to test for balanced plate sections.  It was simply not considered important. As a result, most tube testers will test one plate section stronger than the other, leading the modern user to a false conclusion that the tube is unbalanced.  In all cases that I am familiar with, the plate at the higher-numbered pin will always test higher, due to conventional filament wiring.


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