‘Tubes’ 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


the Reprocessed Tube Racket

I found an interesting tube-related article in the Dec-1955-Jan-1956 issue of NATIONAL RADIO-TV NEWS.  The article exposed a supply-chain problem during the 1950s where unscrupulous vendors would buy “pull-outs” (duds or near-end-of-life tubes) from repair shops, then manipulate the tubes in a manner that would make them look reasonably new.  The tubes were then resold to distributors and repair shops, who assumed that they were buying “factory seconds” or possibly even fresh inventory.

The defect rate was enormous.  The first stealth order of 21 tubes showed 14 tubes to be defective; the second stealth order had 17 defects of 20 tubes purchased.

No doubt that some of these gems are still floating around today.  While the article seems to suggest that many of the tubes were resold with their original brand name intact, I suspect that most of the “off-brand” or “private label” tubes were using the same unseemly source (pull-outs) for their inventory.

When I test vintage tubes labeled as “Standard Brand”, “Rad-Tel”, “Atomic”, etc…, the defect rate is far too high (in my opinion) for them to have been “factory seconds” or “quality used”.  That doesn’t mean that all of them are duds, of course, but that they need to be carefully checked before selling or using.

I would point out that the advice at the end of the article is not completely relevant today, almost 60 years later.  It is not uncommon for the tube designation to be hard to read, or completely missing, because some labeling can be EASILY wiped off while cleaning the glass envelope.  Furthermore, it is common to find NOS tubes with oxidized tube pins that must be cleaned — sixty years and varying storage climates will “do that.”

[ Here is a PDF ] of the article that I scanned & cleaned up with Photoshop for good readability.

Testing VR (glow) tubes

©2011 Bob Putnak.

I am frequently asked how to test voltage regulator (VR) tubes, which are sometimes called “glow regulator” or “glow discharge” tubes.

Few tube testers test VR tubes, and most models that claim to test VR tubes do a worthless job at this task.

  • They use AC voltages instead of DC
  • the AC voltages are beyond spec
  • they offer no ability to monitor or control the operating current
  • they provide no ability to test at the minimum and maximum operating range of the tube
  • they provide no voltmeter to show the exact voltage drop across the tube.

(Hickok 123A cardmatic, Hickok 752, and Precision 10-40 would be notable exceptions.)

Since the overwhelming majority of tube testers will not test a VR tube properly, we need an answer.  As a general observation, if a VR tube lights up, it is probably acceptable.  But, that’s not good enough, so how to test a VR tube?

1.  The best way to test a VR tube is to try it in the actual equipment, and measure the voltage drop across the VR tube’s anode and cathode.
2.  If the first option is not available, you need to create a real circuit and measure the voltage drop across the VR tube at the minimum and maximum operating current as documented in the datasheet.

The datasheet specifications that are most important for VR tubes are:

  • “dc operating voltage” or “average anode drop” — this is the voltage drop across the VR tube
  • dc operating current range — the minimum and maximum operating current for the VR tube to regulate properly
  • average DC starting voltage

Datasheet specifications for VR tubes are not identical for every manufacturer, but all datasheets are close enough to work from, so use whatever receiving tube technical manual that you have available, such as RCA RC-30, GE Essential Characteristics tube manual, or a Sylvania Technical tubes manual.


NOS NIB NEW Arcturus Blue 071A tube (71-A)

Here is something that you will not see everyday, an NOS NIB NEW Globe-style Arcturus blue-glass 071A audio output tube.  Type 071A is also known as 71-A, 71A, 171A, 271A, 371A. This tube has the star-logo on the top of the glass bulb with 071A inside the star logo. Engraved base reads “ARCTURUS – No. 071A – MADE IN U.S.A.” The box has some creases and wrinkles and is missing topside lid. Tube is gorgeous.

NOS Arcturus blue-glass 071A tubeTesting the NOS Arcturus blue-glass 071A tube
Tube has been sold.  Thanks.


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