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

DIRECTIONS:

  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.

NOTES:

  • 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

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repair broken TP-Link TD-8616 ADSL modem

If you are trying to fix a TP-Link TD-8616 modem, chances are good that this is your problem.

On May 21, 2012, I purchased from Newegg a new TP-Link ADSL TD-8616 DSL modem to replace a very old Westell modem that was supplied by Verizon. At that time, the reviews for this product were overwhelming positive. Less than two years later, this TP-Link modem is already broken, and I see that recent reviews are more up and down.

Simply put, this modem had garbage capacitors, which is a common quality-control manufacturing problem with modern electronics.

Symptoms: for the past several months, connection was erratic, sync erratic, speeds fluctuating. Unable to connect to modem admin via TCP-IP.

Repair was as follows: Read the rest of this entry »

Vintage Advertising page

On this page, you will find PDF’s that I create from my own collection of vintage advertising for tube amps, speakers, test equipment, microphones, etc.  The ads often provide production specs and other useful information.

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the Crosley Auto Expressionator circuit

In the 1930s, Crosley designed a tone-volume expander circuit that would, in theory, add fidelity to the music that you were receiving from over-the-air radio AM transmitters.  The circuit was said to boost the bass and expand the volume.  It was used in some of their better 1930s Crosley console radios that are popular with radio collectors today.

I have no personal experience with the circuit because I do not service or work on Crosley radios, but a good friend of mine that services antique radios says that the circuit adds nothing of practical value and is tantamount to an early example of tech-snakeoil.  Looking at the schematic, I am inclined to agree, but without any actual hands-on experience with the circuit, I would not want to prejudge it.

For those of you either servicing one of these radios, or who enjoy reading about esoteric circuit designs of yesteryear, [ this article ] from National Radio News, July 1936 issue, will be a good read.  I scanned this article and cleaned it up with Photoshop as a courtesy to my readers.


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