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

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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: My TP-Link TD-8616 was version 6. It has 7 capacitors, of which 6 were badly defective, brands were “GJT” (which were labeled “Low ESR and 105°”)  and “Leaguer” (which were not specified to be Low ESR but were marked as 105°). They were real junk.  Defects were very high ESR in all, and in a few, low capacitance, and one of 100uf caps was open. Here are photos as a few examples:

100uf high-esr and low capacitance

a 470uf “GJT” capacitor with bulging vent, high-esr and low capacitance.  The other caps that I removed from the modem are piled on the face of the meter.

Very high esr in this 100uf "Leaguer" cap

Very high esr (20 ohms) in this 100uf “Leaguer” cap

All the other caps, except for one, tested similarly bad ESR — extremely bad. These were easy to troubleshoot in-circuit with my ESR meter, so repair was easy to diagnose and fast to perform. The values of these caps (two of 470uf and 5 of 100uf) are all values that I keep on hand, so the modem was repaired in less than a half hour.  Since all seven caps were from same manufacturing lot, and therefore junk, I replaced all of them even though one was still good.

ESR is “Equivalent series resistance” and is measured in ohms. As a comparison, I will show you how a “good” and “better” capacitor will test for ESR. First of all, you should understand that ESR must be compared ( vs. )  a new cap of SAME CAPACITANCE value. In other words, you cannot compare the ESR of a 100uf to that of a 470uf. Second, if you are using an ESR meter, do not rely on the “ESR charts” they give you. They are very loose guidelines and not necessarily accurate. Your best comparison will always be to compare the old cap vs a new cap of same capacitance.

Comparison of new 100uf caps:

a typical general purpose brand new 100uf capacitor being tested for ESR

a typical general purpose brand new 100uf capacitor being tested for ESR

This photo shows testing a typical new “general purpose” 100uf capacitor.  ESR is 0.72 ohms, and in my experience should be less than 1.0 to be of reasonable quality.  This cap is perfectly fine for most uses, but will not work well in a circuit that requires a low ESR capacitor.

Rubycon ZL-series low ESR 100uf capacitor

Rubycon ZL-series low ESR 100uf capacitor

This photo shows testing of a Rubycon ZL-series 100uf low ESR capacitor. As you see, it tests much lower ESR than a “general purpose” capacitor, testing at 0.10 ohms. This cap would be a good choice for low ESR circuits.

One final note: don’t get caught up in thinking that you should always buy low ESR caps for all of your projects. Some regulators (low dropout regulators) require a certain amount of ESR in order to work properly. (Often, if you examine the datasheet of the regulator, it will specify a minimum and maximum ESR for the capacitor.) Thus, installing a low ESR cap will actually cause that kind of circuit to not work at all, or be unstable. In most cases, I only use low ESR caps when they are required, such as in switch mode power supplies.

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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