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: 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.

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.

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