For tube fans, today’s Pluggers comic strip conjures memories of the NATIONAL RADIO NEWS magazine cover, Aug-Sept 1945 issue.

The Pluggers comic is in today’s newspaper July 23, 2011, or you can enjoy the comic online at Comic Strip Nation, comic link is HERE.

I mention this comic because many fans of old tube gear will find it interesting, but from my perspective, seeing this cartoon was a one-in-a-million coincidence.  Yesterday, I was rooting through my storage facility, and I found a quantity of these “National Radio News” magazines published by NRI (National Radio Institute).

This exact issue (Aug-Sept 1945) was on the top of the stack of magazines that I hauled home from my storage facility.  And the very next day, the Pluggers comic appeared.  What are the odds of that?

©2011, Bob Putnak.  This post examines the performance (directly related to the input impedance) of low-cost meters; specifically, I explore a common multipurpose Colluck PM-128E DPM (digital panel meter) and a bargain-priced Cen-Tech #98025 multimeter.

Limitations in the design of these low-cost meters can severely affect measurement accuracy.  First of all, I prove that the input impedance of the PM-128E is 1-megohm, not the 100-megohms or 10-megohms that is specified by the manufacturer and most vendors that sell this DPM.  Second, I demonstrate that the input impedance of the Cen-Tech #98025 multimeter is also 1-megohm.  The conclusion is that either meter will not accurately measure high-impedance circuits, and both perform poorly at measuring low AC voltages.  They can be suitable for other types of measurements, though.

Explanation from a very old Supreme radio course

First a little background –”Input Impedance” as it pertains to a meter — is the load that the meter places upon the circuit being measured.  Ideally, a perfect meter would have no loading effect, but all meters have some loading effect on the circuit they are measuring.  For example, early analog VOM’s had an input impedance of 1000 ohms per volt, which meant that when the meter was set on the 500v range, the input impedance was 500k ohms.  This input impedance (sometimes called ‘meter sensitivity’) is the exact same as placing a 500k resistor across the circuit. Newer analog VOM’s had an input impedance of 20,000 ohms per volt; therefore using our 500v range as the example, the 20,000 ohms/v meter would only load the circuit at 10-megohms.  VTVM’s (vacuum-tube voltmeters) and TVM’s (transistorized voltmeters) commonly had a fixed loading effect of 11-megohms or 22-megohms, regardless of measurement range.  Most quality modern DMM (digital multimeters) have a fixed input impedance between 10-megohms to 11-megohm.  The higher the input impedance resistance, the more accurate the measurement.  Input impedance is a serious issue when measuring high impedance circuits.

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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.

April 1, 2011.  TubeSound, a worldwide vendor of audiophile tubes & test equipment and service center, is pleased to announce the appointment of Snickers T. Dogg to the position of Chief Technical Consultant.

“Snickers will be a valuable addition to our team by growing our service center in a valued-added result-driven manner.  He is a strong strategic fit with our core competencies.”

Snickers brings a goal-oriented approach to servicing.  “The endgame is simple — get it done.”  As pioneer of the Spray-and-Pray service technique, he has been proactive in driving down the cost of repairs.  ”I once stepped on a can of WD-40, and the rest was history.”  This user-friendly servicing technique has empowered millions of technicians worldwide.

Never satisfied with the status quo, Snickers has leveraged the synergies of spray & service to expand the effectiveness of his Spray-and-Pray methodology.  ”If the spray don’t work, you can whack it with the can.”  His outside-of-the-can thinking will allow unparalleled speed-to-repair.  This is a win-win scenario.

“The one thing that impressed us the most was Snicker’s 24/7 customer-service mindset.  His proactive networking creates a strong foundation of trust.”

Snickers also brings to the table a rare ability to find bad transformers without need for any test equipment or powering-on the equipment.  ”I must have a nose for it” quips Snickers.

Competition to land Snickers was fierce.  In turning down a position as a jukebox technical consultant with a Pittsburgh-based music distributing company, Snickers explained “I can’t be associated with nothin’ lame.”

Snickers also plays a mean game of “Bullshit Bingo” and feels that he will have many opportunities to play here at TubeSound.  In fact, he is barking “Bingo” right now.

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.

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This is one of many examples where tube testing requires experience to properly evaluate any tube.  In this situation, using a tube tester would yield a completely wrong answer.

12AX7A tubes - incorrectly factory marked as E283CC

These tubes were factory marked as E283CC, but are actually 12AX7A tubes.  Hence, if you tested them in your tube tester using E283CC settings, you would have concluded that the tubes were shorted and defective, and you would have discarded valuable NOS 12AX7A audio tubes.

The fact that these were not E283CC was readily apparent to anyone with experience.  E283CC is a “special quality version 12AX7 for audiophile” with three distinguishing characteristics: (1) different pinout than 12AX7, (2) only a 6.3v heater, (3) a shield between triode sections that terminates at pin 7.

A shield between triode sections would be easily visible, and these tubes do not have a shield.  Hence, they are not E283CC.

Since E283CC is a “special quality version of 12AX7″, that was a logical place to start.  Experience shows that they look like Amperex 12AX7A tubes, and testing them as 12AX7A verified that premise.  In fact, each tube is well balanced between its triode sections, and they are quality audiophile 12AX7A examples.  (Tube #1: Triode #1 = 36, Triode #2 = 36.   Tube #2: Triode #1 = 31, Triode #2 = 30.  Test scores from my professionally calibrated B&K 707 mutual conductance tube tester, and also without shorts or leakage.  For 12AX7, scores of 22+ good, with scores in 33 range considered typical new).

Lessons: (1) experience matters, (2) tube tester results MUST be interpreted based upon experience, and not blindly accepted as gospel, (3) you should have the experience to recognize when a tube does not look correct as marked, because blindly inserting a wrong tube into your equipment may cause serious damage and/or fireworks.

by Bob Putnak, ©2010.

Of all the subjects that I have discussed, I receive the most inquiries about my HP Officejet “scanner system failure” repair article.  That article also fondly mentions the old-school Laserjet’s, and I had several inquiries about repairing them.  These old dinosaurs are still useful to any small business that prints traditional business text documents or for printing schoolwork.   So here is an article about how I have repaired and refurbished these Laserjet 4L and 4P printers in the past (not so much anymore!)  Do not attempt if you are not qualified to perform such repairs or do not want to risk further damage or inability to reassemble your printer.  The basic concept is similar with other models not shown here, although disassembly will be somewhat different.

HP Laserjet 4L

These printers were built like a tank and page-feed problems are the most common complaint.  A general overhaul will take care of most of these issues.  In summary: the printer is torn down, thoroughly cleaned with compressed air (including the 4 optical sensors on the mainboard), rubber rollers are treated with rubber rejuvenator, the sticky-stuff is cleaned off of the relay coil, the relay coil arm is wrapped once with thin black electrical tape.  If you have a dead printer, you will also inspect the power supply fuses, although a blown fuse is usually the symptom of a real power supply defect.

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Today was my annual warehouse spring cleaning day, so it was a good opportunity to take a “family photo” of my tube tester collection.    My collection changes all the time, sometimes daily.    I did include all of the models that I use everyday for tube sales and bench servicing, and also models that I “rotate in” regularly to keep familiar with them.  Many of these are a permanent part of my collection, others will be repaired/calibrated and sold to customers who want a quality tester that works great.

TubeSound tester collection as of April 2010

Columns are numbered left to right and models listed from top to bottom:

• Column 1: B&K 707. Jackson 648 early tweed case version, Precision 620, Simpson 555, EMC 206, Sencore Continental MU140, Jackson 648R, Simpson 330, B&K 700
• Column 2: Hickok 532, Precise 111 Mutual Conductance, Knight 600B, Hickok 800, Sencore TC28 Hybrider, B&K 550, B&K 747, Hickok 800, Simpson 1000, Hickok 533
• Column 3: US military I-177-B, US military I-177, Hickok 6000A, Precision 10-40, Knight 600C, Triplett 3423, Jackson 648S, B&K 707, Jackson 648A, B&K 707
• Column 4: US military I-177-B, US military I-177-B, US military TV-7D/U, B&K 700, B&K 700, B&K 500, B&K 700, Precision 640, Heathkit TC1, US military I-177-B, Jackson 637
• Column 5: Mercury 1000, Eico 625, Heathkit IT-21, Sylvania 620, B&K 550, B&K 700, B&K 700, Jackson 648-S, mint Western Electric KS-15560-L2, Heathkit IT-17, Supreme 550
• Column 6: Eico 666, Precision 612, Eico 667, Precision 10-12, Eico 666, Precision 10-12, Jackson 598, Philco 9100
• Column 7: B&K 747B, Sencore TC28 Hybrider, Sylvania 220, Accurate Instruments 151, Jackson 648, Jackson 648-S, Hickok 533A
• Column 8: Hickok 6000A, US military TV-10D/U, US military TV-7A/U, Hickok 6000, B&K 707, NRI Professional 70, Jackson 561, NOS NIB Sencore Continental II MU150
• Column 9: B&K 707, Hickok 533, Hickok 533, Hickok 539A, Hickok 752


One nice thing about having a large personal collection is that it makes easier troubleshooting of strange wiring problems in a customer’s tester that they have sent for repair. Being able to quickly examine another unit is often much faster than tracing the circuit. It is also nice to have another unit to compare, especially if I suspect that a component may have been replaced many years ago with a non-factory part.

Here is a truly historical item — a LEEDS & NORTHRUP CO. Wheatstone bridge. It has Western Electric K.S.3011 designation.  I originally called the item a mirror galvanometer but it is more accurately termed a Wheatstone bridge.^[1]

The craftsmanship and build quality of this device is truly remarkable.  I cannot fathom how much money it would cost to build this same device today using identical components.  The unit consists of a large magnet, floating mirror, bulb, magnifier, panel scale, brass and silver switches with no measurable resistance, and a large number of precision resistors that were hand-wound and perfectly balanced using the finest balance bridge equipment available at that time. Truly the highest laboratory-grade accuracy that was possible — BEST of best.  Resistance accuracy should approach 0.1%.  For this reason, the unit makes a fabulous (true) precision resistance box for checking calibration.  This gives it a modern “use” instead of sitting on a shelf as a discussion piece.

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HP Officejet R-series

As a fan of vintage tube equipment, it should come as no surprise that I prefer older quality computer equipment.  In this article, I explain how to repair “Scanner System Failure” problems in a vintage HP Officejet R-series multifunction scanner-printer.  Model R40xi is shown in the photos, but all models (R40, R40xi, R60, R60xi, R80, R80xi) repair the same.  In fact, many similar HP multifunction units of this era are repaired in similar manner.

First, let me explain why I feel that these vintage HP units are worth repairing.  Why fix an “old parallel-port printer” when new models are so cheap?  Like most things in life, you really do get what you pay for, and if HP paid me to take their new models (with the assumption that I actually had to use them), I would not be interested.

During the Lew Platt years, HP was making top quality products.  This period of quality was from the early 1990s (Laserjet 4-series and beyond) through the Officejet R-series of 1999.   The hardware during this time was built-like-a-tank, and when compared to today, the ink supplies were reasonable (although they certainly seemed expensive at the time).

Let us look at the economics: Officejet R-series uses a #45 cartridge, holds 42-mL ink, retail cost $30.  This HP45 can be purchased for $5 on eBay. Modern HP cartridges hold as little as 5-mL ink and cost $15 — which is 4.2 times more expensive using retail price comparisons.  If you consider surplus #45 cartridge prices as your comparison, the cost to operate a new HP printer can be  42-times more expensive.  And if you are an environmentalist (or anyone who hates frivolous waste), ask yourself whether it is environmentally responsible for HP to manufacture an ink cartridge that only holds 5-mL or 10-mL? Certainly not, and those puny cartridges are only sold to pretend that the new cartridge price is cheap, when in fact it holds 4-to-8 times less ink.   That is an insult to your intelligence and an affront to the environment.

In another example, a modern HP toner cartridge (for a current model) costs $99 retail with approx 2,000 page yield.  Surplus HP toner cartridges for the old 4-series, 5-series, or 6-series can be purchased for approx $20-$30 on eBay, and have 3,000+ page yields.  Vintage Laserjets have been known to be run strong with 500,000 page cycles.  Any chance you will get even 50,000 page cycles with a modern unit? Good luck with that.  And how many Nag messages from the new drivers to “buy more ink” would you click on during that time?  Well — no nag messages with old HP units!

So when you analyze the situation, the answer is obvious why I fix these old HP units.  (1) High build quality of the hardware, (2) low cost of use (ie – cheap ink), (3) OS drivers that respect you as a person, without bloat or nag screens to harass extra ink purchases, (4) ink that does not “expire” because some Corporate Executive wanted to sell more ink to pay for his/her 7-or-8 figure CEO salary + stock options + benefits + perks + Golden Parachute retirement package.

Let us compare the “new” models.  By ‘new’ models, I consider any model released in the Carly Fiorina era and beyond to be ‘new’.  The only benefits that I can see in the new models are USB connection (which is easier for most users but otherwise adds no functionality) and much better color printing.  Since I have no problem with parallel ports and only print black-and-white, those issues are not important to me. Otherwise, it is my opinion that the quality of the new hardware is much lower than the older models, the cost of consumables (ink/toner) is outrageously expensive, the computer drivers are bloated and filled with nag messages that harass you to “buy more ink”, and some models “expire” the ink cartridges based upon a random date.   If you feel respected as a customer with that experience, more power to you.   Personally, I am not interested in participating.

With that explanation, let us move on to the repair.  As always, repairs are only suited for experienced electronic technicians with suitable training.  Also, it is useful to take a video of your repair project so that you can reexamine your work if you get stuck during reassembly.

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