©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.
- 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.
©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.
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Here are a few Q&A’s about a Fisher 500-B receiver:
1) Q: “How do I connect speakers to my Fisher 500-B”?
A: This is confusing if you are not familiar with Fisher equipment. The photo below shows correct speaker connections to the 500-B. I have seen other 500-B models with a slightly different layout, but essentially one channel (here, the left speaker) is connected normally — between “C” and correct impedance. The other channel (here, right channel) is connected between its “C” screw and the unmarked screw next to it — exactly as the black lines painted on the receiver instruct. The only thing connected to this channel’s impedance screws is the impedance selector wire that runs through the chassis. The reason for this channel having a “different” connection is because of the speaker phase-reversal switch that you see alongside the speaker connection terminals.
2) Q: “My Fisher has almost no output on either channel, from any input or FM tuner. The sound is coherent, but even at max volume it is extremely low and treble-sounding. Any ideas?”
A: You are probably missing the two reverb jumpers on the back panel that connect together the “Reverb In-Out” RCA jacks for each channel. It is not uncommon for these jumpers to be missing, and if so, you can make your own very easy. You only need to connect the “hot” terminal of the RCA jacks (the grounds are connected internally). The slight sound that you were hearing without the jumpers was a result of capacitive coupling.
speaker connections marked with colored dots