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Vacuum Tube Testing  

Preamp Tube Curve Tracing

For critical preamp vacuum tube positions, in-house computer testing and matching ensures best possible tube performance.

Tubes are tested at real world high voltage conditions as they would be in the final circuit. In these instances, the tube testing report is also included with the shipped product.

We test our tubes across their 4 key parameters:

  • gm (transconductance)

  • Mu (gain)

  • Rp (plate resistance

  • Ia (plate current)

These parameters give an indication of the tubes' overall heath and are the key parameters for matching. We often match to 5% or better across all 4 parameters.

 

Since these parameters are tested at a static point on the tube's operating range, we also curve trace these tubes to look for performance anomalies. With curve tracing we can visually catch bad tubes that would otherwise pass the static conditions above. 

Also, with our curve tracing capabilities, each section of dual triode tubes are traced simultaneously allowing a visual comparison of matching conditions between sections as well as numeric parameters listed above.

Example Tube Curve Trace

Tube Curve.jpg

For more information on the importance of vacuum tube testing, see our technical article below

Example Tube Test Report Included with the Product

Tube Test Data.JPG

Power Tube Static and Dynamic Testing

Curve tracing has proved to be extremely accurate while also providing predictable performance and near perfect channel balance for our preamp products and preamp stages in our power amplifiers. However, power tubes are different.

 

The high power dissipation capability of power tubes requires them to heat soak at high static power levels to get a true measurement of their performance. 

It was this need that led us to purchase, refurbish, and put into continued service, the Hickok Model 700 (later sold as the R&D 1700) tube tester.

To call the model 700 a "tube tester" is a bit of an understatement. This this a vacuum tube analyzer designed and sold to vacuum tube manufacturer's product test and R&D departments with the most stringent of standards.

 

Therefore this unit was/is exceedingly expensive and rare with estimated very low numbers produced. Not only is this unit rare, it is in my opinion the absolute best vacuum tube tester built during the heyday of of tube development (possibly outside of tube manufactures own homebuilt custom test equipment).

Hickok Model 700

Features:

 

  • ​All tube element voltage are user adjustable, allowing testing of nearly any tube type

  • Testing conditions can be set to the standards defined by the tube manufactures data sheet, and the test voltages held at static DC levels for high continued power dissipation.

  • True regulated plate and screen power supplies, which maintain the desired test voltage under large variations in tube current. This leads to very high repeatability and accuracy.

 

Testing Parameters Setup:

  • Plate voltage: Up to 300V

  • Screen Voltage Up to 300V

  • Plate Current: Up to 200mA

  • Screen Current: Up to 100mA

  • Grid Bias: Up to -100V

  • Adjustable Filament Voltage

  • Selectable grid voltage AC level level for gm test to match with tube type sensitivity

Measurement Results Provided:

  • Filament Current - To check filament health

  • Gird Current - To check for gird current issues

  • Plate Voltage

  • Grid Voltage

  • Plate Current

  • Screen Current

  • gm - True Mutual Transconductance 

Hickok Model 700 Tube Tester

Hickok Model 700 Power Supply

Hickok Model 700 Tube Tester

Here we see the massive power supply with vacuum tube regulators hard at work. The regulators maintain the output voltage settings at the tube under test, so even if plate or screen current change dramatically between tubes, the test conditions don't.

 

This allows for precise and repeatable measurements without the need for readjusting of the applied voltages.

With the Hickok 700, we can measure datasheet parameters directly and compare them to the factory test values while the tube is biased at the datasheet defined operation conditions.

 

We then capture and match power tubes across the following parameters:

 

  • gm - Transconductance

  • Ia - Plate Current

  • Is - Screen Current

 

 

With power tubes matched in this manner, we find the output stages in our power amplifiers exhibit lower distortion and higher output power due to both matched static bias conditions and matched dynamic AC performance with the audio signal swing.

KT88 tested at Datasheet Conditions

Vp: 250V Vs: 250V Grid Voltage: -15.0V

Results: gm = 12.25mA/V, iP = 138mA; Is = 6.0mA

Notes on Other Testers:

Nearly every other transconductance type tester uses pulsed AC bias, rather than DC bias, set at arbitrary voltage levels which "should" provide "representative" performance results. Depending on the tester, these results may then be scaled by the tester's meters in an attempt match to the published datasheet gm values, even though the tube was not tested at the bias conditions specified by the datasheet.

 

Hickok actually invented (and patented) "Dynamic Mutual Conductance" testers as an inexpensive way to make commercial tube testers without the need for DC power supplies. They use the AC from the mains wall voltage with a rectifier to pulse power to the tube's plate and screen. While they do test dynamic mutual conductance (aka transconductance - gm), the operating conditions are not held at DC and somewhat random. This leads to better test conditions for some tubes and a terrible test conditions for others. Many folks outfit these testers the ammeters to measure plate current for tube matching, which again is plan wrong since this plate current is pulsed.

 

This is why no two dynamic mutual conductance tester models will ever give the same results, and in fact each model will have it's own lookup table for acceptance values for each tube type. This makes these testers useless since the results cannot be verified on another tester. This also leads to some nefarious practices by tube sellers.

 

A tester of this type can only make comparative measurements with itself, rather than absolute. Calibrations are also somewhat meaningless since the meter readout is simply adjusted to match a known good tube. With the Hickok Model 700, calibrations can be made regardless of the tube under test because the power supplies and AC grid signal itself is calibrated in absolute terms to bench top measurement equipment.

A label with the tube ID and test info is added on the box

See the Model 700 In Action

Rectifier Tube Testing

Modern tube rectifiers have been known to have or hit and miss quality. We have experienced this first hand where nearly 50% of rectifier tubes we ordered in a batch were arcing at less than half their rated voltage. These failures were under very kind loading conditions with low filter capacitance values, high source impedance, and with the addition of solid state backup didoes. These series backup diodes act just like a tube rectifier, passing current only in the forward direction. Since they block reverse current flow, they divide the reverse voltage (known as Peak Inverse Voltage or PIV) between the tube and the diode. This further reduces stress on tube rectifiers and also help quench an arc, should it form in the first place. These failures should not have happened!

IMG_5347.JPG

It was clear that we needed to screen rectifier tubes prior to use in our amplifiers.

To do so, we built a custom rectifier tester which mimics an amplifier power supply by loading the rectifier with a multi-stage filter and internal dummy load. Rectifiers are tested from cold with high voltage immediately applied from a power transformer while the cathode and filament slowly warm up as they would in a real amplifier. This is necessary to find bad tubes. Letting the tube warm up before applying the high voltage will bypass the main arcing failure mode of rectifier tubes.

 

We can test rectifiers at two different voltage levels by changing the AC voltage of the secondary winding of the power transformer which feeds the rectifier.  We can also test rectifiers in a parallel or single tube arrangement. The output voltage of the power supply is measured so we can observe the rectifier performance and make sure its voltage drop is in normal ranges. Fuses in multiple locations protect the equipment and typically open once a tube arcs.

 

Only if a tube passes our test will it go on into one of our amplifiers.

What Causes Rectifier Arcing?

 

The typical cause of rectifier arcing is from the forward voltage when cold, not the Peak Inverse Voltage (PIV). When the tube is cold, there is no electron emission from the cathode, so the tube temporarily "blocks" forward current as well reverse current, as normal. After power on, the cathode and filament start to warm and thermionic emission of the cathode begins causing the tubes forward resistance falls, allowing current to flow only in the forward direction. This current flows into the power supply capacitors slowly charging them up. Once the tube is fully hot, its forward resistance is low, giving the specified forward voltage drop in the datasheet, often in the rage of 20V-50V. At this time, the rectifier only has high voltage across it when reverse current is blocked during the negative half cycle of the AC waveform. This voltage is the inverse voltage, which the rectifier must repetitively withstand every AC cycle. This one-way valve action is how a rectifier converts an AC voltage into a DC voltage.

 

Poor quality control of the electron emitting coating on the cathode and/or poor control of spacing between the plate and cathode can lead to hotspots on the cathode. Before the cathode / filament are fully warmed, these hot spots may be the only regions where current begins to flow to charge the power supply capacitors and load. If the hot spot area is too small and/or the current is too high, then the current density in that region will be high enough to develop and arc between the plate and cathode. Once this happens, the slow controlled startup behavior of a tube rectifiers is lost. Once the tube cathode is fully heated, the cathode emission will be much more uniform along the length of the cathode since the cathode temperature will be evenly distributed. This means lower current density to support the given power supply load so the risk of arcing is now very low.

Some tubes may be more pone to are than other due to a poor vacuum, which leaves excess gas trapped inside. If the tube has excess gas or contaminants inside the envelope, than the high current density can much more easily ionize the residual gas causing the gas molecules to become charged and thus able to carry current, bridging the physical gap between the cathode and plate. This is also how neon bulb works!

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