ELECTRON TUBE TESTING
The testing of electron tubes has long been a subject of disinformation, confusion and false claims - mostly by folks who mean well but just do not understand this complex subject.
It is common these days to describe tubes as "Matched Pairs", or "Matched Quads", or even "Matched Octets" - but what does matching entail and how accurate is the method used?
To set the record straight, this page presents extracts from the RCA Receiving Tube Manual RC19 and the Radiotron Designers Handbook - 4th Edition, that describe RCA's attitude and approach to the testing of electron tubes.
In particular, close attention should be given to the closing paragraph entitled "Tube-Tester Limitations".
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There is of course much more to the subject but it is hoped the above explanation clarifies any misunderstandings or expectations that you may have regarding electron tube testing.
In the case of "Emission Testers" most use AC as the test current and simply convert to DC for metering purposes.
Most apply only low voltage for the inter-electrode test and cannot replicate high-voltage operating conditions.
One thing can be said with certainty - emission testing
cannot determine the likely power output or gain of an electron tube -
but short of installing a tube in a known circuit there is no other convenient
way to test a tube.
An Emission tester is a COMPARATIVE measurement instrument - ie it compares one tube with another of identical type, specification, or known standard of performance.
Emission testing is not an absolute measure, such as "volts" or "amps".
First-off, before commencing any test, ensure that full-scale deflection (FSD) of the meter is calibrated for the mains voltage available during the test.
When calibrating an Emission tester for a particular tube type, ensure that full-scale deflection (FSD) of the meter only occurs with a tube that is known to be 100% good.
The FSD setting can be confirmed by measuring a batch of tubes and determining the best typical performance - do not rely upon the Tester Manufacturers' recommendations. Of course if a particular batch is "all-high" or "all-low" then the setting will be false so it is best to use tubes from as wide a sample of batches and manufacturers as is practicable. Don't forget to record the setting for next-time.
Do not set the scale for FSD with tubes that deliver better than 100% emission - ie those that are on the high tolerance band of manufacturing tolerance variation for emission - which is typically + or - 25%
Military or Industrial versions of common receiving tubes
may display lower emission than their receiving tube versions when the
same tester settings are used - this is normal.
Audio tube performance may be readily gauged by listening.
If the high frequency performance drops off over time then the tube should be replaced - regardless of other test indicators.
If hum is present in a push-pull output stage then one
tube most likely requires replacement to restore balance in the output
Miltary and industrial versions of popular receiving tubes often do not display superior performance in audio applications.
This is because the military or industrial version was
designed for non- audio applications and their type or batch testing did
not provide for desirable audio characteristics - such as low hum, low
noise, low microphony etc
Although it is tempting to require audio twin triodes to be accurately matched between triode halves - because it seems like a good idea - unless the circuit design requires a matched pair then there is no advantage in matching.
Equivalent tubes - ie those in a corresponding circuit location - in each channel of a stereo amplifier should be matched for gain to preserve audio balance in the output signal.
MATCHING is desirable where close tolerances are required for circuit operation, such as in push-pull amplifiers and multiple channel stereo applications.
It should be stressed that in the 50's and 60's, when vacuum tubes were king, due to the additional costs and delays involved, matching was not a normal process for commercial or domestic equipment.
Vacuum tubes were used by the million, so to insert an additional process that produced little noticeable result to the finished product was counterproductive.
However nowadays as a result of hype on the internet, there is considerable demand for matching of vacuum tubes.
Whereas consumers are quite contented to purchase equipment having components with manufacturing tolerances of between 5 and 20 % - and/or with no provision for internally adjusting operating conditions - they want their vacuum tubes to EXACTLY match.
The following text, extracted from the Radiotron Designers Handbook 4th edition 1953, shows why so-called "matched" electron tubes are not matched at all.
Chapter 13.5 page 580.
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This page last amended 07 October 2012
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