Tube Analyzing


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 Some general thoughts on tube testing.

I test all tubes in a radio before I begin restorations.  I am looking for hard shorts, heater shorts (when hot), weak tubes and plain old dead tubes.  I do not want to restore a radio to have a shorted tube put B+ on circuits it does not belong and take out perfectly good parts.  This is obvious.  A heater to cathode short will typically not do too much damage but will send you in circles chasing poor performance and hum.  A dead tube is good to know.  The three above mentioned tubes get replaced to avoid damage, aggravation and tube sub guessing.  The weak tube is the condition needing consideration.

 A weak class A wired audio output tube can cause significant poor performance.  This distortion at low volume usually leads to general irritability (in the owner).  This may or may not present a problem in an AA5.  But it certainly will lead to general irritability if left in a high end Zenith or Grundig.  And so will a Push-Pull configured tube.  A weak audio tube will suffice for restoration activity and alignments.  But weak audio tubes should be replaced before returning the radio to the cabinet.

 Sometimes IF or Detector tubes show weak or questionable on the tube tester.  I put those back into the chassis, complete the restoration and a full alignment.  The final alignment and general performance (within spec or not) will indicate if the tube needs immediate replacement.  Also, I find many detector tube diodes will show weak in the tube tester but perform to specifications.  Check the AGC & tube bias voltages and observer the audio output.  If good then use the weak original. 

 A lot of times a weak RF and first stage RF tube will critically degrade reception sensitivity.  I replace those with fresh strong tubes.  Some aficionados  may be reluctant to spend the money or want the original branded tube but that leads many times to “This radio does not receive good or sounds like crap!”  I don’t usually take the chance on RF tubes or audio output tubes.

 High voltage, high power AF/RF tubes need special consideration.  After testing for a damaging short circuit, a strong or weak indication in a tube tester means little.  The true test of a high voltage high power tube is under full operational conditions.

 For more reading on tube testers there is a great site here:






I was asked to individually test some tube to establish per tube Transconductance.  So I built a small adaptor (old school Ham Radio homebrew style) to insert an amp meter into the plate circuit.  


I use a B&K 700 Dynajet tube tester as a platform.  It conveniently supplies B+, filament voltage, and bias supplies.  The tube tester also has the advantage of testing for shorts and grid leak.  It is very convenient not to piece and part the whole test circuit together.  


Test set up.

IMG_4101.JPG (1046921 bytes)Testing my test jig with an old 12AX7.

12AX7 tests good if the scale is over #20.  The left meter is plate alternating current  Ia, in the tables.  The right meter is the grid alternating voltage 

IMG_4107.JPG (1003173 bytes)

This is an ECC85 (6AQ8).  


EL95 test set up.  

IMG_4111.JPG (355205 bytes) I made up a test socket for the EL95 (6DL5).  I took a dead 7 pin miniature tube, cut the evacuation tip to let the air in, and used a fiber wheel on the Dremel tool to cut the base off the glass.  I then soldered wires to the base and used epoxy to hold the wires in place to the glass tube base.  The amp meter is in line with the plate circuit.  The volt meter is measuring the AC grid volts.  The tube is a weak tube from another radio. 



scan0001.jpg (883616 bytes)  

Plate volts = 212 vdc.

The circled numbers are the tube tester meter readings that represent relative gm (no calibration).  

I believe the gm may calculate to be higher if the plate voltage was 300 volts DC as in a tube manual.   

scan0002.jpg (823451 bytes)

Plate volts = 212 vdc.


scan0003.jpg (483500 bytes)

(EL95)  This sheet shows a max Va of 550 volts. 

I measured the plat volts at 173 vdc (250 typical), and about 1/5 the max plate current, far from the typical value.  So the resultant transconductance is lower than the max possible.  This tube is for mobile audio service and is specified at 3 watts audio max., 12% THD (Not exactly Audiophile grade).  

All tubes passed in the Dynajet 700 tube tester with high readings.  


The equipment list:

bulletA mutual conductance tube tester.
bulletTube test sockets to access the tube tester circuits.
bulletExtra tube socket
bulletSome thing to mount the tube socket firmly to. 
bulletVolt meter and milliamp meter.
bullet300 volt hook up wire.
bulletTube manual for tube pinning.
bulletA book of tube formulas (may be in the front of your tube manual).


Some conclusions:

All tubes passed testing just fine.  You may hear small differences when using a different audio tube in a particular application.  But the differences should be minimal.
The transconductance on the data sheets represents the maximum value at a particular plate voltage and grid bias.  I could not easily duplicate those parameters with out building an extensive test facility.  However those tubes fall within operational parameters both on the calculated data sheet and with the B&K Dynajet 700 tube tester. 
The EL95 is a tube initially developed and designed for automobile radio use putting out a maximum of 3 watts per tube with max plate voltage.   It is not the cleanest tube that faithfully reproduces a sine wave signal as indicated by the specified total harmonic distortion (THD) of 12%. The lowest THD achievable is 3.5.% in class B push-pull operation with two tubes.  Normally an audiophile would prefer less than 1%THD.  So in my opinion do not expect too much from the EL95 in the way of sonic purity. 
Download the free software from this site:  and compare the tubes.  Use the Short Data tab for quick information. Compare the EL95 with a 6L6.  The 6L6 in push-pull operation will yield 0.6% THD.  Other 6L6 configurations render a poor 15% THD. You can use this software to look up tubes, find substitutes and have an idea to predict how the radio or amp will perform.
The ECC83 tubes seem to be just fine.  There are a few with higher gain and one or two on the low end.  But all indicate just fine in the Dynajet 700.  All should be serviceable in your unit.  Noise, tonality and microphonics are a subjective matter.  So choose the tube that you thing sound best for a particular application.  If you are having difficulties with a particular unit look towards the individual circuit parts like capacitor, resistor and the power supply keeping up with the load demand.  

Microphonics is the physical vibration of the metal tube elements.  If it is extreme acoustic feedback can occur.  I have experience this extreme microphonics.  Tubes can be constructed to minimize microphonics with internal changes and supports.  It is similar to an incandescent lamp manufactured for rough physical service (paddle fans).  The manufacturer installs additional filament supports to reduce the shaking of the hot filament.  All tubes will produce noise, vibrations or Microphonics if struck hard enough.  It is the tubes that pick up normally expected vibrations (i.e., guitar amp speaker cabinet vibrations) or feedback that are problematic.  Do not set your sensitive tube amp on top of your Hi-Fi speaker.  And make sure all the rubber standoffs and speaker washers are in good conditions or your radio may experience acoustic feedback.

Tubes have what is called Shot Noise.  That is similar to a bunch of BBs hitting a metal plate.  The electrons impact into the tube elements and cause this shot noise.  It can sound like static.  Some tubes may have reduced shot noise.  Tube testers do not indicate shot noise unless equipped with a head phone monitor jack.  Reducing the impacting electrons will reduce shot noise.  That is accomplished by reduced plate voltage.  But you give up gain (transconductance). There is always a trade off.
Carbon composition resistor go bad with decades of time or if they have been overheated during soldering.  They go bad even in storage.  They will typically drift high out of tolerance when bad.  A questionable carbon composition resistor especially if others around it are bad it will most likely soon go bad with use. I found this out the hard way with an old radio that I replace the resistor only when they went bad.  I had the unit on the bench at least 4 times before I just "shotgun" replaced all the resistors.  That fixed it. I typically replace resistors with Carbon Film unless they operate in RF above 18 Mhz.  An old carbon composition cathode resistor may add shot noise and make the tube sound bad.  Try a carbon film or metal film or metal oxide resistor. 
Other non tube parts can cause problems.  Carbon Composition resistors, by design of the compressed carbon particles and insulation, will be noisier than carbon film, metal oxide resistors or resistors made from a homogeneous strata of material.  One can study the manufacture's data to make these conclusions.


Capacitors do not typically come with noise specifications.  But some military specified (mil spec) do.  Capacitors constructed for severe RF service (satellite spacecraft, medical apparatus, defense systems, etc) would render a design crippled if they contributed large amounts of distortion or noise. So those caps should operate very clean in audiophile circuits.  

With an eye to selection and specifications one can choose parts that will perform (with tested and proven operations and life specifications) admirably in audio circuits with out spending vast amounts of cash.

Power supply "sag" can cause strange problems.  The volume can drop and the dynamic response of the amp can be compressed.  Sag is when the power supply can not keep up with the demands of the amplifier circuit.  Loud, continuous and/or bass passages drain down the filter capacitors (electrolytic) faster than the transformer and rectifiers can recharge them.  There may be bad power supply components including the electrolytic filter capacitors.  If you hear hum that may indicate a dried out electrolytic capacitor that needs replaced.  A small amount of sag us usually attributed with vintage tube power supplies.  Sag can be a desirable artistic quality in musical instrument amps.   But it is hardly tolerable in an audiophile or PA system.  


The ECC85 is for RF service.  There is a lot more going on with these tubes than just pure gain.  In the RF world interelectrode capacitance plays a big roll.  Some times more than gain.  So one tube may work just fine in one radio and be questionable in an other.  They all pass in the Dynajet.  Tube #15 is a bit unbalance from side to side.  It may not work well in circuits that need a matched set of triodes.  But #15 will do fine in circuits using the triode sections in different stages.  Keep in mind this is only an outside guess by looking at the numbers only.


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