Auto Bias Cards on L1 and L3 tester

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These cards will do a quality test.

Test test set up is a real tube circuit, with a programmable cathode resistor and programmable capacitor, inside the L3-3 tester. As it has this option, and it is fully programmable. The circuit is 'Auto Bias', so there is no need to set grid voltage.

There is great misunderstanding about what tube testing is for. Normally it is to find out the use condition. The one and only method for this is AUTO BIAS. Any information about how the tubes comply against the curves is low with this method. So it cannot be used for matching.

Adjustable bias is only to verify a tube against the tube curves, or against other tubes. (As needed with matching). Precise information about lifetime cannot be generated from adjustable bias testing, because the result is compromised by the grid voltage tolerance.

This is why auto bias testing is probably the most important for used tubes.

Somes tube can be tested in the manufacturer's recommended Auto Bias Circuit. This requires the auto bias resistor to be the similar, and it does not have to be identical. So the nearest possible step is taken, which is not always the datasheet value fully exactly. However, a tube which will show for instance 83% condition, will do so over a wide range of settings, and we use at least a close setting. So this works well.

New: Auto Bias testing was always thought to be impossible for the L3-3 with directly heated tubes. However, I have found a way for that, which works not on all sockets, but it does work on the B4 socket and UX4 socket, and probably a few others. So the most common used HiFi Triodes can now be tested in auto bias.

You will see some test result option on the card.

  1. 'Bad': If below, the tube failed to bias normally in this circuit. You might consider to verify this tube with a Funke-W19 card. It can be an emission problem, but also another problem like grid damage or drop damage. Tubes like KT88 or EL34 can have such mechanical damage quickly, while emission is reasonable still. If it is an emission problem, the W19 card will find this out, like no other. We can make such cards for many, if not all tubes.
  2. Above 'Good': This is the minimum value a tube should have to work in a random circuit.
  3. In between Good and Bad. You may see a tube is below 'good' and above 'bad'. This is called the '?' range. This means a tube may work in circuits that are less demanding. If such a tube works in a particular circuit, it can be used further. .
  4. 'Ideal' This represents the value for an unused tube
  5. Above 'Max' A tube should not draw as much current as possible in an auto bias circuit, as the circuit aims to set the bias to the ideal range. If that is not working, there is something wrong with the tube, like grid deformation by drop damage.

Note1) What does it mean if a tube is above maximum or close to it? Generally this is bad! Such a tube will may give amazingly nice plate current on the autobias card, and you think it is a great tube. However, when testing such a tube with the Adjustable bias card, you MAY see, transconductance is much lower than normal. This combination of too high plate current, and lowered transconductance is just what indicates plate deformation, or grid deformation. Or, when testing such a tube with the Funke W19 card, it may come out as low emission tube even, simply because a emission tube may be able to draw a lot of current in auto bias still. However, the W19 card is not fooled by that, and will reject the tube, whereas the auto bias card may accept it. Also such a tube will be difficult to match in a push pull circuit. For matching always use the Adjustable bias card though.

Note2) Although it is possible to measure Gm with this card too, it is recommended rather to take the Adjustable Bias cards for this, as Gm must always be measured at the 'Ideal' plate current of the tube, and not the lower plate current as results with used tubes in an auto bias circuit.

The Auto Bias cards can be used for burn in conveniently. Only after good burn in, a tube will test nicely as it should. If NOS tubes were stored for 50 years, they may come out if the boy testing fine or testing weak. The weak ones may improve greatly after burn in, and also the spread in paramaters will become smaller. Leakage will go away, and transconductane will go up. Note, after already 6 months of storage the tube will begin to loose burn in already. Even factory new tubes are normally not burned in, unless they are SQ types. So what comes out of NOS boxes, many times is initially disapointing. Burn them in first, and then judge again!

So each test card, auto bias, adjustable bias, and emission test, has it's own advantages.