Technical Bulletin 11. Cathode Issues

Technical Bulletin TB-11 Part 1


by Jac van de Walle


The white Spark Problem.

Sometimes, a tube amplifier can cause an output tube, or rectifier tube to spark.  This can be an impressive pink flash of the whole tube, or just some little blue or white sparks of inside the anode. What causes this?

If the local current density of the cathode is exceeded, it is clear this generates heat locally. The cathode has a powdered surface, and if we would look at that under a microscope it looks like a moon landscape, with craters and pieces of junk. Because of this, It is obvious some microscopic small parts emit locally more electrons that others. Specially when the heater is in the warming up phase, it is full of not yet working spots, and some other spots are working already. These spots which begin to work first, logically carry ALL of the current. This is particularly difficult, when the rectifier capacitors are too large, the transformer inner resistance is too low, and the amplifier itself draws a surge current at switch on, due to faulty design.

The local over heating of the cathode, during these few seconds of warming up time, may cause very tiny fractions of cathode powder to become loose. These very fine dust particles, once free in the vacuum, get charged negative by the free electrons. Once charged, they moved away from the (negative) cathode, and want to fly to the anode. Before they get there, they get massively hit by all of those electrons, which equal the electric current of the whole tube. The dust particle gets white hot, and evaporates. This gives the white spark you can see. This evaporation for a very short moment fills the tube vacuum with (very few) ions, and this is the audible effect. In very rare, severe cases, this ionization can cause an avalanche effect, and then the whole tube lights up violently pink. This is called a plasma effect, and it causes the whole tube to short for as long as the flash lasts. OSRAM called it the Trigger Effect.

Understanding things better

Fully understanding this issue requires some understanding of physics, but I will try to avoid referring to this, when possible. It is a little bit common, unfortunately, some people just swap the "bad" tube with a "good" one, and from that they believe they can tell, if the cause is a bad tube, or a bad amplifier. It is a bit strange to see sometimes how little they understand of what they are doing, and then know everything better. I whish I would have this talent too, not doing my homework, and still come up with a qualified answer.

There is a common misunderstanding, tube manufacturers are supposed to build products which can withstand some small level of abuse. While at the same time, amplifier manufacturers do not have to do so. So at a tube issue, of the kind we all know these can happen, the amplfiier may fail or self destroy, and then it is not called an amplfier issue. Both things are not true! Jean Claude Verdier (Yes from VERDIER company) could not have explained it better :)

Though many times, tube and amplifiers will survive the attacks of the one to the other, product quality is not defined like that. The nicest example I once had, was by a tube dealer in Rome, Italy, who mistakenly tested a 2A3 tube on 300B settings, with his tube tester. Note, 300B has 5Volt heater voltage, and 2A3 has 2.5Volt. Apart from that, 2A3 will test well on 300B settings. He had been doing so ever since, and did not notice. Until, one day it went wrong. The first tube of an expensive AVVT 2A3-Mesh pair survived, and test results were fine. The second 2A3 he tested, blew up the heater wire. We had a lovely discussion about tube quality afterwards. His reasoning was, if the "good" tube can survive it, the "bad" tube has a quality issue. Those two tubes he said, were not the same. For him, one was good and the other was bad. He never changed his opinion, and quit the relationship for ever.

It gets more difficult when we talk about microphonics or noise. That is because for this, you will have to understand circuitry. Some tubes will be noisy, some not, and it is close to impossible to make people understand why that is an amplifier issue, when it is clearly related to one tube of a pair. However, just correct the circuit design error, and voila: The "bad" tube will operate dead silent. Every tube dealer knows, you can get returned "bad sounding" tubes from angry cistomers. You re test them, and they are fine. The next buyer of what we sell then as a demo pair, with option to return, is totally happy with it. The only change? Another amplifier. What else is there tp say about it! In this light we must also see most cathode issues.

What does it mean when we have a white spark issue?

The good news is: The tube is still good. The flashing may seem terrible. but in vacuum a short is not possible. The tube takes no damage, but rather the amplifier could, or the SE transformer, or the tweeters. Though this risk is small, and most of the time it will only give a small crack noise in the speaker.

The bad news is: The amplifier has a design problem. To say it clearly up front, a white spark is always an amplifier problem by definition.

What causes the white spark?

This is explained at some other places in this text as well, but quick and simple: It is caused by too much current, with a not fully warm tube.

The most seen cause is excessive in rush current with tube rectifiers, far above the maximum peak value. There are a few ways to do this wrong.

The second cause is DC coupled circuits, which work good by coincidence, and sometimey by some other coincidence fail.

The effect gets larger when the tubes have better emission, so it can happen after a tube exchange.  

To prevent any damage do not use the amplifier any more, and contact your amplifier manufacturer to have it checked up, or better say repaired. 

Read the next part: The white Spark Problem. Part2
For good amplifier designers, the below links are usesless, because they already know all of this. But don't feel ashamed to read it anyway.

1) Warning by OSRAM, about design issues,
which causes sparking tubes

2) Spark Warning by RCA, about design issues,
which causes sparking tubes

3) Another Spark Warning by RCA.

4) Table of lowest allowed transformer
windings resistance

5) This Application Note, Part 2
Important Notes for amplifier manufacturers

6) Rectifier tube killer schematic

7) Circuit to calculate minimum series resistance of heater winding.