Technical Bulletin 11. Cathode Issues

Technical Bulletin TB-11 Part 1

Cathode-Issues

by Jac van de Walle

The white Spark Problem. PART 1.

Sometimes, a tube amplifier can cause an output tube, or rectifier tube to spark.  This is a bright  white or pink spark, or blue or pink sparkling inside the anode. It causes a very loud  noise from the loudspeakers. If this happens, you may think it's a short circuit in the tube, specially if it  seems to be connected to one tube of a pair. However, this is always an amplifier problem. Here, some of the background information will be discussed, and at the end some links to historical documents are given.

Understanding things

Fully understanding this issue requires some minimal 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 they already can tell, the cause is definitely the bad tube, and definitely not the good amplifier. I whish I would have this talent too, not doing my homework, and still come up with a qualified answer. But I remember already in school, it didn't work this way for me.

A very common misunderstanding is, tube manufacturers are supposed to build tubes which can withstand some level of abuse, and amplifier manufacturers don't have to do so. Both things are not true.

Though many times tube and amplifier will survive the attacks, quality is not defined like that. The nicest example I once had, was by an Italian tube dealer, who was by mistake testing 2A3 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. The first tube of the pair survived, and test results were good. The second 2A3 he tested, blew up the heater wire immediately at 5 Volt. 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 were not the same, so one was good and one was bad. He never changed his opinion, and quit the relationship for ever.

It gets more difficult when we talk about microphonics or noise. Because for that, 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.

In this light we must also see most cathode issues.

What is the white spark issue?

Sometimes a rectifier or output tube can produce a violent spark inside the bulb. This gives a cracking sound, from the tube itself, or from the speakers too. With the above text in mind, I don't have to tell you the reasoning of most people, when this occurs with only one tube of a pair, or when this occurs only with the new bought rectifier, and the old rectifier works good.

The good news is: The tube is still good. The flashing may seem terribly violent. However the tube takes no damage, but rather the amplifier could, the transformer or the speakers. Though this risk is small, and most of the time it will only blow a fuse of the amplifier, or tweeter fuse.

The bad news is: The amplifier has a design problem. To say it clearly up front, a white spark is always a design problem by definition. Only a pink illumination effect may be a gassy tube.

What causes the white spark?

Quick and simple: 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. The second cause is totally wrong constructed DC coupled circuits, which work good by coincidence, "designed" by trial and error, and failing protective circuitry.

The effect gets larger when the tubes have better emission.  So you may observe this with large power tubes, but it can happen to smaller tubes as well, when the switch-on effects in the amplifier are very bad. 

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. 

For good amplifier designers, the below links are not helpful, because they already know what is written there. But don't feel ashamed to read it.

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

Read the next part: The white Spark Problem. Part2