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 spark,  in are case also blue or pink. 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 Here, some of the background information will be discussed, and at the end some links to historical documents are given.

Fully understanding this issue requires some minimal understanding of physics, but I will try to avoid this, when possible. It is a little bit common, unfortunately, when issues do not necessarily occur with every tube, that people who in fact understand very little about the subject, "already know" the cause is the tube, and definitely not the amplifier. Their misunderstanding is, the manufacturer does not build tubes which are intended to withstand abuse. Though many times a tube will survive bad attacks, still quality and sure not lifetime is 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 to his opinion. Those two were not the same. One was good and one was bad. He never changed his opinion, and quit relationship afterwards. It gets more difficult when we talk about microphonics or noise, when tubes are used the wrong way. Because you will have to understand the circuitry. Some tubes will be noisy, some not, and it is close to impossible to make people understand why they do something wrong, when the issue is related to one tube of a pair. However, just do it right, and both tubes will work fine, is not what people want to hear. 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 the power supply fuse of the amplifier, the tweeter protection fuse, if it was not forgotten to use one, or even none of that, and the whole set up plays normal at the next switch on. Just the problem will randomly reoccur.

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.

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 amplifier designers, the below links may be helpful.

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