why tubes valves wear out image

Most folks are just not sure when should you change your valves?

Valves do wear out.  How do I know?

We need  explanation of how they wear out and what to look for. Then you will know when to replace them

There is a lot of stuff written on forums on why tubes valves  wear out.  There is a lot of confusion.

Here is an example of the confusion on one typical forum .

I have previously written in another article about failure mechanisms in valves/tubes.  We all know as valves get older they get duller sounding. The amp looses its attack and the high frequencies start to turn mushy.  Ever wondered why valves tubes this happens?

Ok, so just bear with me and read this physics bit.  There is no exam at the end! It is worth knowing so read on..

A valve works with electrons passing through  a vacuum The anode has a high positive volts and attracts the electrons. This creates the current going through the valve.  But where do the electrons come from?

There is a heater inside the valve. That lovely orange glow that you see.  It heats up a metal plate called the cathode. It gets it really hot. The cathode is coated in a special material which gives off electrons when it is heated.

All sound good so far? 

Keep in mind that there is a fixed amount of this special coating on the cathode so there is limit to how many electrons it can produce over the lifetime of the valve.

As the valve ages, fewer and fewer electrons are given off. This is called the valve or tube emission dropping.

What does this mean in the real world?

Higher frequency notes need much faster response times to track the signal.  With fewer electrons available it effectively gets slow and lazy. The result is the top end frequencies are effectively filtered out by the poor old worn valve.

The same is true for output valves.  They tend to start producing break  up at lower volumes than they used to do.  The headroom on the amp gets lower and lower.

This effect of the electron bearing material wearing out is therefore related to the number of hours when the amp is switched on.  The load on the preamp valves ( such as the ECC83/12AX7) is relatively constant compared to the output valves that are doing all the heavy lifting.

Output valves are doing all the hard work in amp as they produce the power output.  So, playing at higher volumes  will mean more demands put on the output valves.  This leads to a shorter life on the output valves as compared to the preamp valves

A good tell tail sign of wearis that you are finding you are having to crank up the treble on your amp to higher and higher settings

A power soak may be handy for keeping the volume down but the penalty is that the output valves are worked much harder.  The trade-off is that you can expect a much shorter life from output valves running into a power soak. Of course it is good for us because you become a regular customer buying output valves.

In some cases, certain vintage amps cannot withstand long term power soak usage and the output transformer will fail.  Us guys that fix amps are more than happy to replace that expensive transformer in your amp.

You can start to hear crackling and popping in the amp. This is caused by the insulation in the valve starting to break down.  It unusual starts with what. can “wiping ” sounds then moves on to crackling and popping. This is usually the output valves and it is time to replace them.  If you leave them eventually the insulation will totally break down, the valve will short circuit. The main and internal fuses will blow and maybe burn out a screen resistor or two and even burn the tracks off a PCB based amp.  

Your amp can become really microphonic. Your amp case becomes sensitive to tapping it.  The electrodes in the valves have become loose with all the vibration in the cabinet. Loading and unloading plus shaking around while being transported causes the valve innards to become loose.  V1 is at the start of the gain chain so usually is the most sensitive.  Time for replacements.

Loose electrodes in larger output valves, for example EL34’s, KT88’s can actually be heard while the amp is being used. You can actually hear the buzzing of the electrodes inside the valves coming form the valves themselves and not the speakers !!

So the question is how long will my valves last?

Now you will no doubt have heard stories like the valves in my amp have been in there for 20 years and are still working. Three things here:-

  1. The amp has hardly ever been used so it is more of an ornament than an amp
  2. The owner has got so used to the cloth eared tone that he thinks this is how the amp should sound
  3. Even NOS valves wear out

With all the above taken into account , it is hard to give a fixed answer on how long valves last . A rough rule of thumb, is about 1200-1500 hours operation for output valves WITH NO POWER SOAK !!

The above applies to tube valve rectifiers too so do not forget the rectifier  will loose emission. The volts will drop and that adds to the loss of headroom too.

Preamp valves last longer usually around 2000-3000 hours. 

There are exceptions, I heard of one guy who accidentally left his amp on standby ( heaters on) for three weeks when he went away. He came back to a set of totally worn out valves.  They had been in the amp for some time, and 500 hours of continuous running finished them off. Although, the amp was not operating, the heater was making the cathode emit electrons who were all dressed up and nowhere to go. it still exhausted the material.

Amps that receive a lot of mechanical vibration due to transportation can expect loose electrode  ( microphonics) to appear at less than the lifetimes quoted.

So now you know why valves tubes wear out  and what to look out for and decide when to invest in some new valves.  
Check your treble control setting and use that excellent test instrument….your ears !

The good news is that we have extensive stocks of Russian and Slovakian valves from Genalex, JJ Electronic, EHX, Sovtek and tung sol all tested by us in addition to factory testing. Single valves, matched pairs and quads plus complete valve sets





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