Why did that valve suddenly stop working?
I thought it might be useful to look at what the common failures in valves are as they age.
I have had a number of folks ask me why their amp suddenly started blowing fuses. My first recommendation is to replace the fuse , unplug the valve rectifier and see if it still blowing fuses. More often than not, the amp stops blowing fuses when the rectifier is removed. the heater in the valve as it ages and when the valve is near the end of its life the heater sags due to the metal fatigue of all that heating and cooling. I have seen a rectifier working fine in an amp and when you tilt the amp to check the valves the fuse blows. What has happened is that the heater has touched one of the anodes and that is a dead shot. You now have an ex-rectifier ! A similar thing occurs in output valves and they get particularly hot.Some time back, I wrote about rectifiers and you read that article by clicking here
Be wary and do not remove and re-insert a pre-amp valve too many times. Output valves have usually octal bases ( except for the EL84) and this is not an issue for valves with bases. In valves where the pins come directly out of the glass, you are depending on the seal between the glass and the metal pins to keep the vacuum inside. No vacuum = dead valve. With multiple insertions the pins can be moved to a point where you make a small leak so that gas eventually gets into the valve and again it becomes an ex-valve.
Some budget amps use valve sockets where the pins in the socket are totally fixed and they put extra strain on the pins when you fit them. Be very careful inserting valves into this type of socket because there is much more chance of creating a minute gas leak. Good quality sockets have the pins which “float” in the socket so they align with the pins on the valve. There is much less likelihood of failure due to gas leaks with these types of sockets
Valves take a lot of vibration punishment in especially in a combo amp which is close to the speakers. Over time the mechanics of the valve inside can become a bit loose and this shows up as being microphonic. It literally becomes a microphone !
This happened to me this week. When I switched on my amp on there was suddenly feedback as if I was holding a microphone too close to speakers. I gently tapped the valves starting from the input forwards and found a super microphonic ECC83 . It had been in the amp for about 15 months and basically it had rattled itself apart. A quick visit to my storeroom and it was fixed. That is one advantage of selling valves – your choice of instant stock ! A delivery time of 4 seconds .
You might be wondering what causes valves to age and how do you tell? Valves work by the heater , heating up the cathode which is coated in a special material. This coating is designed to emit electrons and that is how the current flows in a valve. The negative electrons are pulled towards the very positive anode. Over time the coating on the cathode basically gets worn out and emits much fewer electrons. This is what is described as a “low emission” valve.
The current flowing in the valve drops. Some amps have auto biasing systems on the output valves to keep cranking up the current as the valve ages through reducing the negative bias voltage. This to my mind is trying to get a quart out of pint pot.
I use my ears as the first test tool with an amp. When valves are ageing you get a combination of the low emission and going microphonic too. What I hear is that a note ends up with a “fizzy” tail and the amp is just not as clean sounding as it should be. This gradual deterioration can catch you out because you just don’t realise that the amp is deteriorating then one day it hits you and you think to yourself, “this amp sounds crap!” . The notes are murky and fizzy and maybe the volume has dropped.
I left the most obvious to last, the valve heater is just like light bulb and the heater can fail next time you switch on. Usually you play a whole gig and everything is fine. Next day you switch the amp on and there is a deafening silence. Its easy to check for a dead heater. Just check all the valves are lighting up when on. You might need to take the pre-amp metal shield covers off by pressing down and anti-clockwise twist. BE CAREFUL THERE IS 500 volts flying around in there and you don’t want that up your right arm or else YOUR “heater” may blow – permanently !
When an amp starts to get into the mode of one valve goes, you replace it and then 3 weeks later another one goes to the great valve factory in the sky, then its time to bite the bullet and replace them all. Don’t forget the valve rectifier.
All is not lost though. When you replace your valves , you now have a spare set of valves, so your amp will still keep running even though you have to put one of the old ones back in. When at a gig its better to have a slightly poorer sound than zero ! And make sure you have a little case which is part of your gigging pack. It is as important as spare strings.
Why did that valve suddenly stop working ?
