What is Valve Bias?
Valve bias is a confusing subject for the non-technical. Putting it simply, bias is the ‘idling current’ the valve draws in the absence of an input signal. You can think of it like the tick-over rate of a car. All valves in the amp are biased, but we are only ever concerned with the output valves when we talk about ‘biasing an amp’. If the bias current is set too high, the valves will glow hot (‘red plate’) and will have a much shorter life. Too low a bias current and the valves will last longer but the sound tone will be affected. We aim to set the bias somewhere in the middle – a compromise between long life and good tone. Bias is only an issue when changing valves as even the same make of valve will draw different bias currents.
“How do I Use The Standby Switch on my valve amp?”
Many guitarists do not really understand the correct use of the standby switch (if fitted). This is how to use it correctly. Valves have heaters which make them glow hot. They also have very high voltages across them (400V or more) to make them work. Ideally the valve should be allowed to get to working temperature before the 400V is applied as this will extend the valve life. This is what the standby switch does. In the OFF position, there is no 400V applied to the tubes. In the ON position, the 400V is applied. So to use the standby switch correctly, first turn it OFF, then turn the amp ON. Then wait about a minute (ideally two minutes), then turn the standby switch ON. If you take breaks during playing, you do not need to switch the amp off altogether, just flick the standby switch OFF instead. When you have finished playing, turn the standby switch OFF first, then the power switch and always give the amp time to cool down before you move it. If yo can get into this discipline it will extend the life of your valves enormously.
What is ‘Re-capping’ and do I Need it?
Re-capping means changing various capacitors in your valve amplifier for new ones. The older (more vintage) your amp is, the more essential a recap is. If your amp is under 5 years old it is unlikely to need a recap. At 5-15 years it is possible that the main electrolytics might need changing. Capacitors wear out and get ‘leaky’ and this degrades amplifier performance. Again using a car analogy, a vintage car will require new hoses, tyres and wiper blades from time to time. This in no way detracts from its ‘vintage’ value.
My Valve Amplifier Has ‘Scratchy Pots’ What Can I do?
This is a weakness of almost all amps. Manufacturers use fairly cheap ‘carbon track’ pots which are open to the elements (not sealed) and pick up dirt. They need cleaning. But the problem is that this is far from straightforward for most amps. It usually means completely dismantling the amplifier to get at the pots. Also it’s really important what you use to clean them with. I recommend DeOxit.
What Situations Should I Avoid When Using my Valve Amplifier?
The most important one to avoid is playing your valve amp in fields, marquees and suchlike which need a petrol generator. These things put out highly variable voltages often way in excess of 240V. Many amps are destroyed by being run on generators. If you have no choice, make sure your valve amp is protected with a surge protector, something worth doing whatever the venue.
Where Should I Store my Valve Amp?
The rule of thumb here is easy. If you wouldn’t sleep there yourself, don’t store your valve amp there either!
Is There Any Difference Between Makes of Amplifier Valves?
Some say yes and swear by their favourite valve brand. Others say no, there’s no difference. Our personal view is that you would be unlikely to hear the difference between one make of valve and another. It follows that you should buy a good quality, sensibly priced set of valves.
Do I Need my Valve Amplifier PAT Tested – And What is That?
PAT testing is a quick and cheap safety test. You definitely need it every year if you gig with your amp. If you only use your amp at home, it becomes a matter of personal caution. If you feel happier with a PAT test, it’s best to have one. A PAT test does a few things. Some of which are: it tests the chassis earth to make sure it is solid; it checks the high voltage (‘breakdown’) insulation of your mains cable; it checks the correct fuse value in the plug (this should usually be 3A); it checks the mains plug is wired correctly and safe. The test engineer will fix a green label to the back of your amplifier showing the date it was tested and when it is next due a test.
I Have a Genuine Vintage Amplifier. How can I keep it sounding like new?
Get a technician to:
- Do a complete re-valve (assuming not done recently).
- Do a recap job (assuming not done recently).
- Remove the speaker and replace with a modern one. You should keep the old speaker in case the next buyer wants it for ‘genuine vintage’ value. But an old paper cone vintage speaker is likely to give a very poor tone.
- If it has a valve rectifier, change this to solid state (it does not affect the sound of the amp as it is not in the signal chain).
I Heard That Re-Capping Ruins The ‘Original’ Tone of a valve amp. Is This True?
There is a lot of myth and rumour about this. The first thing to say is that your vintage amp will need recapping at some point anyway, so this is not optional. It is like wanting to run a vintage car with the ‘original’ bald tyres and split hoses forever because you don’t want to sacrifice ‘authenticity’! Secondly, it is highly likely that you would prefer the sound of the re-capped amplifier. Old leaky capacitors degrade amplifier performance (just as old, leaky hoses degrade car performance). Eventually a capacitor will fail and cause a fault on the amplifier. Prevention is far better than cure.
Can I Leave my Valve Amp in Storage For an Extended Period?
Valve amplifiers need to be used at least twice a year otherwise the electrolytic capacitors inside can dry out. Think of it like driving a car. Cars don’t like to be garaged for extended periods, they need to be run. Some of the worst sounding vintage amps are the ones which have been kept ‘showroom new’ and almost never played. Some of the best sounding vintage amps are played regularly.
Turn Off Your Valve Amp Between Sets
Valve amps get very hot and this will give it a chance to cool down. By turn off, I mean completely off, not just ‘standby’.