How to change the valves (or tubes) in your guitar amplifier

Two EL34-B output valves
Two EL34-B output valves

What to find out first - and what to do next

Is your guitar amplifier starting to sound a bit flat and lifeless? Or maybe its already stopped working and you know one of the valves doesn't light up any more... or worse glows cherry red!

The beauty of a valve based guitar amp (apart from that wonderful unique valve sound!) is the fact that the valves can be easily unplugged and changed, so you can try different makes of valve in different positions, experiment with slightly different valve types to change the sound of your amp or even fix a faulty amp if a valve does go down.

What may be putting you off though is not knowing how to do it safely.... valves run at high voltages often around 500 Volts, and dangerous voltages can even lurk inside your amp after its been switched off, as components called capacitors can store these very high voltages for some time. Apart from this, valves get very hot.... not a good idea to grab one with bare fingers after its been on for a while!

Another consideration is that some of the valves in your amp may need to be "matched" in pairs or in fours (quads) for balanced performance with the best sound, and it may be necessary to make an electronic adjustment called "biasing" the output valves to set them up for best performance and long valve life..... at worst, a badly baised set of output valves may last only weeks or even hours if running too hot!

This article is written to give you the information you need to decide whether or not you want to change your own valves, or take your amp to a technician to do the work for you.

So, where do we begin? It's obviously not possible for me to cover every type of amp here, or go into the specific details of how you change the valves and set them up in your amp, but I can go through what you need to know and what you need to do once you know it!

First, you need to decide what you are going to change. Most of the specialist guitar valve suppliers stock complete valve sets for many different types of amplifier, or you can choose to buy the small valves individually and get the larger output valves in a matched pair or quad to give nicely balanced and even operation of the output stage.

The small valves in your amp... and there may be anything from none to five or six of them... handle the small signals, the inputs, preamplification, tone circuits, reverb drive and the drive for the output valves. All these valves work on their own (except driver stages) and they all run at pretty low currents, and you can change any of these singly without worrying about matching.

Some dealers will sell sets with "balanced" driver valves (where the two triodes inside the driver valve have been measured and draw very nearly the same current each), but whereas in a hi-fi amplifier this may give some advantage, it really isn't necessary in a guitar amp, where there are all sorts of tolerence imbalances in the surrounding components anyway and after all, most of the characteristic and much loved sound of your amplifier come from some sort of distortion anyway!

So, feel free to buy the small valves as ones, or just change one or two of them at a time... or buy a complete set. The main thing to consider is that when you change a small valve, you can just change it without having to set anything up afterwards, but if you change the big output valves, you may have to rebias the amp. I say may, because some amps have an auto bias system and no bias controls, and some have small preset controls (potentiometers or pots... these are variable resistors) that may well need tweaking.

I will assume that you have bought a complete set so that we cover everything.... and obviously you will have checked your amp first if you didn't already know to make sure you order the right valves.... many valve amps have different options, particularly in the output stages, so this is well worth checking.

You will find that there are several different manufacturers valves on offer... and any of them with the same main type number (for example "EL34" can be replaced with the Shuguang EL34-B, which has an uprated anode maximum power dissipation for longer life and greater reliability... a simple plug in upgrade!) will work in your amplifier. There are some differences between them though, particularly if they have a different suffix... usually something like "A" or "B" or a couple of other letters after the main part number. Mostly these mean a slightly different gain, noise performance or extended life or working voltage/power range, but you can use any of them in your amplifier.

How different any of these variations or makes will sound is a matter of endless debate which I am NOT going to add to, these things are very subjective and, as usual, one mans meat is another mans poison!

OK, so you know what valves your amp takes, you have bought the valves and you have those lovely little cardboard boxes in your hands... its time to begin.

First things first, the amp should have been left switched off for some time, at least an hour or so to let it cool down entirely and let those capacitors discharge (most capacitors have something called "bleed resistors" across them which harmlessly drain those high stored voltages away within a few minutes, but once you do get inside the amp its best not to touch any more than you have to.... and ALWAYS UNPLUG your amp from the mains supply before you begin! Double check this... especially if your amp is plugged in with a load of other stuff, its all too easy to unplug the wrong thing and plunge your hands inside a live cabinet!

Now, depending on the type of amp you have, access to those valves is more or less easy. Those of you with an open backed combo have an easy life, because those valves are just hanging down exposed inside the back of the amp. Those with sealed back combos will need to undo a few screws to remove the back plate to get at the valves, and if you have a head you may either have a removable cover, or you may need to take some time removing the chassis mounting screws and sliding the whole chassis out of the cabinet. If you do this, try to avoid putting your fingers into the inside of the chassis as you withdraw it.... just in case of stray voltages!

The larger valves may have spring caps on their tops, or spring clips gripping their bases to keep them firmly in their sockets when they are being carted about and sometimes bashed or dropped in and out of vans etc, so you will need to gently unhook or prise open these before you remove the valve.

Lets start with the small valves.... grasp the glass with three fingertips and rock the valve from side to side firmly but slightly as you pull it out of its socket, which should happen fairly easily. Take each valve out one at a time and NOTE CAREFULLY which number goes in which socket! Also look at the pins on the base of the valves and look for where the gap in the circle of pins was in relation to the socket as you can't always see the socket clearly when you replace a valve and they only fit one way around.

There may be a faded and yellowed valve chart stuck to the inside of the cabinet... very useful.. or someone may have written the valve number on the chassis by each socket... but double check what you take from each socket anyway (if the letters are still readable, they can come off with handling and heat over time).

If you can't read one or two of them, its not the end of the world, just Google your amp and find out what goes where.

Output valves are the big ones.... there are most likely two of these for amps of up to about 60 Watts and probably four of them for 100W amps. Beware that your amplifier may (especially vintage amps) have an additional valve rectifier, which doesn't amplify but changes the AC from the HT supply transformer into DC for the amp, and this is likely to be a big valve too. Look out for it (there may not be one and some amps use modern small diodes instead) and leave it alone... unless your valve set included a rectifier valve too.

The output valves can be a little harder to remove, but the same rocking action as you pull firmly on them.... and if you can reach the hard plastic (Bakelite) base to grip that whilst you pull and wiggle then this might be better as old output valves can get a little fragile on their grip with the base.

Again, check which way around the valve was fitted... the sockets are probably going to be large Octal sockets with eight evenly spaced pin sockets on, but note the plastic centre post on the centre of the base has a locating lug, and this needs to match the cutaway on the hole in the centre of the socket when you replace it.

OK... so once the valves are out, open the new small valve boxes and take a look at the pins. Hold the first valve the same way around as the old one came out... or match the gap in the circle of pins with the socket if you can see it... and wiggle and push the new valve into the socket. Note that some vintage amps may have Octal sockets for the preamp valves too, and if so you can rotate the valve slightly either way as you offer it up until you feel it drop slightly into the socket as the lug lines up with the cutout, then just rock and push the valve in firmly. This may take quite a few wiggles and rocking the valve in a slight circular motion whilst pressing firmly is the best way to seat it fully in the socket.

If you don't intend to use a bias meter with plug in Octal probes.... more about that below... then unbox and plug in the output valves in the same way, rock and push until firmly seated. Refit any spring caps and give spring base clips a squeeze to close them up a little before fitting the valve in place to make sure it bites the base and holds it in firmly when upside down.

Make sure that all of the valves are sitting straight and the first part of the job.... all of it if your amp is the self baising type... is finished!

Lets assume that you do need to bias your amplifier though. At this point I must give you a clear warning. Biasing most valve guitar amps involves running the amp with high voltage circuits exposed measuring close to live and potentially lethal voltage points. If you are sure that you can tackle this then do so with care and caution and if not, leave it to a skilled technician!

If you do decide to set up the bias yourself, you will need either a multimeter with a 100mA DC current range, or a 100mV DC Voltage range, depending on your amp circuit, or one of the special dedicated valve bias meters which make setting up the bias of an amp with no special bias measurement points a lot easier. I will describe how to use both.

Using a standard multimeter, digital or analogue, before you begin, you will need to know if your amp already has bias test points fitted. If you are very lucky, your amp will have external bias measurement points on the back of the chassis, and maybe even a rear chassis mounted bias adjustment pot. If that's the case, check whether you need to measure millivolts or milliamps (probably millivolts as these test points often connect back to a low value resistor, often 1 Ohm, which is in the cathode current path of each output valve). Then set your meter to its 100mV DC range (or 250mV is fine) connect your meter to the test point pair, keep your hands well out of the way of the amp, power it up (you can leave any standby switch "Off" if fitted) and wait for the valves to warm up. Now is a good time to just check that they all light up and glow nicely.

If anything crackles, flashes or goes pop at this stage, or the valves don't light, switch off immediately.... its almost impossible to fit the new valves in their sockets wrongly but something is certainly wrong... technician time!

Now switch the standby switch to "On" and you should be rewarded with the usual slight hum/hiss your amplifier makes, as well as a reading on your multimeter. Now careful adjustment of the bias pot with a suitable small insulated screwdriver or trim tool can be used to set the bias to the correct setting. This varies between one type of output valve and another and may be anything between about 25mA (reads as 25mV across a 1 Ohm resistor) and 40mA for larger valves.

The setting will slightly affect the richness or tone of the output, or affect the overdriven distortion sound at very high volumes, and you are free to experiment within that sort of range running the valves colder (lower current) or hotter (higher current). If there is more than one set of bias points and bais pots, measure and adjust these in the same way.

It's a good idea to leave the amp running just sitting there for an hour or so then recheck the bias setting, as new valves do "burn in" and you can expect to see your initial setting drift away and need to readjust it.

Those of you without the luxury of bias test sockets need to get close up and intimate with the inside of a live 500V amplifier chassis. This one is ONLY for experienced electronics buffs and technicians. If you do need to put a hand into the exposed or removed chassis to adjust an internal bias preset pot, then keep your other hand well out of the way and under no circumstances lean on or hold the chassis (or any other possibly earthed metal item nearby) with it as one slip could easily result in an electric shock up one arm, through your heart and down the other arm, possibly fatal.

There are probably internal test points for the bias measurement and a small preset on the board, if there is a board! Worst case is that there are no test points at all, in which case you could start playing with 1 Ohm resistors and a soldering iron, or you could switch to a dedicated bias meter. I am not going to go into any more detail about the 1 Ohm resistor and multimeter method, as I am not going to encourage anyone who doesn't know how to do this already to try it. If you do need to remove the chassis completely to gain access to the bias points or bias pot, do remove the output valves at least first, as amp chassis are very heavy and its all too easy to crunch and smash one of those big valves as you try to heft the chassis out!

Instead, lets look at the use of a valve bias meter. These are lovely little pieces of kit which consist of a little meter in a box (or sometimes a set of sockets to plug into a standard multimeter set to read 100mA/mV as previously) and a lead or set of leads that go from the box to your amp and these have Octal double sided plug and socket type headers on them.

With the power off and unplugged, you remove an output valve, plug the header into the amps chassis valve socket and plug the valve into the top of the header. Hands out of the amp, plug in and warm up, and the valve bias current will be displayed on the meter. Adjust (carefully, watching what your hand is touching) and leave to soak test, then readjust and the job is done. Switch off, unplug, wait for the amp to cool down and discharge, remove the valve and header and plug the valve into its chassis position. Repeat in the other valve positions if there is more than one bias pot, referring to the diagram or instructions for your amp.

Once complete, reassemble any chassis and cover plates you may have had to remove and enjoy your revitalised amp in all its glory!


Valve Bias Meter

Valve Bias Meter with probes
Valve Bias Meter with probes

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Comments 2 comments

akune profile image

akune 5 years ago from Surrey, England, United Kingdom

Useful.


Jellyfishaudio profile image

Jellyfishaudio 5 years ago Author

Glad you found it so... will you be having a go at changing some valves?

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