bmusic - Tech Tips

Notes on Replacing Power Amplifier Valves
by Bob Charman

In the first of our 'Valve Head' articles, Bob asks the question "Can I just by a set of power amp valves and replace them in my amp?" That question in itself poses many more questions and Bob guides us through them all in 'Notes on Replacing Power Amplifier Valves'.

"Can I just by a set of power amp valves and replace them in my amp?"

Well, it's your amp so you can do what you want, but, if you owned a '63 Mustang, a Corvette, or a tasty Jaguar, would you buy a new set of spark plugs, replace them and not tune the engine? Same thing with valves.

First things first, which of the potentially many valves in the amp are power amplifier valves? They are usually the larger ones, however, if you have a valve rectifier, that may be a similar size and if your amp uses EL84's they are not much bigger than the pre-amp valves.. The common power amp valves are, EL34/6CA7, 6L6, 6V6, EL84, and 6550. These will be in pairs (duets) or fours (quads) or, if you own a Super Six Reverb or you are one of the lucky bass players who own an Ampeg SVT, sixes (sextet). No matter how many there are in your amp ALWAYS change all the power amplifier valves together. Sure, if a valve fails during a gig, replace it for that show, but, as soon as you can, get a complete matched set, from bmusic of course, and replace the lot. But that's not the end, what about the tune up?

Valves are made up of a large number of very small, fragile components that are assembled, and then enclosed in the glass envelope, which is then evacuated. The significance of this is that every valve is an individual. This means that firstly, sets of valves have to be matched so that each one carries an equal amount of the work when they are operating. Then the amplifier must be adjusted to suit the particular characteristics of the set. This is commonly called biasing and should be done every time a set of valves is replaced.

Some of you are probably already nodding off with boredom with because all this technical stuff is of no interest to you. If all you want is THAT SOUND, not the tech crap. In that case, just make sure that when you need valves, you get your friendly local amp technician to put them in and bias the amp for you. You can now skip the rest of this article and jump to the reviews or what's new at bmusic.

However, if you are interested in what makes it all work, read on.

The standard output section, or power amplifier, of a guitar amp uses a technique called "push-pull". Basically, each valve can only produce about 25-30 watts. Since most players want 50-100 watts we have to use more than one valve. They are connected so that each amplifies half of the signal and then these two halves are joined together in the output transformer. It is necessary that the two 'halves' of the signal match up properly when they are joined and this is what is achieved by biasing.

There are two ways to incorrectly bias an amp. Underbiasing, which causes the valves to run hot and shortens their life. This will also cause the amp to lack punch, and it may hum. Or, overbiasing, the amp will produce "crossover distortion". This should not be confused with the nice warm "harmonic distortion" we all love, which comes from driving the valves. Crossover distortion makes the amp sound sick and it will be low on power.

So, how is the amp biased?

NOTE: The adjustment requires exposing the wiring of the amp that is mains powered, 240 volts AC here in Australia and 110 volts AC in the US, there is also the high voltage DC to the valves, which can be up to 450 volts, or even 750 volts on MusicMan® amps. All are potentially lethal and, therefore, should only be worked on by qualified technicians.

On all Marshall's® and most Fender's® the bias can be adjusted by a bias adjust control inside the amp. Other amps such as HiWatt, Mesa Boogie® etc., are fixed biased and the adjustment requires the replacement of a 'bias resistor' with one of the correct value. The adjustment also requires some special test equipment, a multimeter, an oscilloscope, an audio signal generator and a high power dummy load. The amp must be run at about 70-80% of full volume during adjustment, that's why most tech's use a dummy load rather than the speaker, a hundred watt amp running at 80% volume at close quarters while you adjust the bias does not aid the concentration or conserve the hearing. With the amp running and an input signal of 1000Hz, adjust the bias for maximum negative voltage. The amp is now over biased. The bias is now adjusted, while observing the output signal on the oscilloscope, until the crossover distortion, which looks like a flattening of the signal between the half cycles, is removed. If the bias is reduced even more the valves become under biased. This process is difficult to do correctly and requires some experience to achieve the desired result.

Alternatively, there is a device called a Bias Probe, which is plugged in between the valve and the valve socket and connects to a digital multimeter. The amp is then turned on and allowed to run for fifteen minutes to allow the valves and other components to settle. The bias is then adjusted to achieve the optimum cathode current for the particular type of valve. This method also allows a certain amount of variation, so that the 'sound' required by the player can be approximated. Some guitarists like to bias 'hot' which seriously shortens valve life, but does achieve a 'vintage sound, often associated with early Fender and Vox amps. However, in almost every case the amp can be biased within its normal operating range and still achieve the required sound.

It can be seen from the above that this is task that should be left to qualified personnel. Considering that you are going to pay $80-120 per pair, for your valves in Australia, I think that $25-30 to adjust the bias correctly is well worthwhile.

From the land of big crocs, hard rocks and dirty socks keep rocking.

Bob is available for all valve needs, from repairs and servicing to finding you those elusive components. Bob is also a wealth of information and would love to be tested with your curly valve questions. Based in South Australia, Bob can be contacted by clicking HERE.
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