Valve amplifiers require a little more TLC than transistor amps - the valves themselves age, and can be changed and upgraded. One of the tasks you might need to carry out at certain points in your journey as an owner of a valve amp is to check or set the bias on the amplifier. In this article I'm going to explain a little about why we need to do this, and how to do it practically.
First, let's have a quick refresher on how valves work. A thermionic valve (or as our American cousins like to call them, a vacuum tube) is an electronic component that is used to control the flow of current and to amplify signals in audio applications. There are lots of different sorts of valves, designed for different purposes, but they all have, inside a broadly cylindrical container (which is effectively a vacuum) the following
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A heater
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A cathode
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A plate
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A grid
The heater is the bit that glows like a lightbulb. Its job is to generate heat. The heater warms the cathode, which is a negatively charged terminal. Once the cathode is hot, electrons can pass from the cathode (which is negatively charged) to the plate, which is positively charged. The grid sits between the cathode and the plate. By varying how much voltage there is on the grid, the electron flow from the cathode to the plate can be controlled. This level is the grid bias.
Biassing in a valve amplifier simply means ensuring the optimal and same grid bias for the valves in your system.
The word valve is convenient, because a helpful analogy is that of a tap. Imagine your taps in your kitchen - if you have them screwed right off, you'll have no water, and won't be able to make tea, or do the dishes. And if you have them on full blast, you'll make a terrible splash everywhere when you try to do anything, and you'll waste a lot of water. Under normal circumstances you wouldn't want your tap to drip - you'd just want it off, if you're not actually using the water.
In the case of a valve amplifier, we actually want a small standing electric current to flow through each valve. Not too much - this would lead to loss of impact and clarity, and your valves will wear out quickly. We also don't want too little - this will lead to distortion, which will sound unpleasant. So we need to get it right. We do that by setting the voltage supply to the grid - which is something we can measure and compare, so we can be sure each valve is set correctly.
Simple enough in theory - how do we do this in practice?
Well, when you buy an amp from a respected dealer, it will come already biassed either by the manufacturer, or by your dealer, or both. However, you might want to do it yourself for a few reasons. It's not a bad idea to at least check the bias once every three months or so. Of course if you have bought replacement valves, you will need to bias them. So - to bias your valves - there are basically two ways - the easy way and the hard way!
The easy way
Some amps have a simple bias feature built in - many of our Ming Da amps, tuned by Audio Detail, have a little switch on the side, and a potentiometer (a little screw-like thing that varies the voltage). With the switch set to bias, the VU meter on the front of the amp will instead function as a volt meter, and you can line it up to the right level (typically about 0db on the VU metre) by twiddling the pot. Repeat for each side, and you're done!
The hard way
If you don't have an amp with this feature , you'll need to use a multimeter. Your amplifier should have a test point for each valve you want to bias, somewhere visible. Check the documentation, ask your dealer, or give us a call - we'll either know or can easily find out. For each valve, you need to touch the test point with the positive probe of your multimeter, with the negative probe touching the negative speaker terminal of the amplifier, and measure the voltage.
You want to set your bias to the same level for each valve. The value you want will vary depending on the architecture of your amplifier - again consult the operating manual, or consult your dealer, or, of course, give us a call!
The general procedure to follow is this:
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Power off the amplifer, and anything providing a signal to it (preamp, music source etc).
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Locate each bias pot corresponding to the valves you will be biassing, and turn it all the way to the left, anticlockwise. This will set the bias to the minimum. You don't want this, but it just means you're starting at a known state.
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Set your meter to a level of sensitivity where you can measure of voltage of about 2V - typically this will be around 20VDC.
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Poke your negative probe into the negative post of one of the speaker terminals - a good tip is to unscrew the fastener enough to allow you to poke the probe through the eye of the binding, and then tighten up the terminal again, holding the probe in place. Leave the speaker connected.
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Power on the amplifier, and let it warm up for a while - a minute should be fine, but if you have time to leave it for an hour, that would be even better.
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Insert the positive probe into the test point for the first valve you want to bias. You should see a voltage of zero. Now very slowly turn the pot clockwise, until the right level is reached.
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Repeat this step for all the other valves until they're all at the same level. If you were able to leave the amp for an hour, the valves will be pretty stable by now, so maybe just check it once more. If you left it for a minute, you might need to do this again after five minutes, and then maybe again a bit later too.. Don't get too worried about slight differences - if your readings are within a 10th of a volt, you should be fine.
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Disconnect the meter, and power on the sources, and enjoy your music
I hope you found this helpful - as I mentioned, if you have any questions at all, just get in touch with us - give us a call on 01507 499057, drop us an email at humans@expressiveaudio.com, or check us out on Twitter or Facebook.
If you want to listen to a valve amp, we can arrange a demo for you, and we can also service and repair all valve amps, provide replacement valves, and advise on upgrades and modifications.
We also interviewed Mark Manwaring-White, UK distributor and tuner of Ming Da Valve amps, and builder of the fantastic Audio Detail valve range, which is worth watching if you'd like to learn more.
