![]() Check the documentation, ask your dealer, or give us a call - we'll either know or can easily find out. Your amplifier should have a test point for each valve you want to bias, somewhere visible. If you don't have an amp with this feature, you'll need to use a multimeter. 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. 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). So - to bias your valves - there are basically two ways - the easy way and the hard way! Of course if you have bought replacement valves, you will need to bias them. It's not a bad idea to at least check the bias once every three months or so. However, you might want to do it yourself for a few reasons. 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. Simple enough in theory - how do we do this in practice? 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. We also don't want too little - this will lead to distortion, which will sound unpleasant. Not too much - this would lead to loss of impact and clarity, and your valves will wear out quickly. In the case of a valve amplifier, we actually want a small standing electric current to flow through each valve. 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. 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. 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. The word valve is convenient, because a helpful analogy is that of a tap. This level is the grid bias.īiassing in a valve amplifier simply means ensuring the optimal and same grid bias for the valves in your system. By varying how much voltage there is on the grid, the electron flow from the cathode to the plate can be controlled. The grid sits between the cathode and the plate. Once the cathode is hot, electrons can pass from the cathode (which is negatively charged) to the plate, which is positively charged. The heater warms the cathode, which is a negatively charged terminal. The heater is the bit that glows like a lightbulb. ![]()
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