Output Tubes in a Valve Amplifier

If you own a tube amp, you will be well aware that they are very particular for sound, tone and headroom. Tube overdrive can sound great, but you really need to be aware of how to work the tubes in the best way, depending on the headroom of your guitar amplifiers, to get the right amount of clipping at volumes appropriate for a venue.

Much of the actual power of an amplifier comes from the end stage (or power amp). This is the last psrt of the simplifier which processes the signal from the guitar. Some tube amps – with less wattage – will have only one valve in the end stage, others will have two, but for larger watt amps with four tubes or more, there are some interesting tricks that you can do to alter the overall power output.

Why would you want to lower the power output? Let’s say that you play two gigs a week, one in a restaurant where you play quieter songs to provide background music, and one is in a rowdy bar on a Saturday night. To maintain your tone and levels, do you go all out on buying two separate guitar amplifiers of different amounts of power, or would it be much more convenient (and not to mention more cost effective) to buy just one and adapt it to the circumstances of the show?

 

In an amp with at least four large tubes in the end stage, the two tubes on the right control half of the sound, while the two on the left control the other half. If we think about the tubes being numbered 1-4 from left to right, it is actually possible to remove two tubes and have around half of the power coming out of the end stage. To keep the

 running properly and avoid damaging the coils in the transformer, each part of the sound should have at least one tube to go through. That means that you can remove tubes 1 and 4, 2 and 3, 1 and 3 or 2 and 4 to achieve a stable lower powered amplifier. The difference is around half, so removing two tubes on a 40w amp will bring the power output down roughly to the level of a 20w amplifier.

The advantages of doing such a thing include better overdrive at lower volumes, and a slightly grittier overall tone, as the two remaining power amp tubes are pushed further and respond great.