If you have read any of the previous articles from my amplifier series you will have noticed that I am particularly fond of the vintage tone which can be attained from a real valve amplifier. For me, they are the personification of everything rock; the attitude, the raw power, the overdriven grit and the image all rolled into one box. The most important component which goes into the construction of a good tube amplifier is pretty obviously given away in the name; it’s in the vacuum tubes that most of the magic happens.
As mentioned in my previous blog post about the different stages of amplification, a tube amp works with two main stages, the pre amp and the power amp. The pre amp is responsible for the gain and EQ among other things, while the power amp stage adds the power to the sound to prepare it for the loudspeaker. Each has different kinds of vacuum tube which work well for the specific purpose, and the key to a great sounding amp is largely in the choice of tubes.
For the voltage amplification stage of the amplifier (the pre amp), most amplifiers rely on a number of smaller tubes. Some popular tubes are 12AT7, 12AU7 and 12AX7. Depending on which ones are being used, the overall tone of the guitar distortion, voltage headroom and tonal harmonics will all be different.Larger tubes are used in the output stage (power amp), typically 6L6 or EL34. While the sounds from these two popular types of big tube are different, much of the tone and gain characteristics are more dependent on the pre amp stage than anything else. They do however have different tonal characteristics. 6L6 tubes are traditionally for the ‘American’ sound. They offer a broad, well balanced range of frequencies from low bass all of the way up to the highs, and companies like Fender and Mesa Boogie use them almost exclusively because of the fact that they deliver bright cleans and lend themselves well to distortion when supplied with enough gain from the pre amp. The EL34 on the other hand is the typical English Gentleman. First introduced in Great Britain by the Mullard company, this tube is the sound of British rock, favoured by companies like Marshall, Orange and Hi Watt. Tonally, the EL34 is a little less balanced than the 6L6, which means it is sharper sounding – particularly in the high-mid frequencies – than it’s American cousin. An EL34 will have that bite and crunchiness that takes you all of the way back to the days of Led Zeppelin, Pink Floyd, The Rolling Stones, Cream and The Yardbirds.