Why are multiple power tubes used in guitar amplifiers?

Posted by admin in Saturday, February 14th 2009

Topics: Engineering

frickadella asked:

I’m not an engineer, so can someone explain why you would design an amp with multiple tubes… what does this give you in terms of power, volume, or tone control? I’ve read about tubes being paired or matched, and I would guess pairs work in series then.

Thanks.

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2 Comments

NordicGuru said in February 16th, 2009 at 5:30 am

It’s better to amplify a signal in multiple stages because one large gain stage usually leads to distortion. Multiple stages allow each stage to do something different. Usually an amplifier can be optimized for noise or power, but not both. So, for example, the first stage amplifies the small signal using a low noise amplifier. The (now bigger) signal then passes through one or more power amplifiers to boost the signal further. Because the signal is already larger the noise added by the power amplifiers has less of an effect on the signal and hence, less distortion.

Bob D said in February 16th, 2009 at 9:04 pm

Tubes and transistors have power ratings (maximum operating power). These devices can be made to operate at higher power than an individual tube rating by connecting tubes in parallel.

Depending on the type and class of the tube output state, two tubes may be connected in a “push-pull” circuit configuration; thus, requiring two tubes. There may be pre-amp or driver tubes before the output stage.

Beyond gaining higher output power (louder sound) by paralleling tubes, I don’t think the arrangement will add anything more to tone are overall frequency response—depends on how the circuit is designed, of course.