Frequency response of a guitar amplifier

Hello All Does anybody know, or have measured, the frequency response of any good quality guitar amplifiers. I am not interested in bass guitar amplifiers here.

By frequency response I mean the low & high frequencies where the output power is -3db down on its centre frequency, measured using a dummy load and a signal generator. For example with HI-FI amps this range is typically 20Hz to 20KHz.

Having just measured my home made amp - which is still under construction (my excuse) - I found out why it has very low hum - it cuts off in the low hundreds of Hz.

Any tips or pointers welcomed.

The 'speakers are the obvious limiting factor. Freq. response will be someting like 50Hz to 5KHz (perhaps less). Amplifier frequency response isn't normally quoted - different amps tailor frequency response to create a diferent 'sound'.

Are you talking about the amp itself or the speakers?

From recollection: - Guitar amp response should be 50Hz or so up to about 12kHz or so. Can often be higher (essentially HiFi). - Guitar speaker response should be around 150Hz or so up to about 5-7kHz or so. (Unless you use horns on your guitar amp, which is generally not a great idea.)

(All above are very variable and are subject to correction by the better-informed .)

Summary: speakers are usually the main limiting factor.

Steve http://www.fivetrees.com

Thanks Steve

It was the amp on its own I was asking about. Will get the scope & soldering out again. I was most surprised what a dramatic effect the speaker has on the sound when I changed mine from an Eminience to a Celestion Vintage 30 recently.

Who cares?

Which explains why amps are designed to work with particular speakers. The frequency response of the combination (amp and speaker) is far more important.

Well, yeah, except it sounds like the OP really does have an amp problem (cutting off below a few hundred Hz - it should go lower than that, otherwise the bottom-end response will be, er, absent).

Steve http://www.fivetrees.com

I was unaware that guitar amps are designed to work with particular speakers. Presumably the amplifier design also takes account of the speakers enclosure type (vented/open backed/tuned/..) as well? Is the aim of the amplifier design to compensate for the non - linearity of the speaker in question or what? How would I obtain this information - from the speaker manufacturers?

Perhaps I was a little short in my response. Most amps hum (AC heaters) and many amps have a low frequency roll off (to keep the distortion sounding 'nice'. What, specifically, is the problem?

I have an old (1960 ish) 25 watt valve PA amplifier made by Grampian which I have converted to a guitar amplifier by replacing the pre-amplifier/tone controls circuits with the 'Verberator' by Harmonic Appliances front end. The phase invertor / output valves / OP transformer & PSU remain unchanged. Upon testing the modified amplifier with a signal generator / dummy load / scope I found the lower -3db point was approx 500Hz with the controls flat! There may be a wiring error causing this early cut-off however I realised I did not know what frequency response I should be aiming for - hence the question. The speaker to be used is a Celestion Vintage 30 in a open back cabinet made of 15 mm plywood.