On Mon, 29 Dec 2014 16:32:16 -0800, Phil Allison wrote:

**Hi,

what specs does an audio generator need to make it ideal for servicing
guitar amps? I suspect most would say practically any budget priced
bench generator will be fine - but is it ?

Most guitar amps have very high gain, wide range tone controls and built
in treble boost. They also can output plenty of hum, noise and harmonic
distortion all of which is unlike regular audio gear.

So, there are some special needs the generator ought to meet.

1. Ability to adjust the output level to well below 1mV and smoothly up
to several volts.

2. Circuitry not linked to the AC safety ground to avoid hum loops.

3. Have a fixed level output to connect to the external trigger input on
a scope - so the trace remains in synch despite noise, hum and
harmonics.

3. Square wave output for checking tone circuits and amplifier
stability.

4. Frequency from 10Hz to 100kHz, preferably in four overlapping ranges.

5. Amplitude remains steady when frequency setting or range is changed.

6. THD not more than 0.1% with low order harmonics.

If #6 is not met, then those amps that have large amounts of fixed
treble boost (like Mesa Boogie) will amplify the harmonics coming from
the generator far more than the fundamental - resulting in a dirty
looking wave on the scope. Function generators are best avoided.

BTW: use an analogue scope - using DSOs with guitar amp repairs just
makes life hard for yourself.


.... Phil

Do you have a favourite?