Harmonica players hit two problems when they try to amplify themselves through a guitar-type amp. One is howling feedback through the microphone, and the other is the lack of volume that happens when they turn down low enough to cut the feedback.
The big problem with using guitar amps for harp is that they have too much gain. Harp mics put out far more juice than guitar pickups and they don’t need the gain, it just makes them scream.
Guitar amps also have a built-in mid scoop. This was no doubt the result of Leo Fender, whilst chucking his first amps together in a hurry to get down the pub and spend the proceeds, not really trying very hard to get a flat eq, but it has become the standard guitar amp sound and for most people guitars don’t sound right now without the mids taken out (jazz guitarists being, as usual, the main exception). The trouble with that for harp is that the harp really does need the mids to sound right (being mainly made up of mids).
These things I can deal with. We can cut the gain by means of circuit changes and/or valve swaps, and we can level out the eq. Good candidates for this sort of conversion are the Fender Blues Junior and, for huge amount of volume, Blues Deville. Most non-high-gain Fender amps of whatever vintage can be converted to a loud, sweet harp amp, usually for less than £100.
But what if it’s five minutes before gig time and you still can’t get a decent sound, because you’re forced to use a guitar amp for harp. What to do?
1. Turn the treble right down and the bass right up, use a long mic lead and cross your fingers.
2. Berate yourself for not reading this earlier.
3. Pull all the output valves but one out of the guitarist’s amp. Watch your fingers now.
4. Give me a call after the gig. Preferably the next day, I do sleep at nights.