Preamp gain and feedback
Excessive preamp gain is the bane of the amplified harp player, as excessive preamp gain will in all cases increase feedback. Guitar amps are designed for the lower input that is generated by the guitar's pickups; for guitars, this gain is necessary. But when a harp player connects a microphone to a guitar amp, the level generated by the mic is much higher and the preamp is overdriven, making feedback much more likely and a muddy tone possible. Feedback is technically an acoustic problem arising when there is coupling between the speaker and microphone. It can be eliminated by relocating the speaker and/or the mic and by decreasing the volume out of the speaker. This solution is not always viable for musicians because they need to be heard and they need to hear themselves.
The odd thing is that if there is not enough gain in the preamp and your amp does not feedback at all, it will most likely not have enough gain to overdrive the tube finals and give us the distortion we want. If your amp feeds back between 6 and 9 on your volume control, you are good. At this point, increasing preamp gain will only create feedback trouble for you, and you will have less volume control range with which to work. However if you typically can turn your volume control to 10, then you can benefit from increasing preamp gain, but no matter what you do to the preamp, a good harp amp will still have some feedback.
To understand what really happens when we study preamp gain, we must first understand what causes power amp distortion and the relationship between the volume control setting and the sound level out of the speaker. This can be covered in two concepts: the first dealing with bias voltage and the second with perceived volume.
The first is easy enough; to overdrive the power tube, the preamp output must exceed the bias voltage, and then drive the tube into saturation, cutoff, or both. Your preamp should have enough gain to overdrive your power tube at a ratio of 2.5 to 1 as compared to the bias voltage. With a bias voltage of 16 volts, you need your preamp to put out a max of about 40 volts peak to peak. This does not have to be exact but close, and more than 40 volts just increases feedback problems. In order to achieve this gain a typical guitar amp will use 12AX7 tubes with a gain of 100x. In the event you do not have test equipment, all you need to do is answer this one question: at what volume setting does your amp feed back in a typical playing environment? If it is between 6 to 9, you are good. If your amp feeds back too early, say around 2 to 4, then you will have a lot of trouble with feedback. When feedback comes in, it will be fast, loud, and difficult to manage. Lowering the threshold will make it easier to manage and less troublesome. There are several ways to accomplish this: lowering the volume control on your mic, tube swaps, adding voltage dividers in your preamp, and/or reconfiguring the preamp circuitry to lower the tube's gain.
The second seems to be hard for some people to grasp. But the bottom line is that the volume controls the preamp output level and the number on the volume control provides a reference to the sound level out of the speaker. The reference will change when you decrease your preamp gain. When you decrease the gain in your preamp, you will have a corresponding decrease in the sound level out of the speaker. So guess what? You will have to turn up the volume control to bring the sound level back to where it was before. So now your control is set at 8 instead of 4, but the sound level out of the speaker is the same; but, feedback is now more manageable because you are operating in the linear range of the volume control where it is not as touchy. This is why tube swaps, from 12AX7 (gain of 100x) to 12AY7 (gain of 20x), for instance reduces feedback. If you cannot or do not want to change preamp tubes a simple resistive pad or some other method of lowering the signal from the microphone will work.