Preamp Gain and Breakup Threshold
Increase preamp gain for earlier breakup; we see it on all of the forums and it is a claim of many mods.
The presumption is that your amp will start to break up at a lower volume, but that is not what happens.
Increasing preamp gain can have pros and it certainly has cons for the harp player. If your amp feeds back
between 5 and 8 on your volume control your are good and increasing preamp gain will only create feedback
trouble for you and you will have less volume control range to work with. In the event you typically can turn
your volume control to 9 or higher before feedback then you can benefit from increaseing preamp gain, but
no matter what you do to the preamp your amp will not break up at a lower volume.
To understand what really happens when we increase 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.
The first is easy enough; to overdrive the power tube the preamp output must exceed the bias voltage and drive
the tube into saturation or 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 this just increases
feedback problems. 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 eviroment? Between 5 to 8 and you are good.
The second seems to be hard for some people to grasp, but the bottom line is that the volume control controls the
preamp output level and the number on the volume control provides a reference to the sound level out of the speaker
and the reference will change when you increase your preamp gain.
When you increase the gain in your preamp you will have a corresponding increase in the sound level out of the
speaker, so guess what? you will have to turn down the volume control to bring the sound level back to where it
was before and so now your control is set at 4 instead of 8 but the sound level out of the speaker is the same.
Do you have earlier breakup? No, the sound level out of the speaker is the same at 4 as it was at 8 and all you
have accomplished is a loss of volume control range and increased feedback potential.
Here are a few drawings that may help to explain. In the first we have an amp that provides overdrive, with good
useable volume control range, and minimum feedback. In the second we have increased our preamp gain and lowered
the volume control, notice that the sound level out of the speaker ends up unchanged, this is an important point.
In conclusion I would like to say that you certainly need sufficient preamp gain to provide adequite overdrive for any mic and any venue but too much will decrease your volume control range and greatly increase feedback trouble.