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1. Turn the main and monitor volumes all the way down (off). Plug a cd player into channel 9/10 and play a track of music of your liking. With the main/monitor volumes still off, adjust the gain on 9/10 until the clip light turns on, then back it down until the clip light just turns off. Put the channel volume at 12 o' clock.
2. With the music still playing, turn up the mains to the desired listening level first, and then adjust the graphic eq until the music sounds good in the room. Remember or mark the position of the volume control. Do not use the channel eq on 9/10 for music that has been mastered properly, leave the eq flat. Once this step is completed then you have now set the main eq.
3. Repeat the same for monitors. Turn off the main volume and then bring up the monitor main volume to the desired level first, then set eq. Now your monitor eq is set properly. Remember or mark the position of the monitor volume.
4. Set up microphone - plug a mic into channel 1 with volume all the way down. Speak or sing into the microphone and adjust the GAIN until you see the clip light, then back down a litttle on the gain. Put the monitor and main volumes back up to the mark from step 2. Now adjust the volume and monitor send on the mic channel to the desired loudness first before adjusting the mic channel eq. Use subtractive eq method to minimize distortion and feedback. ...i.e. if the mic is bassy then turn down the lows, do not ADD highs. If the mic needs bass, turn down the highs.
This is only a guess on my part, but if the potentiometer you replaced is part of the feedback loop and either of the outside legs are open, the circuit will be infinite gain and have a parasitic oscillation you describe; many gain circuits in pro gear are configured this way. Also, since you did replace the pot, it would be easy for you to check this first to make sure these connections are secure as it may not be a co-incidence, but still related to the original fix. Hope that helps.
Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.
There is not enough info here to provide a complete answer, but let me give you some background:
There will always be some hiss. This is due to general amplification where a small amount of noise is amplified along with the general signal. In most cases, the signal is strong enough to overpower any noise that is present (vocals in your example). This noise should not be that noticable in normal cases. If there is an automatic gain control in the line, this could account for it as with "silent" times, the gain will automatically be increased potentially to the degree where the noise is noticable.
Barring that scenerio, if the input signal is too low, the overall gain necessary to produce reasonable volume at the speakers will also be such that the noise is noticable. In order to track this down, please provide details about the current setup including:
1) Microphone make and model being used
2) Input being used on the mixer
3) Gain level being used for the mic channel
4) Gain level being used at the master level.
You can cut the treble gain some BUT most importantly is to send adequate signal to this speaker so volume level does not have to be set high when sending LINE LEVEL signals to it. Hissing noise is a product of the statistical noise that occurs in electronics. Running any device at high gain invariably brings up the hiss level. Managing thee signal levels is the responsibility of the sound engineer.
ALSO feed ALL interconnected equipment from the same receptacle or power source, even if it means running a power cable to a mixer alongside the snake. ALWAYS USE BALANCED audio cables whenever possible.
Sounds like your master volume is turned up too much. The channel volume will make the squealing go away(caused by high gain and humbuckers). The best thing to do is dial in your favorite presets and change the channel volume to the correct level with the master volume where you usually keep it. I use my Spider Valve at master volume at about noon and the channel volume between nine o'clock and noon. Let me know if this does not solve the problem......
There are 2 channels, Which means there are 2 sets of knobs ... One for clean and one for crunch. Start out with a low master volume and set up the crunch, Then the clen and switch between the 2 and make adjustments until they match volume wise ... Then turn the master up.