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I'm not sure which amp you're playing thru. but generally the settings you want for a clean sound. check your drive (or Gain ) knob. You want it about half way.and your Channel volume, half way. those are good starts. then let your master volume do the rest. Also, make sure, if you have active pick-ups on your guitar. check the batteries. if they're low, it can cause distortion. if this doesn't resolve, then its internal. could be anything at that point.
Well, the thing to shift between those is more of a marketing ploy than a useful function. The difference from 7 to 15 is ONLY 3 Db... which is about the minimum step one can hear in sound level... The amp might be able to deliver more power at the 15 setting than it can at the 7, but you have to crank it up to do it...
Since you have tried all external possibilities, this will require troubleshooting. If you are not adept in electronics, you best take this to a shop familiar with this model.
If I were troublleshooting this, I would suspect the power amp transistors or connections to them might be bad, but the testing would likely use an oscilloscope to trace the signals in an expedient manner..
One schematic for Crate I found uses darlington transistors TIP142 and TIP147.
You MAY be exceeding the capability of the unit causing the thermal protect of the PA chip to shutdown. A 15 Watt amp is NOT going to make a lot of noise unless you have super-efficient speakers that are say 15 inch ones in decent cabinets. You still will be limited in sound level. Just because the gain control goes to a paticular number doesn't mean the amp can take it !!! Mpost of these use 7 terminal amp chips about the size of a TO220 transistor. Heat sinks are OFTEN very inadequate.
You can reach the limiting IRRESPECTIVE of where the volume control is set when you have a guitar or other source that has HIGH output.
Guitars that have battery in them with internal amplifiers are an example of a high output guitar. Plain inductive pickups may require the volume control to be set at say 70% to reach limiting.
Please NOTE that the volume control is NOT linear, but is an "audio taper".
Limiting means you are probably driving it too hard. Many learn the hard way when speakers and thee power amp fail.
When the cone is driven too far beyond the pole pieces of the magnet, then the IMPEDANCE of the speaker becomes relatively low and can damage the power amp. The limiter tries to prevent this by sensing the current and or voltage and will back off the gain internally. This causes distortion when it happens.
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.
guitar amps are all set to a low Z resistance configuration. wich are made for guitar only. make sure you are using the right cables.
you need sheilded guitar cable not unshielded speaker cable in many cases these cables look exactly the same. look at your cable look for the words speaker or instrument. you need the instrument cable. if you still have that problem. then you may just not be satisfied with the quality of the amp it self.