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previous: Mike Burgundy c´Mon, yáll can do better than that... -- 8/7/1999 12:03 AM view thread

Re: This is really a very cool amp, anyone?

8/7/1999 2:14 PM
Re: This is really a very cool amp, anyone?
Intermodulation distortion is what causes this amp to be muddy. There's a good description of distortion on the LXH2 page- Check it out. Basically when distorting all the harmonics of a guitar equally, the intermodulate each otherand cause alot of noise. The two theories to reduce IM distortion are:  
Boost the high frequencies- distort then cut the high's a bit and EQ the whole mess. This is whatmost modern amps do, and the bass is nice and tight.  
The second way is just the opposite! Cut the treble and distort the bass frequencies. This sounds cool too.  
So here's my recomendation, based on the boost the high end theory:  
Those cathode bypass (polarized electrolytic) capacitors are too BIG! In fact only replace the first one or two, permanently leave out the others (atleast for starters). Popular ,and nice sounding amps (Marshall, Soldano, PV,) run cathode bypass caps for a break frequency of around 86 hz. The guitars low E is about 82 hz, so 86 hz limits the thumps and some pick noise. Gain can be restored through the interstage resistor divider's, or by lowering the value of the cathode resistors slightly... I guess you caould also raise the value of the plate resistor too!  
Next comes the interstage resistor/ capacitor frequency shaping. I personally think that the midrange around 350hz sounds particularly un smooth, so I like to boost the high frequencies above there abit. The same popular Co.'s use between 677 hz and 721 hz often. First try only between the first and second stage, then mod the other's if necessary!  
The amp schematic shows interstage high freq boosting, but I couldn't tell the values... (1/(2*pie*R*C)  
Now the tone control... Alot of the interactivity of the tone control seems to be in the treble. Some say to use a 1M pot and a small capacitor, 47pf-56pf are typicall suggestions. If it is a 250k treble pot., try 220pf to 470 pf. 220pf sounds smooth in the shop but has a hard time cutting through a jam session. 470pf, again to my ears, lets in too many harsh midrange frequencies. My amp has a 360 pf. I think the schematic shows 1nF, no wonder the tone control is slightly unresponsive.  
Ok quik and dirty. If you like Marshall, Soldano, mesa, modern tube distortin sounds, here ya go ( for starters):  
First stage- (cathode resistor*bypass cap)~= 1.8  
for instance 1.8k*1uF= 1.8 .... 2.7*.68=1.836  
Those guys also use the 470pf/ 1M interstage shaping alot too.  
And lower that treble cap in the tone stack and you should be rockin' Good luck! Eth