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Weird ugly distortion in tiny homebrew


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3/12/2003 1:32 PM
Henrik L
Weird ugly distortion in tiny homebrew
Hi all  
I put together a small amp based on a 12ax7 and a el95 for low power. It also has a ez90 rectifier for good looks... : )  
 
Well, it works quite good set very clean (in the preamp) but I want to be able to crank the preamp, and whenever it is hit by a hot signal (such as my Gibson LP with original pickups) it distorts in a very ugly way, grainy, farty bassy behind the normal sound, which becomes fuzzy distortion when cranked. The problem primarily appears with the neck pickup on this guitar, I also have an old Musicmaster with a very low output pu, which sounds quite nice through it, as long as the preamp is not cranked.  
 
Recently I acquired a cheap used oscilloscope to look at this distortion, and as it seems it primarily appears in the second preamp stage. But it behaves strangely, with a 100hz signal it first looks normal, but when the volume is raised the waveform does not clip and cut off, but rather "bends" and flattens around origo, and ends up being totally squashed with very high, sharp peaks. It seems I have created a beast with inverted distortion, any suggestions what might be the cause?  
A schematic of "Myggbett" can be found here: http://w1.316.telia.com/~u31638516/myggbett_1.3.jpg Considering how many different things I have tried to remove the trouble it might not be prefectly up to date, but it's close.  
 
Thanks for looking!
 
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3/14/2003 11:49 AM
Chris ( CMW amps )

Hi Henrik  
 
Altough I don't have experience with the EL95 here are some suggestions :  
- lower the voltage for the ECC83 ( enlarge the resistor in the power-rail )  
- larger cathode-resistor for the 2nd stage and smaller bypass-caps ( all stages )  
 
Don't know about your OT ......  
 
Chris
 
3/15/2003 3:42 PM
Henrik L

Hi Chris, thanks for your reply, I think I actually stumbled on a possible solution yesterday, I had been trying with another OT to see if that could be it (nope), and then I started to check all currents around the el95, and it was very high on the anode (55ma, instead of normal 24), and very low on the cathode. It seems like that current went off into the heater circuit, which was referenced to the cathode instead of ground through two 100ohm resistors. I removed them, and the problem pretty much disappeared (haven't checked on the scope yet).  
Only problem is, now the heater circuit is not referenced, and it's potential against ground is now something like 175 volts! It is also a bit high (7 volts). Any idea why the heater circuit would pull away my cathode current? The PT is from an old taperecorder, which used a lot more power than my tiny circuit. But I'm pretty ok. :)
 

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