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JCM 800 pre-volume puzzled

6/8/2004 2:10 PM
JCM 800 pre-volume puzzled
Just completed a JCM 800 clone, sounds great, just one bug.  
With the gain set between 1 and 4, it seems normal, clean to bluesy tone.  
Gain between 4 and 7, sound is very weak, distorted and sound seems to die off abruptly (sounds like when you turn the amp off while your still playing, that weird highly distorted/buzzy dying out sound).  
Gain between 7 and 10 is normal again, loud with good pre-amp overdrive.  
Amp really sounds great except for this mid-range area on gain control.  
Note: You can actually hear this kinda dead area as I sweep the gain control full travel back and forth with no input signal.  
Thanks for any suggestions.
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6/8/2004 2:27 PM
Bruce /Mission Amps

If it's not just a bad pot, ultra high freq oscillations.  
Move wires around to and from the pot and see if it changes the spot where that happens. If it does, you have lead dress issues.  
6/8/2004 4:19 PM

Thanks for the help.  
I noticed strange voltages on v1b cathode during the dead area sweep of gain pot, sure enough, moving the wire from v1b grid to gain pot caused changes in these readings. The fix, simple as re-routing this wire.  
I had checked signal routing for problems on initial power-up but didn't think to check with amp at various settings.  
Thanks again, the best thing to do is live and learn and sometimes ask and receive good help.  

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