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Radio signals in my Marshall


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5/26/2000 4:49 PM
Mark Chatterton
Radio signals in my Marshall
I recently got my Marshall 2204 out of storage, retubed it and installed Trace's TOT mod. While using it this past month, I've been picking up radiostations through it (country music, UGH!!). If my memory is correct, the amp did this before when I used to use it during the hair band days. It's not loud, but is annoying. Is it just a matter of moving some wires or replacing some signal wires?  
The amp sounds great, but I want to kill the radio noise. HELP!!  
 
Mark C  
 
PS - I just picked up a Mangnavox era SVT w/new tubes for $450.00. :)
 
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5/26/2000 7:58 PM
Rick Erickson

quote:
"I've been picking up radiostations through it (country music, UGH!!). If my memory is correct, the amp did this before"
 
 
RF is most likely riding in on the ground lead of the plastic input jacks. Sometimes moving the ground lead for the first stage 12AX7's cathodes to the volume pot and running a separate ground lead from the jacks to the same ground point on the pot will reduce or cure it. If that doesn't work, my preference is to replace the plastic jacks with metal Switchcraft jacks that ground directly to chassis. Part #14B and #12A jacks will do the job in a MV Marshall. I use a shielded wire to the first preamp stage and put the 68K swamper resistor on the tube socket. Another thing I do esp. when modding the gain is to rearrange the wires on the socket so that the first stage connects to pins 1,2 & 3 and the second stage connects to pins 6,7 & 8. This keeps the stage progression moving from one side of the chassis to ther other, always a good idea in a MV amp.  
 
quote:
"PS - I just picked up a Mangnavox era SVT w/new tubes for $450.00. :)"
 
 
Good score!  
RE
 
5/28/2000 1:34 AM
Peter S

This is very rare, but I've seen it a few times.....sometimes a cold solder joint or a bad connection can act as a diode, which then acts as a detector for radio signals.  
PS
 
5/30/2000 11:26 AM
Ian Anderson

I had this with my (now sold) Marshall JTM60. It really used to bug me and could get really intrusive especially if I was recording.  
 
That was an average amp in any case - good riddance to it I say!  
 
... Ian
 

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