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Input jack proximity question

3/2/1999 11:02 AM
Steve Jones
Input jack proximity question
Last night I located a cause of some of my hum, by moving the shielded cable carrying the input signal from the jack to the preamp tube. There was a bit of slack in the cable and it was arched toward the tube socket. If I bent the cable farther from the socket, it reduced the hum. I cut the cable a tad shorter and moved it as far from the tube as possible, which cut out some of the hum. This made me wonder if some of my hum is caused by the fact that my input jack is located real close to the tube socket (2-3 inches). Does this make any sense at all? If so, do I need to shield it somehow?  
Or is it more likely that by bending the cable away from the socket, that I was minimizing the hum induced by the filament wires?  
Any advice appreciated.  
3/2/1999 12:20 PM

A nice thing about a 12AX7 is that you can run the 12 volt heater at 6 volts because a center tap is provided. The tube manufacturers even recommend running the tube's heater via the center tap when using AC because it helps reduce the hum level inside the tube.  
The (series) 12v connection uses pin connections 4 & 5, which are right next to each other. So when using twisted pair to supply the AC heater current, the wires stay twisted in a hum-cancelling configuration right up to the tube socket. But when you use the 6v center tapped connection, one heater wire goes to pins 4&5 tied together, and the other goes to pin 9, which is all the way around the other side of the socket. Thus, there are short lengths of untwisted AC wiring able to couple hum to any signal wires in the vicinity. Socket wiring layout becomes more critical.  
Keep the heater wires down tight to the chassis, or up and over in a bridge affair like is done on older fender amps (These wires actually end up close to the chassis bottom cover shield stapled to the underside of the cabinet top). Keep any signal wires as far away from the heater wires as practical, and cross paths at right angles when they are in proximity. Also, don't leave too long of an unshielded input lead end near the tube socket. The shield does have to be trimmed so as to not short against one of the tube socket pins, but you can trim it to about 1/4" exposed center lead by also finishing it off with a short piece of heat shrink tubing to seal the end of the shield.  
Are you sure the cable shield is actually grounded in your amp? It can get overlooked if input jacks are insulated from chassis.
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3/2/1999 12:32 PM
Steve Jones

Also, don't leave too long of an  
unshielded input lead end near the tube socket.
Ok, here's what I've done and maybe this is the problem. Since there is a 68K resistor between the hot lead of the jack and the grid, what I did was run shielded cable to an ungrounded lug on a terminal strip mounted about 1" from the socket. Then there is the 68K resistor connected from the lug to the grid pin. This leaves the unshielded resistor lead vulnerable. Sounds like I need to reverse the scheme and have the resistor go from the hot lead on the jack to the lug (mounted as far from the socket as possible), and then shielded cable from the lug to the pin?  
3/2/1999 1:07 PM

Right. your "reverse scheme" is the most practical way to go.
3/2/1999 9:20 PM

"Sounds like I need to reverse the scheme and have the resistor go from the hot lead on the jack to the lug (mounted as far from the socket as possible), and then shielded cable from the lug to the pin?  
yes, sorta... first remove all of that stuff.  
What you should try is soldering the 33K resistor right to the tube socket(pin 2 or 7), with as little lead length as possible and then solder the shielded cable to the other end of the same resistor with as little lead as possible showing. Then slip the heat shrink tubing material over the cable down to the resistor and tube lug and heat it up.  
The go over to the input jack with the other end of the shielded cable and connect the center conductor as you would normally do and also connect the shield to the ungrounded jack's barrel lug.... and then a seperate ground wire from there over to the circuit board where the cathode of that section is grounded.... (sucking air)..... whew!  
Now you have nearly NO unshielded gaps and no ground loops per section either.  
Mission Amps
3/3/1999 7:06 AM
Steve Jones

Thanks Doc and Bruce!  
The reason I used the terminal lug was so the resistor was firmly soldered to something on both ends, but I guess that's not as important as getting rid of the damn hum. I'll take your advice and see what happens.  
Sometimes on these preamp tubes you see the metal shields that cover the tube and mount on the socket. I always assumed that was more for radios. Do those shields make sense for guitar amps?  
3/3/1999 7:10 AM

Thanks for that post, I've been so close to this but never pictured the perfect method... until now... I especially like the shrink tubing over the whole resistor idea, very sanitary!  
... Whit
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