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Re: First question for the new bbs


 :
8/3/1999 3:01 AM
Steve A.
Re: First question for the new bbs
Mark:  
 
 
 
    That is interesting because sometimes I will shield wires that don't conduct the audio signal thinking that they may be radiating crap that can be picked up by the audio circuit. (And I thought I was just getting carried away with shielding everything!)  
 
 
 
    Just make sure that the center conductors inside the shielded cable can handle the voltages and currents of the leads in question- for heater leads I might look into the thick shielded cable used for microphones (something like 18ga or 20ga).  
 
 
 
    Thanks!  
 
 
 
Steve Ahola
 
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8/3/1999 4:06 PM
Gil Ayan

quote:
"Just make sure that the center conductors inside the shielded cable can handle the voltages and currents of the leads in question- for heater leads I might look into the thick shielded cable used for microphones (something like 18ga or 20ga)."
Well, Fender was pushing the envelope with the 18 AWG wire they used in their amps. Given that Marhshalls use EL34s, which draw more filamente current, use 16 AWG (yes, it is thick and awkward unless you use the solid type) for the filamente harness. Of course, if you waire say one set of theiwres, from the transformer, to the power tubes, and another set of wires from the transformer to the preamp tubes, then you can get away with a thinner wire (since no one piece will carry all of the current required).  
 
Also, note that the wiring should be "in-phase" to minimize noise.  
 
Gil
 
8/4/1999 2:58 AM
Steve A.

Gil:  
 
 
 
Also, note that the wiring should be "in-phase" to minimize noise.  
 
 
 
    Many of the BF/SF Fenders would alternate the heater wiring from tube to tube... perhaps hoping that it might have some kind of hum-cancelling effect? If there is any truth to that idea (which I really doubt!), it might make sense in a tweed amp, with the tube stages laid out almost in a straight line, but the BF/SF reverb amp circuits go all over the place in terms of signal polarity from tube socket to tube socket.  
 
 
 
    However, if you switch the preamp tube filaments over to DC it is important that you check the wiring (Torres says to wire the positive (+) to pins 4&5 and the negative (-) to the #9 pin.) With that possibility in mind, it'd probably be best to wire up the heaters "in-phase" just like when you wire up a 6L6 output socket you might as well wire it up so that you could put in EL34 tubes later...  
 
 
 
    Or did you mean something else when you said "in-phase"?  
 
 
 
Thanks!  
 
 
 
Steve Ahola
 
8/3/1999 4:36 AM
Carl Z

Mark;  
 
 
 
That's an interesting comment. I'm not exactly seeing how unshielded heater wires would cause motorboating. I suppose anything is possible though. It would seem that the greater problem would be the rats nest of wires stuffed along the back edge of the chassis  
 
 
 
all those heater wires, opt and pt wires crammed together it's a great recipe for all kinds of problems.  
 
 
 
Regards;  
 
Carl Z  
 
Summit Amps  
 
http://www.geocities.com/CapeCanaveral/Station/1655
 
8/3/1999 12:40 PM
Mark

Carl, It was Don Butler(toneman) that posted this. He couldn't stop the problem, called Marshall in England and was given this tip by a tech there who mentioned this was a fairly frequent problem. I'm doing this from memory. I saved the post somewhere but I'm in the process of moving my shop and well... Mark
 
8/3/1999 4:08 PM
Gil Ayan

quote:
"I'm not exactly seeing how unshielded heater wires would cause motorboating."
 
Well, Carl, it sounds unusual to me, but if Marshall says it's a fairly common problem with their amp's so it must be. What would happen is that the filament windings may radiate soem 60Hz stuff that gets in there somewhere and modulates the audio signal with that low frequency pulse, and hence the motorboarding.  
 
Gil
 
8/3/1999 5:30 PM
Farrow
Marshall heater wire
I just had a '73 Super Bass in here and the filament wires were TOAST! The black wire had bubbled half of its insulation off and was touching (intermittently) the plate pin on the bottom of one of the power tube sockets. I rewired with some 1000V 20A test probe wire and no more problems. Of course, I took great pains to twist and route the wire neatly along the chassis corner. You should always route heater wires in a chassis fold because you get shielding on two sides... BTW along the lines of the earlier post, the Marshall had heavy gauge wire (16AWG?) for the power tube heaters and thinner (20AWG? Maybe 18?) for the preamp tubes.  
 
 
 
Farrow
 

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