faraji
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Post by faraji on Nov 13, 2008 21:26:02 GMT -5
Hi... I'm looking at the isolation capacitor on the guitar nuts site: and I can't figure out why the cap is between star ground and body ground. The volume knob is connected to body ground already via it's shell / front plate, so doesn't this effectively bypass the cap? Shouldn't the cap be between star ground and the output jack? Thanks!
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Post by newey on Nov 13, 2008 23:26:53 GMT -5
Faraji- Hello and Welcome! The shell of the volume pot is connected to the shielding foil -"body ground", if you must call it that. The "negative" terminal of the vol pot is not connected to the shell of the volume pot (at least it shouldn't be- if the terminal was folded over and soldered to the pot shell originally, you have to undo that to do the star grounding scheme). In the QTB mod, the string/bridge/trem ground wire is attached to the cavity shielding. If your tube amp fails, lethal DC voltage could be placed onto the signal return path- but the risk to you comes from touching the strings/trem/bridge at that exact moment. In theory, the cap isolates the string ground, via the cavity shielding, from the signal path. self-destructing before you do. It needs to be between the star ground and the shielding to do that. The QTB mod does not isolate the other metal parts which may be about your guitar (the jackplate, for instance) so if you happen to be touching one of those at that magic moment . . .BZZZZORRPP!! It would work equally well (or better) at shock prevention if it were, as you suggest, placed between the star ground and the output jack. But then it would be in the signal path, and would not be benign to your sound.
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Post by ashcatlt on Nov 14, 2008 0:31:40 GMT -5
Newey is correct about this. Conventional wisdom tells us not to put the cap in the signal path between pickup and output in order to avoid loss of low frequency content. Just for fun, I went and ran some 5spice analysis on just what might be expected to happen, and I came up with some pretty surprising results. First, here's the circuit. I'm using a pot to bypass the isolation cap because I can't seem to find a switch in this program. The value of the pot is large enough to where it's not affecting the results any. Could, in fact, be much smaller and accomplish the same thing. Now, here's the frequency analysis across the "audible spectrum" as the knob is turned in 2 steps from no cap to all cap. The red line represents the signal running directly through cap. The black line, which can't be seen very well because it hides behind the red, is a normal guitar with no isolation cap. Here's a zoom in around the area of the fundamentals of the low strings. Again, red line is with the cap. Notice that the difference is quite a bit less than 1db, which is supposed to be the smallest volume difference which the human brain can percieve. And here's a zoom into the region of the actual cutoff for this particular hi-pass filter. Seems to be down almost 3db at 0.5Hz. Have I done something horribly wrong here, or is this issue of tone loss from the isolation cap completely overstated?
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faraji
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Post by faraji on Nov 14, 2008 2:01:31 GMT -5
Thanks guys, this is extremely helpful. Good points on both accounts, particularly that spice simulation! I think I understand - the mod decouples the front hardware from the jack for DC. If the guitar encounters a bad amp, the cap effectively blows open? The jackplate would still have dc voltage across it. If I were to put a similar-sized cap in series from the jack , it would decouple the guitar as well, but steal a bit of tone. Do Les Paul dual humbuckers require caps on both volume pots? Also, what happens when it blows? Your guitar pops and smokes? Is that the signal to repair the amp? And I assume: turn the amp off before reaching down and unplugging the guitar!
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Post by newey on Nov 14, 2008 6:52:45 GMT -5
The cap is not "on the volume pot", it is merely contacting the shielding through a ring connector slipped over the pot shaft. The pot is merely a convenient way to hold it in place. It could just as easily be on the shaft of one of the tone pots, or screwed directly to the cavity shielding. In a Strat, putting it on one of the pots, as opposed to directly into the cavity shielding, allows one to remove the pickguard without having to disconnect that additional wire.
The QTB mod has to be further modded for an LP style guitar.. If it's a 2 HB LP, probably doesn't need shielding anyway. An LP with P-90s might need shielding. Either way, only one cap is required.
If you experience this type of spectacular tube amp failure, the breaker on your house's (or venue's) AC circuit should blow in short order, assuming the breaker works properly. The cap simply provides a few milliseconds of safety until the breaker trips out. You would still get a bit of shock through the cap, but hopefully not a lethal voltage. Whether the breaker trips before the cap expires is a matter of conjecture.
Your amp is probably beyond repair, in the sense that the extensive repairs needed would probably exceed the value. Unless it's a classic something or other, like a '59 Bassman, in which case only your pocketbook can be the judge.
And Ash-
Nice work! I don't see anything obviously awry in your model, except that I don't understand how adding a 150MΩ pot is a transparent change. Couldn't you just as easily have done 2 separate sims, one with the cap and one without?
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Post by ashcatlt on Nov 14, 2008 14:28:07 GMT -5
Well, I could have, but then I wouldn't have gotten two lines on each graph, and I think it would have been messy. The 150M is big enough that its contribution to the load on the pickups is minimimal. In this case it really is pretty much just a switch.
2 x 500K pots in parallel = 250K effective resistance. With the 1M amp imput it comes down to 200K. With the 150M "switch" it is 199.73K.
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faraji
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Post by faraji on Nov 15, 2008 4:25:15 GMT -5
Thanks again. You guys have been a huge help!
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