What is that brown wire that is by the bass pot going to? It looks like it has too much insulation stripped off.
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JMI VOX AC30 Motor Boating on Normal Channel - Help
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Originally posted by guitician View PostWhat is that brown wire that is by the bass pot going to? It looks like it has too much insulation stripped off.
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Fix the term problem with a new 12AU7 Can't seem to solder anything on to that broken lead. Solder won't stick Now, can anyone tell me where I can get a 33uF 450VDC cap to replace it. Thanks to everyone for your help. BTW, I just ordered that book, "A Service Engineer's Guide to the Vox AC30 Valve Amplifier". Hope to read up on AC30 circuits so I can trouble shoot these things in the future.
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So I've been looking at different schematics for top boost and that cap value can vary from 22uF to 25uF, etc. Can anyone tell me what would be the best value in there? I take it, the higher the value the more bass cut? is that right? Also, my cap is a 450v cap. Does it have to be that hi for a bypass cap?
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Isn't that the "filter" cap? It will have high DC volts at one end and it WILL block that DC from getting to ground, but only if it can handle the amount trying to get there. It's not really a bypass cap. It's a decoupling cap. It allows the AC on that node to go away (good thing). It would also explain the "ghosting" you've been hearing. No decoupling cap (which is what an ungrounded decoupling cap is) will do that. It seems that decoupling cap shares a node with the normal channel decoupling cap OR a lack of decoupling on the top boost circuit forces it to interact with the normal channel and the normal channel cap isn't decoupling well. And that's what caused your motor boating. At any rate, I called it in post #3 (Woot Woot!). It was an easy one though. Glad it was just a tube for the trem fix. You can get a 22uf, 20uf, 25uf or even a 33uf axial lead cap rated for 450V from almost anywhere. Any of those values would work fine."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by Chuck H View PostIsn't that the "filter" cap? It will have high DC volts at one end and it WILL block that DC from getting to ground, but only if it can handle the amount trying to get there. It's not really a bypass cap. It's a decoupling cap. It allows the AC on that node to go away (good thing). It would also explain the "ghosting" you've been hearing. No decoupling cap (which is what an ungrounded decoupling cap is) will do that. It seems that decoupling cap shares a node with the normal channel decoupling cap OR a lack of decoupling on the top boost circuit forces it to interact with the normal channel and the normal channel cap isn't decoupling well. And that's what caused your motor boating. At any rate, I called it in post #3 (Woot Woot!). It was an easy one though. Glad it was just a tube for the trem fix. You can get a 22uf, 20uf, 25uf or even a 33uf axial lead cap rated for 450V from almost anywhere. Any of those values would work fine.
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http://bmamps.com/Schematics/vox/Vox_tbmod.pdf
Ghosting is more commonly due to high signals on a power amp with an underfiltered B+; a signal heavy in low frequencies, eg the normal channel, may be more prone to it.
AC30 traditionally only have 16uF on the reservoir and screen grid B+ nodes.
Ghosting can be reduced by increasing the value of either / both of them, eg to 22uF, 33uF.My band:- http://www.youtube.com/user/RedwingBand
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Originally posted by pdf64 View Posthttp://bmamps.com/Schematics/vox/Vox_tbmod.pdf
Ghosting is more commonly due to high signals on a power amp with an underfiltered B+; a signal heavy in low frequencies, eg the normal channel, may be more prone to it.
AC30 traditionally only have 16uF on the reservoir and screen grid B+ nodes.
Ghosting can be reduced by increasing the value of either / both of them, eg to 22uF, 33uF.
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Yes there's a 25uF 25V cathode bypass cap, but also a 32uF B+ de-coupling cap; its voltage rating should be somewhat higher than the static VB+ across the reservoir cap.
Best to be ~1.4 x that, in case it's powered up without any el84 in place, so the 450V rating seems good to me (320 x 1.414 = 452)My band:- http://www.youtube.com/user/RedwingBand
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Originally posted by cluster View PostSo I've been looking at different schematics for top boost and that cap value can vary from 22uF to 25uF, etc. Can anyone tell me what would be the best value in there? I take it, the higher the value the more bass cut? is that right? Also, my cap is a 450v cap. Does it have to be that hi for a bypass cap?
So for any conceivable MI application, 22uF IS 25uF.
The cap's application is cathode bypass, it is shorting all signal frequencies to ground, its value is nominal, anything 10-1000uF would be fine, its actual value has almost no practical affect on performance.My band:- http://www.youtube.com/user/RedwingBand
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Originally posted by pdf64 View PostThe cap's application is cathode bypass, it is shorting all signal frequencies to ground, its value is nominal, anything 10-1000uF would be fine, its actual value has almost no practical affect on performance."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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