JMI VOX AC30 Motor Boating on Normal Channel - Help

[guitician]

What is that brown wire that is by the bass pot going to? It looks like it has too much insulation stripped off.

[cluster]

What is that brown wire that is by the bass pot going to? It looks like it has too much insulation stripped off.

Good question. A year ago or so when I was at a different location, I was getting RF AM/FM radio in the top boost channel. The only way I got rid of it was to custom make a shield and place it around the Tone pot (top chassis) to separate it from the 2 wires coming to it and ground the treble pot that you see on the bracket to the ground bus on the chassis. Sounds crazy but it worked and killed the RFs. I'm going to re-test the term channel tubes to make sure I didn't miss anything. Thanks for the help so far. BTW, is the normal channel on a vox know to have ghosting? I might be hearing things but sometimes I think I hear ghosting. Thanks again

[cluster]

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.

[cluster]

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?

[Jazz P Bass]

25uf is the current standard.

22uf is the old standard (ie: no longer manufactured)

The difference is very minimal.

That said, the higher the value, the more bass will be passed along.

[Chuck H]

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.

[The Dude]

25uf is the current standard.

22uf is the old standard (ie: no longer manufactured)...

(for clarity) Isn't that bass ackwards?

[cluster]

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.

thanks so much for your help. i didn't want the amp sitting around for weeks at an amp tech and I'm luck you guys pointed me in the right direction. looks like I have to buy a new cap. I can't seem to solder anything to that stump. I tried sanding it down but still no luck. Too bad.

[cluster]

BTW, does anyone have a schematic for that top boost bracket section? I've looked around but no luck. Thanks again.

[pdf64]

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.

[cluster]

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.

Thanks so much. Will definitely consider that if it persists.

[cluster]

http://bmamps.com/Schematics/vox/Vox_tbmod.pdf

In that schematic I see a 25uF cap at 25volts (not 450v). It also appears as a bypass cap. I guess the 450v is not necessary. Thanks.

[pdf64]

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)

[pdf64]

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?

Electrolytic caps have a tolerance spec on their value typically at least +/-20%; in the old days it was commonly +50-20%.
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.

[Chuck H]

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.

I just want to be sure there's no mistake here. The caps purpose as a filter/decoupling cap as well as it's purpose as a cathode circuit bypass cap have both been used. I think it's a filter cap. In which case the schematic specs 32uf (probably why it was 33uf) and also why there was an oscillation problem when it wasn't grounded. If it were a cathode bypass cap I don't think the builder would have used the spec in both voltage rating and uf value and I don't think that a ground fault would cause oscillation, ergo, connecting the ground shouldn't have stopped the oscillation. Further, the spec for the cathode bypass cap is indeed 25uf and doesn't need to be 450V. And the builder wouldn't have used that voltage rating and as noted any reasonable value and even some unreasonable ones could be used. What I'm not understanding is why this cap in question with the busted lead that caused oscillation is being called the cathode bypass cap and the concern of 22uf vs 25uf has been raised. I'm pretty sure we should be discussing the 33uf/450V filter cap that caused the problem. Where did things get confused here?