Need advice from a Vox AC30TBX genius

[Chippo]

Hey guys

I was wondering if anyone has some tips for me regarding my AC30TBX (korg era vox - 90's). I've read a tonne of threads about hum and buzz but nothing seems to be specific to my experience

Mine is reletively quiet until i jumper the channels. I plug my guitar into the high input of the normal channel then use a patch lead to go out of the low input of that channel to the high input of the brilliant channel. this causes an pretty annoying buzz. It's not loud enough to make the amp un-usable like this but it would be great to diagnose and cure it. If i link either channel with the vibrato it doesn't mind. Only when the normal and brilliant is linked. I have to figure out what is going on in the circuit as i know this is something people do regularly and I've never had troubles before with AC30's.

Other people seem to just have general hum/buzz so I thought I would ask my specific question.

Thanks so much for any help. I really appreciate it

[HotBluePlates]

I'd have to look up a schematic for your version, but you wouldn't be able to jumper channels in a 1990's AC30 with/without reverb. The Normal and Brilliant channels have a different number of stages, which puts them out of phase and means they'd have a tendency to cancel each other.

Doesn't answer why you get buzz, but suggests even without the buzz you'd get some unsatisfactory results.

[kagliostro]

Is this your amp schematic ?

K

[SoundmasterG]

Maybe it has something to do with the fact that those channels come into different places at the phase inverter? The LTP has three inputs, and each channel uses one, but I think the normal and vibrato channels are in phase and the top boost channel is out of phase with those two?

Greg

[HotBluePlates]

Maybe it has something to do with the fact that those channels come into different places at the phase inverter? The LTP has three inputs, and each channel uses one, but I think the normal and vibrato channels are in phase and the top boost channel is out of phase with those two?

Before the Top Boost circuit was added to the AC-30, the Bright and Normal channels each used a single 12AX7 stage and fed the same side of the phase inverter. The Vibrato channel fed the other side of the inverter; the amp doesn't use a feedback loop so that side of the long-tail inverter wasn't needed for that function.

That would make the Vibrato channel the odd man out, because it was opposite-phase due to feeding the other side of the inverter. It's not 100% clear to me what phase the Vibrato channel will be compared to the Normal/Bright channels, for the old amp. Easy way to find out is feed a test signal to each channel and check them at the inverter with a scope.

Anyway, the later circuit like Kagliostro posted shows that the Top Boost circuit added an extra 12AX7 stage and a cathode follower. Net result is it's now opposite-phase of the Normal channel, so you can't jumper them. Both feed the same side of the phase inverter, just as they did in the previous Bright/Normal channels.

[SoundmasterG]

Maybe it has something to do with the fact that those channels come into different places at the phase inverter? The LTP has three inputs, and each channel uses one, but I think the normal and vibrato channels are in phase and the top boost channel is out of phase with those two?

Before the Top Boost circuit was added to the AC-30, the Bright and Normal channels each used a single 12AX7 stage and fed the same side of the phase inverter. The Vibrato channel fed the other side of the inverter; the amp doesn't use a feedback loop so that side of the long-tail inverter wasn't needed for that function.

That would make the Vibrato channel the odd man out, because it was opposite-phase due to feeding the other side of the inverter. It's not 100% clear to me what phase the Vibrato channel will be compared to the Normal/Bright channels, for the old amp. Easy way to find out is feed a test signal to each channel and check them at the inverter with a scope.

Anyway, the later circuit like Kagliostro posted shows that the Top Boost circuit added an extra 12AX7 stage and a cathode follower. Net result is it's now opposite-phase of the Normal channel, so you can't jumper them. Both feed the same side of the phase inverter, just as they did in the previous Bright/Normal channels.

Well then he should be able to swap the inputs to the phase inverter between the normal and vibrato channels and whichever one is on the opposide of the top boost channel should then be in phase when they are jumpered together right?

Greg