Strange Vox AC30 problems

[Jimmy74]

Well after a long break and further work, I'm back here still having the same problem, I replaced all the tube sockets and heater wiring. And I actually made the single ground point for the whole amp as R.G. suggested. I also remembered that R.G. explained that the 3.3Mohm resistor off pin 7 on V12 should be physically as close as possible to that pin so I tried getting it as close as possible but that made things worse. So I'm actually back to scratch....

anyone have any further ideas?

thanks

[Jimmy74]

Just found out something very strange, the whole power cable is shielded..... wonder if that has to do with the hum I'm getting?!? The amp works fine, reverb, tremolo, EQ all works fine and the volume is loud and twangy specially on the brilliant channel, it's just that hum that you can hear when you're not playing through it.

thanks

[R.G.]

Just found out something very strange, the whole power cable is shielded..... wonder if that has to do with the hum I'm getting?!? The amp works fine, reverb, tremolo, EQ all works fine and the volume is loud and twangy specially on the brilliant channel, it's just that hum that you can hear when you're not playing through it.

Bummer, Jimmy. I was hoping that no news was good news. I've about exhausted what I could do without actually laying hands on the amp. I do think that the things you posted about no end bells on the transformer and the shielded power cable are interesting, and possibly related, but I can't come up with exactly what to do about them, other than putting on end bells and tracing out the shield on the cord.

I hope someone else has more.

[Jimmy74]

Well the only original parts left are the PCB's and reverb PTP board, the power switch, power led, most of the wiring, tremolo switch, the chassis and the power cable. I've changed out everything else for new parts and the hum is still there. I did put a bell cover over the OT but as you can guess that made no difference, the new tube sockets don't have the metal center holes so I couldn't solder the that shield junction for V12 but that didn't make much difference either. I'll try changing out the power cable tomorrow and that's really my last luck and I'm at a dead end. Though I did read once somewhere that if ground or heater junctions are placed too close to the PT that can result in hum and interference..... what do you think?

thanks

[Jimmy74]

I changed out the power cable with a modern plastic type one, it made a little difference but the hum is still there. If you come up with any other ideas let me know and I'll try them out.

thanks

[rf7]

Bump! I'm chasing the hum in one of these also.

[rf7]

Could the fact that the chassis is made up of 2 different pieces of metal bolted together have something to do with this hum problem?

The 1st filter cap ground is on the lower metal (Steel) piece, while the preamp star ground and the AC inlet ground are on the upper chassis (aluminum) piece. The two chassis pieces are bolted together with 3 bolts.

The reverb tubes (V11 and V12) and the output tubes are on the lower chassis part and the preamp tubes are mounted on the upper piece. But the ground for the reverb tubes is on the upper chassis section.

It seems like any output from the reverb section that is fed to the preamp causes hum. I went around V12b by tying V12 pin 7 to the PI input V5 pin 2 (disconnecting V12 pin 6 from the coupling cap) and it still hums.

The last guy who worked on this amp, disconnected the reverb section from the amp completely. I can see why he did that.

[Alex R]

AC30 reverb sections in amps from that period always hum somewhat in my experience.

[circlotron]

The buzz in these amps is the input circuit to V12B picking up 120Hz from the HT supply, caused by the bizarre circuit layout in combination with the 3.3M/10pf reverb mixing network

To kill the hum you just need to delete the existing reverb mixing network and its accompanying recovery gain stage (V12B), and instead use the phase inverter as the reverb mixer.

Remove 3M3/10pf network. Take the junction of the preamp output mixing resistors/reverb send coupling cap and connect to input coupling cap of phase inverter pin 2 (after disconnecting that end from V12B plate).

Remove 470k mixing resistor from reverb pot output and tie wiper directly to the other PI input cap on pin 7 (after lifting that end from ground)

V12B input should then be grounded as it’s left floating, or you can simply disconnect V12B and wire it in parallel with V12A to bump up the reverb return gain (keep Ra/Rk values the same). If more is needed you can add a cathode bypass cap to the reverb return stage and/or increase the value of the reverb pot.

I can draw out a schematic if it would be helpful.

[g1]

I can draw out a schematic if it would be helpful.

Please do. We love schematics here.