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Vox AC 30 How quiet can it be ?

12/5/2004 3:15 AM
Alf Vox AC 30 How quiet can it be ?
I have a question , how much hum does a perfectly good Vox have ? Can it be completely silent ?  
The topboost channel is very quiet but the vibrato and normal channel give some hum when the volumes are turned up.  
My customer uses all channels chained together. I already put the grid resistor from the normal channel directly on the tube. Would it be benificial to do this with all the channels ?  
Or does it always hum a bit ?  
Regards Alf
12/5/2004 3:56 PM

you may encounter tone changes if the hum is reduced or eliminated. i say keep it.  
12/5/2004 6:17 PM
Greg Simon
Because the Vox AC30 doesn't have a feedback loop, it will hum more than most amps that do. Examples would be pretty much any blackface Fender, or a Marshall. The AC30 will hum more than those amps. If it is a vintage AC30, there were some wiring mistakes that can be fixed. They grounded one end of the filament string for example. Changing that around to an elevated filament with a couple 100 ohm resistors making an artificial center tap will certainly help. Also, your customer may not be aware, but the top boost channel is out of phase with the othe two channels, so chaining them together doesn't help his tone all that much. Moving the grid resistors to being directly on the tube is a good thing, but won't help the hum rejection as much as using an artificial center tap on the filements and elevating them by about 50v DC. Another useful mod would be to change the random chassis grounding to a star ground or true ground buss as Randall Aiken and others recommend.  
Hope that helps?  
12/6/2004 8:34 AM
Thanks for your useful replies. I'm going to find out if one end of the filament string is grounded.  
It's from 1970 ( date code on speakers).  
Regards Alf
12/6/2004 9:27 AM
Chris @ CMW amps

Hi Alf ( from Eindhoven? )  
Check the grounding 'cause there are some groundloops in that channel.  
Good luck!  
12/6/2004 11:54 AM
Yes Chris , that's me !!  
I found one mustardcap that had been unsoldered once and it had a bad solderconnection but it was covered with heatshrink tube. After I solved that the hum went down to an acceptable level.  
Can you be a bit more specific about the loops ?  
Do you have a layout for the wires under the boards?  
I don't want to take the entire amp apart.  
I do already have a layout for the componentside of the boards.  
12/6/2004 1:06 PM
Chris @ CMW amps

Hoi ( Hi ) Alf!  
I don't have diagrams but rerouting some ground-wires/connections does help.  
Check if some groundpoints from the vibratochannel are connected to other groundpoints.  
Otherwise email me at  
greetings from 3mountains :)  
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