17/07/2026
What's on the workbench today?
So, today I overhauled a 1995 VOX AC30 from the "Korg Era". The amp came in for a general service from a customer that bought it used.
We found some pretty "artistic" work from a former tech that changed the first 2 filter caps in the PSU, modded the Bias to "too cold" (yes, that's possible in an AC30) and also tried to cure the notorious hum problems of the Korg-Era AC30s with an added 100uF Cap for the Output Section (nice try, but that's not the problem).
So, we went with the following works to bring this amp back to s***f again:
- Checked the whole Tube Set and replaced the GZ34 Rectifier and one 12AX7
- Gave the amp a nice full set of new Nichicon Electrolytics
- Dialed the Bias to 95% PD with 82Ohm as Cathode Bias Resisitor
- Corrected the wiring of the GZ34
- Upgraded the Screen Grid resistors from 100 Ohm to 1k
- Installed an elevated virtual center tap for the heaters
- Modified the Normal Channel into something useable
- And of course the usual housekeeping
There's so much more you could do to improve a Korg-Era AC30, as they are notorious for their loud hum and noise due to some bad layout decisions of Korg on the PCB, GND-Loops and a generally bad grounding scheme. In this case, I didn't go for it, as it is quite some extensive work (and additional costs for the Customer). The Customer will be using the AC30 in combination with his Fryette Power Station, which is a great help in not just reducing the insane volume of the AC30, but also in reducing the hum & noise of it. In that combination we end up with a pretty convenient noise floor and are good to go. For the moment... 😉
