Technical info resource for amps:

Vox AC30BM amp modifications: Peter Michalowski version

During 2008 I worked with both Mike Ryde and Peter Michalowski on modifications to their Chinese made Vox AC30BM Brian May amps. Both Mike and Peter are very experienced with the Brian May guitars pedals and amplifiers, and feel that their AC30 BM amps have been much improved soundwise by these mods.

Please click here for Mike Ryde’s website where you can download a pdf file about modification of the AC30 BM to the Dave Petersen BM spec. Mike Ryde’s site is very informative about his experience with the Brian May equipment in general.

In late 2008 Peter Michalowski from Sweden asked his amp tech Ola Lehnert do the mods that I drew up, and together through 3 versions of the mod they experimented to tweak the amp to what Peter thought sounded best. Ola Lehnert is a well respected amp builder and repairer in Sweden.

For Ola’s website see

Click here to read Peter Michalowski’s comments about the BM AC30 mod and to hear his soundclips of the Greg Fryer and Dave Petersen modified BM AC30s.

Below are my schematics of the Peter Michalowski version of the AC30 BM amp mod.

Warning: please bear in mind that this sort of modification voids the warranty of your Vox AC30 BM amp, and that amplifier repairs and modifications should only be carried out by experienced qualified technicians.

Double warning: guitar amp modding can be a fun home hobby if you become experienced in valve electronics, take it VERY seriously and read up bigtime about it, but because of the high voltages involved it can also kill or seriously injure you and its not much fun playing your super hot amp with your super hot guitar when you’re dead! So if you’re interested please ask your amp tech to do this work for you.

BM AC30 schematics: please excuse the rough hand drawings

BM AC30 preamp schematic

BM AC30 EL84 Output stage schematic

Note: screen grid resistors can also be 270 ohm/3 watt metal oxide or 330 ohm/3 watt metal oxide

BM AC30 power rectification schematic

Mike Ryde’s website pdf document gives a very clear view of mods to make.

Vox AC30 TBX standby switch tip:

For a GZ34 rectifier AC30 you really don’t need a standby switch at all. Just leave your standby switch in the ON position permanently, and turn the amp off and on by using the Mains On/Off switch. Your GZ34 will last a LOT longer this way.

Click here to read Lyle Caldwell’s excellent in-depth article about why you don’t need a standby switch on a GZ34 rectifier AC30 at

AC30 suggestion: if you need to mute the AC30 for guitar or pedal changes or whatever, simply pull the guitar lead halfway up out of the black plastic input socket. This will earth the signal going into the first valve thereby muting the amp. And you don’t have to scramble around looking for that guitar lead on a dark stage…


More tech stuff coming soon