N5X to AX84 HO build

N5X to AX84 HO build

The conclusion of my living room amp build. My previous post gave some details on an issue I was having with the build: a funny fuzz tone behind notes and chords. I scoped the signal path and tried to work out where it was coming from, but it was proving too difficult and time consuming to track down. This process is detailed here.

It was taking up too much time so I decided to get back to a ‘known good’ circuit: the AX84 High Octane (the lower power version of my other build, the SEL).

I implemented star grounding (complicated slightly by the VVR) with help from the AX84 forum members, detailed here.

UPDATES: Since this post I’ve been using the High Octane a lot at home – it’s been great. Although I did make a couple of tweaks.

  • Reduced the value of R19 (dropping resistor to preamp) to 1k to keep the voltages higher, and then increased R65 to 10k to give a difference in load lines between the 1st two stages and 3rd stage.
  • Altered the grounding of the power tube (the Alex mod) so that it now works… Both the capacitor C11 and resistor R64 need connected to a single point and from there to ground through the output transformer or directly, via switch S6.
  • I thought the amp lacked a bit of bass through my speakers, so I increased the coupling capacitor C56 by adding a 10nF in parallel to the 2.2nF. This seems to work nicely.

That’s it for now, but I’m sure there will be more. I’ve updated the schematic below to reflect these changes.



On the grounding, all unfilled stars connect together at the solid star. The sole signal ground point is by the input jack.

Sound Clips

To be added.


Can be viewed here.