So I decided to build a guitar amp for my brother as a birthday present...

I had built lots of Hifi gear but this guitar amp had me scratching my head. I can't even play guitar! Could I actually build an amp that might sound decent and wouldn't blow up? Searching for a seemingly simple design left me wondering if there was something that had more tone control options without being technically way over my head. That's when I found the AX84 site. The Hi-Octane took the basic P1 design and added another gain stage to play with and a cathode follower ahead of the El84 tube. 

I had the chassis drilled by the time his birthday came around, but that was a far cry from having anything that even made noise. It took a long time to tackle the layout and start soldering . I decided to use point to point wiring since that's what I was used to with my audio gear. Everyone else building guitar amps seems to use turret boards, even the big boys. I wonder if there's a simple reason why that might be a better option? I also opted for a tube rectifier (5Y3) in a choke input power supply.

 I never did find the correct values for the pots locally, so the values I ended up using don't agree with the schematic. I was reassured that things would work out fine by one of our engineers where I work, so I plunged ahead. I found some vintage caps for the tone control circuit in the old Tektronix scopes I bought at the surplus store. Carbon comp resistors might help give it a more vintage tone too. It all went together pretty quickly. Then the smoke test...

What's that piercing sound?? Lots of hum too! It took me an entire Saturday sifting through the message board on the AX84 site and moving things around to get rid of the squealing. re-routing the wiring and grounding the secondary of the output transformer proved to be the cure. But I still had some hum. On Sunday I moved the whole ground buss away from the OPT and changed the star ground scheme a little. Finally...no more hum. Nearly dead quiet.

While poking around I had 2 simple ideas. Many people had said they removed a cap on the grid of the second stage because it made things too bright. Also, in the schematic there is a note stating that removing the bypass cap on the third stage cathode will lower the distortion. Why not install switches on these caps so my brother could play around without having to open the thing up? I had no idea what it would sound like anyway. It proved to be a great option that cost nearly nothing.

The amp was actually running when he came down for Christmas...6 months after his birthday! When he plugged in his custom built Les Paul, we realized it was worth the wait. I had mixed sound for many bands over the previous 20 years and this little amp really rocked!  Very diverse tonality, depending on where you set the 2 switches and early gain stages. My brother was shocked that this thing was built from mostly spare parts and sounded so great.

Now the cabinets are nearly finished...with a 2 year old loose in the house, it's hard to pry yourself away to finish up those projects. I just need to finish sanding the speaker cabinet and apply a few coats of poly. The 12" speaker was pulled from an old organ that had been set out for the garbage crew to pick up. There were tubes in that thing so I had to drag it home. This speaker sounds great but I don't know much about it. Very light weight cone with a hole in the center of the center dust cap. There are some numbers on the inside rim and if anyone has any info on this speaker, please drop me line.

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