KT[XX] – The Big Amp

26 Jan 2009

After finishing the Princeton I realized I was already addicted to working with valves and would need to find another project to sink my teeth into. I wasn’t sure what kind of amp I wanted to build, but I knew I wanted it to be loud and clean. Loud enough to play gigs with without it distorting.

With that idea in the back of my head I kept an eye out on eBay and saw a hefty power transformer about to end with a very low highest bid. From the specs I could see that it would be enough to power some big valves, so I grabbed it.

power transformer

But what on Earth am I going to build with this thing? After plenty of great advice and expert help from the guys at AGGH (especially Mark “Tubeman” Lautizar – thanks Mark!), I settled on a variation of the October Stage from AX84. The October is supposedly voiced like a classic Marshall or ’59 Fender Bassman (of which the first Marshall amps are a direct copy) and has some cool extra features:

  • Parallel “bright” and “dark” preamps with separate gain controls
  • vintage/modern switch
  • Bias trim-pot per output tube – allows mismatched tubes to be used
  • Designed for KT77 tubes, but will work with KT66 or even KT88/6550 if your transformers are suitable

Where my build differs from the design:

  • Solid state rectifier. Higher voltage, higher output, no sag. Better for bass or snappy funk guitar
  • 100W output transformer, capable of either pentode or ultra-linear operation care of switch on the back of chassis (UL = higher headroom, sometimes preferable for bass or jazz guitar)
  • 1000V Paper-in-oil caps throughout the preamp just because I bought them in bulk for less than 400V poly’s cost.

Having decided what I wanted to build, I went through my parts bins and put together a parts list of what was still needed. Mal and Carole at EVATCO sorted me out with everything I needed, including the 5.5kg 100 watt output transformer and the gigantic steel chassis required for this build — 17×12 inches and 3 inches tall.

WIth all the parts acquired or at least decided upon, I drew up a scale layout plan using Inkscape:

october-layoutLayout complete

Having checked all measurements twice, there was nothing left but to just go for it and cut some steel.

drill plan on chassisDrill plan on chassis and ready to go
hardware mounted to chassisTest fit of top hardware
under chassis fit testTest fit of eyelet board. Lots of room to work!
eyelet board and wiresAdding off-board wires to eyelet board

The amp is designed to run KT77 valves in the power amp, which are the larger valves pictured above. But I was interested in something louder, something cleaner, something larger. On Christmas day, while most people were off being sociable I snuck away from a family lunch for long enough to hit eBay and score a matched quad of SED 6550C for only €5, a saving of more than $100.

When they arrived I couldn’t help but marvel at the shiny little monsters. If tubes kick ass, which they most certainly do, then it stands to reasons big tubes kick substantially more ass. I can’t wait to see – and HEAR – these all lit up in the finished amp:

test fit of 6550 power tubesTest fit of 6550 power tubes, with 12AX7 in front for size comparison
6550 cooking at ~70% dissipation

Finished amp


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