Hal Harvey
Well-known member
- Build Rating
This was a long one, 8 months from start to end.
A large part of that was the design process of trying to figure out how to fit everything I wanted into the chassis. My main concerns were protecting the transformers because I intend to ride it hard, and put it away wet. But also a couple of useful, but defeatable mods. Some high points:
I added internal pre-rectifier fuses on the B+ in addition to the mains and post rectifier B+ fuses. UF5408s for the rectifier.
MOVs on the mains to protect the PT from power surges.
MOVs on the OT primary to protect it from spikes.
Failsafe load for the OT.
Quad bias supply so each power tube can be biased on it's own.
Separate elevated heater supply for the preamp tubes.
All non-transformer wiring is Tefzel or Teflon.
TOCOS pots and heavy duty Carlin switches.
Isolated input and output jacks, one true ground, and one ground connection to the chassis.
Defeatable arc-protection and grid-stopper for the cathodyne PI.
A switch to lift the EQ. I also had a switch for some mods to NFB, the EQ lift turned out to be much more in character of the amp (garage-stoner-doom) and one of the mods needed to go.
Slave output gone.
Effect loop send gone.
Effect loop return remains but is relocated to the lower input position on the left. The stock effect loop circuit is crap, but the effect return is useful to bypass the premap to use an external preamp.
I changed it so the 220K resistor on the grid of the input is only active on the lower two throws of the gain switch.
I have 100 feet or so of shielded Teflon wire and figured I'd need to use it in a number of places but really the only wire I had to shield was the NFB wire. The amp is very quiet (when it should be).
The Hilbish chassis is stunning. 14 gauge steel. High quality powder coating. Roll cage is awesome.
The Mercury Magnetic transformers, although pricey AF, are the real deal and worth it.
First electricity was a pucker moment for me but it went smooth. I started with the variac around 30V no tubes to sanity check with voltage measurements, and then moved to a lightbulb limiter with tubes. Bias was easy. Mercury did a good job cloning the transformers. My plate voltage, after biasing, turned out to be the same as what was measured in this early 70s OR120:
Here's the guts. Starting with the power section that is under the main board.
It's done aside from the shell and some additional labeling on the chassis. I won't be building the shell until later this year when all the amp projects I have going are finished and I'll build shells for all of them at once.
It sounds fucking great, all smiles. It's awesome when something actually delivers to the level you anticipated.
Next up are some tube preamps, starting with VH4 (channels 2,3, and 4), and Wizard MTL (full preamp). Thinking SLO and Mesa Boogie Mark I for the next two. Next amp is a Sunn Model T clone, I have the transformers in hand, and a custom chassis is being built
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