An update: Got a bunch more fuses from Home Depot. Tried only the rectifier and the power tubes. There is arcing inside the rectifier tube (which I know to work fine in an Orange AD15) for a minute or so before the fuse blows. Clearly there is something wrong with the tube rectifier wiring...

Switching to solid state rectifier, the fuse does not blow. However, the amp produces a loud hum regardless of the positions of the controls and the channel plugged into. The hum is louder and bassier on the hi power setting. Must be a grounding issue somewhere...?

I'll be struggling to sort out these issues, so if anybody more experienced has any suggestions, I'd love to hear them!

Oh, one question... Are the control pots supposed to be grounded to the chassis?

Thanks! I'll get one of those current limiters.

Just figured out the problem. Turned out to be really simple! Just a few nearly invisible strands of copper wire were sticking out and shorting pin 3 of the rectifier to pin 2. Now to see if the rest of the circuit seems to work ok.

Whoops, I mean 0 ohms to ground

As the cab I ordered to house this amp finally came, I put on a finishing touch of making the bright cap on the clean channel switchable and was about to install the amp in the cab... However, I noticed that there was a (fairly quiet) low frequency hum when using the tube rectifier in half power mode. I've been emailing back and forth with Nik about this and so far I'm not sure what could be the issue. Here's what's going on:

Amp in full power mode, tube or ss rectifier: no hum, everything seems to work fine

Amp in half power mode, ss rectifier: no hum, everything seems to work fine

Amp in half power mode, tube rectifier: low level low freq hum, is not affected at all by any of the tone, gain, or volume controls

Measure the voltages and things seem to be running at appropriate voltages...

Any ideas on what to look for?

Unfortunately, increased hum is normal for triode mode. But you can treat the symptoms.

Hum, a change of tone (especially a loss of hi-end), and shorter tube life are all unavoidable side-effects of triode mode. I'm surprised you don;t have hum with the SS rectification.

In a dedicated triode setup (hi-fi and such), you would (ideally) take the signal off the screen pins and retain the screen filtering, but that doesn't make for easy pentode/triode switching. If you look at the layout Nik provides, you'll see when you switch to Triode mode you disconnect the screens from their filer cap (wire S1) and connect them to the plates. You've reduced ripple filtering in the circuit.

People tackle this a variety of ways, a simple fix may be increasing the resistor value between the plate and screen, I've read where some people draw B+ voltage from a latter filter in triode mode, but that gets a bit complicated. Do a Google search on triode switching hum and you'll be overwhelmed with people's triode hum problems and suggested solutions.

I add a caveat, I don't know how loud you amp's hum is, it may be abnormal, just know that increased hum is a normal part of triode switching. One issue that may create abnormal hum is if only one tube is successfully connecting the screens to the plates, then the amp will be horribly unbalanced. But from the sounds of it...your amp is working as designed.

Honestly, I've had amps with triode/pentode switching and hated triode mode. Besides hum, everything turns to mush, it gets darker sounding, less focused and the feel gets rel "spongy" (for lack of a better term).

So, its not something I've spent too much time researching.

If you get hum in pentode mode, it may just be normal for the Bad Cat circuit, or it might be a lead dress issue. If you can reduce the hum in pentode mode, it should reduce the hum in triode mode. BUT, typically fixes to tame some aspects of guitar amp design (which is rather primal) can neuter the responsiveness of the amp.