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A PDP-11 system fits in a 19" rack so is in the capability of a hobbyist or small museum to operate. I agree that mechanical systems are likely the hardest things to keep operating. IIRC, there are hobbyist communities passing around advice for keeping tape drives alive, etc, but tape itself is quite fragile. Availability of software is an issue. I think DEC was pretty good about making the software available, but there are other old computers where either the software, source code, or the media are hard to obtain or can't be licensed.

A typical PDP-11 would have been programmed in a high-level language. The users would have sat at a printing terminal (DECwriter, descendent of a teletype, used accordion-fold "computer paper") or a CRT terminal like the standard VT100. I think they were usually booted off mag tape or disk though apparently, one could boot them from paper tape. Because the OS was good for real-time control (more so than many timesharing OS), they were often used for hardware control; my advisor programmed one in Forth to control the instrument we used.

I doubt the TLAs are passing supercomputers along to surplus in an accessible way; anyway, I think the storage space and power requirements would put a contemporary supercomputer, even civilian ones, out of reach or interest for a hobbyist. For some time, the academic supercomputers I know about have been massively parallel, and I don't know if the individual nodes are particularly interesting from a history or hobbyist point of view.

When a circuit has different output volume with different inputs, often that means that the circuit has relatively low input impedance and is loading the input source.  This usually causes the volume to drop if the source has a high output impedance. 

In the case of the Sea Urchin, it appears to have an input impedance of 180K, the value of R2, the input resistor leading to the inverting op-amp.  (It's not strictly caused by the inverting op-amp itself, but that does lend itself to low impedances, see https://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-input-impedance.php.) This is a bit low compared to say a guitar amp, which would typically have an input impedance of 500K to 1 Megohm.  It could be loading your guitar, since the output impedance of a passive guitar is quite high (also depends on your guitar's volume pot and where you have it set).  So guitar into delay might have a volume drop.

It's possible that you set the volume trimmer based on the guitar into the delay, and then when you put another pedal in front you get the volume back, so it seems like a boost.  If this is the case (and I'm not sure from the information provided), then a high-impedance buffer at the input might help, but you have to be sure that it's a really high impedance.

I have mixed feelings about this. On one hand, I remember an old story from a industrial lab I did an internship in, where one of the secretaries told my friend, "The scientists are ok until they get a PhD, then they get full of themselves." I resolved not to do that. I did get a PhD, but I never expect to be called Dr. something.

On the other hand, I have lots of friends who also have doctorates in scientific research, and the women are constantly being undervalued, condescended to, taken for students, mansplained to about their own work (I dislike that phrase, but it's accurate here), and so on. So more of them have started insisting on using the professional title. However, they want to use it in the professional context, not at the coffee shop. That's where this went off the rails, I think.

I actually used a PDP-11 once upon a time in the mid-90s, but it was an LSI-11, which is the more modern/compact variant that used LSI integrated circuits. Yeppers, the original PDP-11 used no LSI chips, meaning the whole thing was made out of ICs probably about as complex as a CMOS 4xxx chip, discrete resistors, and so on. Of course, by the mid-90s the LSI-11 was a legacy item; they were easy to interface to hardware control, so had been used a lot for lab equipment.

Somewhere I have one or two "Flip Chip" boards and a magnetic core memory board, from a DEC-10, I think, that I was given as a souvenir. You can see a whole lot of these Flip-Chips with their plastic handles at 2:00 in the video. https://en.wikipedia.org/wiki/Flip_Chip_(PDP_module) The number of components on a single flip chip is impressively small; less than a medium complexity pedal.

The Flip Chip boards are plugged into a wire-wrapped backplane, which appears at 3:30.  It looks nuts but was a sensible fabrication method at the time. Wire-wrapping is now nearly extinct, but I remember seeing a few hobbyists build PCs out of it when I was a kid. It's actually incredibly robust. The wire is very thin and has a thin insulation coating, like magnet/pickup wire.

I think wire-wrapping likely made sense when you had a lot of pins that needed to be connected directly from one IC or board connector to another; it's easier to route than a single sided PCB because you can cross wires. (That is, in digital circuits you often see a complex set of connections between a small number of ICs or boards with a lot of pins, while in analog circuits there are more discrete components and the traces reflect the topology of the circuit.) Also, wire-wrap was used for prototyping and hobby builds because BITD PCB fab was complex and expensive.

The Rat is not a bass booster. It's not in the nature of the circuit. The Ruetz mod replacing the 47R with a 1K trimpot is the correct answer to reduce the fizzy quality of the Rat (you will also reduce its gain somewhat, I think), but it won't be a bass booster. You don't need to change any of the caps. The 47R + 2.2 uF cap has a corner frequency of 1500 Hz and because of its location - it's part of the feedback voltage divider for a non-inverting op amp amplifier - it makes the gain of bass frequencies less than treble frequencies.

If you want more control over EQ, you can build an Idiotbox Blowerbox, which is a Rat with a Baxandall/James tonestack, giving you treble and bass tone controls. I did this, but I also still did the Ruetz mod to it.

I'm guessing somnif means the style of passing the wire through the lug in the attached picture, as opposed to bending the wire around the lug (both are acceptable by NASA standards if done right).

NASA soldering standards are on the web, you can look at the inspection standards where I got this picture, http://www.sal.wisc.edu/docs/Soldering%20Basics.pdf,
or the soldering technical standards document https://nepp.nasa.gov/docuploads/06AA01BA-FC7E-4094-AE829CE371A7B05D/NASA-STD-8739.3.pdf. Few of us would literally solder everything to these standards.

Solid state, used: Peavey Studio Chorus 210, Fender Princeton Chorus (210).
I've never had either of these, but they are probably pretty solid choices if you want a clean, solid state stereo chorus amp. I think Peavey and Fender both made a range of sizes when stereo chorus amps were fashionable. I doubt they're as loud as the Rolands, which are legendary for being loud and heavy IIRC. If I had infinite storage space and some talent, I would get a stereo chorus amp, because stereo chorus damnit.

Don't let me stop you from building a chorus, but also, try out the Axl chorus. Chorus is one of those effects where sometimes cheap cheesy versions are actually kind of good. (Effectsdatabase actually thinks the Axl is related to a Starcaster chorus, which is supposedly similar to .. a Boss CE-5. I have no idea if this is true.)

Then of course you'll have to build a madbean chorus for comparison and build quality.

Like one of these with the 10K trimmer used to adjust the collector resistor of Q2 (the part that is an 8.2K in a stock Fuzz Face). Maybe the trimmer is in series with the 4.7K resistor.
https://www.sabrotone.com/?p=1419
http://www.generalguitargadgets.com/effects-projects/fuzz-tones/fuzz-faces/

Series of boards shaped like each state. 50 in all

I absolutely swear the following is true:
Before you posted that, I asked myself what would be an especially weird shape for a PCB, and quite randomly, "Oklahoma" is the answer that popped into my head. 

Alabama is covered: http://1776effects.com/product/3pdt-boards/

If you center punch accurately where the hole is to go, use quality drill bits, not big box store junk, start with a drill bit sized to fit the center punch dent, then creep up on the final size with intermediate sized bits until you reach your step bit minimum, you can drill just as accurately with a hand drill as with a drill press. A drill press is great and i'd never give up mine but it's total overkill for pedal enclosure drilling.

I couldn't live without my center punch. Even though it is a $2.99 Harbor Freight centerpunch, one of my better investments. I can drill ok with a hand drill, but think what I'm really lacking is a table and fixture, more so than the actual press. Especially drilling the sides of enclosures is sort of annoying. I should at least try to make a fixture to hold boxes, with some 2x4 and furniture clamps.

While I love the convenience of board mounted pots, I really don't like committing an enclosure to the effect before I've tested it out.  With my last couple of builds - Dangerzone, Naughtyfish, Megatron, Arcaditator - the best practice has been  to solder the pots while they're mounted to the enclosure for proper fit later.  This means that if an effect doesn't set me on fire, I've drilled out a box for an effect I may not really care for - it seems like a bit of a waste.

When a board has mounted pots but i dont want to commit an entire enclosure what i do is take an old enclosure lid left over from a bad drill job and just drill out what i need on the lid. I have a lid around here somewhere from a 1590BB with 2 or 3 projects drilled out of it.

I also have been suffering with this problem - part of it is that I haven't come to grips with any finishing method and don't want to make a lot of bare boxes with squiggled labels. But also, I am better at building electronic circuits than drilling. I can drill ok, I just find it messy and annoying to do by hand. I don't own a drill press or a vise/fixture, which would improve matters.

Recently I found a sheet of plain FR4 and started using it to experiment with Eurorack front panels for a few modules I am trying to build. Drilling flat panels is easier than enclosures. It just occurred to me that the FR4, or acrylic sheet, etc would make a great fixture for soldering board mounted pots in the way you describe. It's cheap and it's super easy to drill.

via Imgflip Meme Generator

One step to take in debugging is to measure the DC voltage relative to ground at some convenient test points.  Most people measure at each pin of all the transistors and ICs.  Correct voltages will be tabulated in Madbean's excellent build documents. You don't have to worry about being off by small amounts (less than a tenth of a volt), but if a voltage is off by a lot, it usually indicates and helps locate a problem. Often the problem is not with the IC but with components around it, for example an op-amp that isn't getting supply voltage, or if a component in its feedback loop is not soldered.

Measuring the DC voltages helps trace the flow of DC levels through the circuit. After that, the next step is to trace the audio signal path. The audio signal is an AC voltage superimposed on top of the DC voltages. Most people use an "audio probe" to follow the signal through the schematic. The audio probe is basically half of an old guitar cable with a capacitor that blocks DC, and you plug it into an amp to listen for the signal as you touch points in the circuit. It is easy to make and everyone should have one - instructions can be googled from various websites.  The audio probe will typically help you find where a signal disappears, localizing the problem.

I think of the Madbean Eagle part library as a dark, richly carpeted room, sort of like in the Clue board game, with leatherbound volumes and overstuffed upholstered chairs. Probably with Nick Offerman sitting in one of the chairs and sloshing a whisky glass in one hand while he stares contemplatively at a few TL072s in the palm of his other hand. So a tour of that would be great.

Shatner of Muppets.