LGP-30 had 113 tubes (for logic switching? or memory?) of mostly type 5687, which elsewhere had industrial uses and were used in hifi audio (I'm unsure how), but are not appropriate for guitar amplifiers. Apparently they take a lot of plate voltage and get pretty hot. I found this brief writeup[1] on them (with pictures) mildly interesting. They can be readily found on eBay in a wide variety of pricing of various brands with NOS pretty expensive, as usual. And there's a neat page dedicated to LGP-30 here.[2]
I can fill in since I recently dug through the LGP-30 schematics[0] on my current (incomplete) journey of designing my own tube based computer.
Tubes on computers from this era were used almost exclusively as amplifiers, inverting and otherwise. [1]
Inverting amplifiers are super helpful to turn the and/or gates you can build out of diode logic into nand/nor gates which then give you access to all of the other logic functions. But if you only need and/or for a particular circuit diode logic is much cheaper with much higher MTBF. [2]
Non-inverting amplifiers are useful to buffer signals when the required current draw is too high for a single tube/diode to push.
So in the LGP-30, you primarily see them being used as the inverters for the nor gates on it's flip flops, the amplifiers needed for the read/write heads of the drum, and inverters/buffers needed in the general logic. [1]
And FWIW, I don't really see why it wouldn't do a good job within a guitar amp. It wouldn't be a good fit for the final output stage before the output transformer because of it's relatively low heft compared to a beam power tube or something, but it'd be a good fit for an input stage before that. They really don't take that much plate voltage (the LGP-30 runs them with a B+ of 150v). Maybe they're simply too clean of an amplifier and guitar amps like that distortion of tubes that are more prone to voltage sag?
[1] I want to add in two places using [1] that in addition to what I'm saying about amplifiers, diode tubes are also used as rectifiers in the power supply. There'll only be a handful of the in each design, but because they're used in the power supply of pretty much every design they're still ubiquitous even if they don't count for much versus all tube use in these computers.
[2] This use of germanium diodes in logic came pretty late in tube computers though, really in the late 50s at the cusp of transistorized machines. Royal Precision's next computer, the RCP-4000 was basically the same design as the LGP-30 but with tubes swapped out for transistors. Leads to some goofy schematic choices like the negative voltages being at the top of the schematic pages because it lines up better when compared with a tube based design.
The 5687 has a low mu and high transconductance. If you put it on a chart with the 12AX7 and the 12AU7, the 12AU7 would be in the middle. The 5687 has even higher transconductance and a little lower gain.
If it ran hot, part of that was probably the fact that the 6.3 V heaters ran at 900 mA, instead of the 300 mA. This is a full 3x as much power, just for the heater.
You could use it in a guitar amp, but I would probably just use a 12AU7 instead. The 5687 has a different pinout.
> It wouldn't be a good fit for the final output stage before the output transformer because of it's relatively low heft compared to a beam power tube or something, [...]
There are guitar amps out there with less powerful dual triodes used as output. Yes, they're low-power. There are amps out there using a 12AU7 for 500 mW of output power. It seems like such a small amount of power, but it can be loud--loud enough to damage your ears, loud enough to get evicted, not loud enough to keep up with a drummer.
> Leads to some goofy schematic choices like the negative voltages being at the top of the schematic pages because it lines up better when compared with a tube based design.
I'm guessing this is because it was using early germanium transistors which would most likely be PNP. It was a long time before NPN transistors became the "default", because they were easier to make in silicon which came along later.
Probably right. It's actually still normal/correct to draw a PNP circuit between negative rail and ground with the negative supply at the top. Schematics should convey how the engineer thinks about the circuit, and inputs come in the top and left and outputs exit at the bottom/right. Except no one really designs PNP-oriented circuits with negative supply voltages these days because eugh.
I don't know if 5687 are still made, but there are tube manufacturers, Shuagang and Linlai in China, JJ Electronic in Slovak Republic, and Reflektor Corporation in Russia.[1][2] But 5687 are not rare.[3]
[1] http://vinylsavor.blogspot.com/2020/12/tube-of-month-5687.ht...
[2] http://ed-thelen.org/comp-hist/lgp-30.html