May 25, 2024...

I'm building a second one.  I've posted some YouTube videos in this playlist though I imagine it's quite boring and I'm not much of a good video maker, but at least it's something.

And btw, there is a source of more of those 6J6A tubes, search in Google for 200 6J6A if you dare!

By ramping the filament voltage with something like the Meanwell SPV-300-48 power supplies and the little SPVCTL circuit it may be OK to use plain 6J6 and not need 6J6A.  Also I've read that an alternative to the 6J6 is the (cheap Russian) 6N15P.

April 25, 2024...

I updated the PCBs to include all my patches and posted them to the github page.  There are two categories for the PCBs:  One category is 'generated' where the partially-routed PCBs are generated from the Verilog-like module files then the routing is completed manually.  There are no schematics as such for the generated PCB files, see the page for details.  These are the main processor circuit boards, ACL (accumulator/link), ALU (arithmetic logic unit), MA (memory address register), PC (program counter), RPI (raspberry pi/leds), SEQ (sequencer).

The other category is 'manual' that have manually entered schematics via Kicad, ie, all the other boards.  The important ones are BACKPLANE and EIGHT (8-tube boards).

Recently (Apr 7, 2024)...

Have been getting maindec stuff going again, by updating the github repo (no hardware changes).

Also did a little with the TC08 dectape emulator, though it doesn't boot OS/8.  Reading on the web about TC08 and dectape, sounds like the drivers do a lot of cycle dependent stuff.  For example, the OS/8 boot block switches the field number in the middle of the transfer.  So I'm not going to put much more effort in it, sounds like too much of a tar-baby for me.  The RK8JE emulator works with OS/8 so that's good enough.

Source code breakdown...

  • asm - pdp8 assembler (allows long labels) and linker
  • cc,ctrl - left-overs from hode, no pdp-8 stuff
  • de0 - out-of-date DE0 FPGA implementation
  • driver - autoboardtest, raspictl programs that run on PC,RasPI to control the tubes
  • filesys - an aborted attempt to write a PDP-8 file system
  • focal - focal-69
  • freqcounter - arduino-based front panel frequency counter (pcb in kicads/freqctr)
  • iowarrior - makefile that downloads and builds IO warrior library - not used
  • kicads - various PCBs
  • maindec - scripts to run maindec programs on simulator and on tubes
  • modules - processor source code
  • netgen - takes modules files and generates PCBs, real Verilog, C++ simulator, TCL simulator
  • scheme - a scheme interpreter
  • silly - silly programs
  • zynq - up-to-date Zynq/ZTurn FPGA implementation

About the tubes...

So back in June or July 2022 I didn't know what tubes to use, I was thinking 12AX7 but they are $10..$20 apiece.  I went to MIT flea market and saw someone with a couple tables of tubes, and looked at what he had.  I noticed a carton of 5 6J6s and wondered what they were.  So I looked on my phone and saw that they were dual triode, common cathode.  I thought I could use them.  So I gave him the $10 he wanted.  I put one on perfboard and did measurements and it seemed to work pretty much as the datasheet said.  Then I had some small circuit boards made that would have one tube and places for the resistors, diodes, capacitors and connectors to make a pair of and-or-invert gates.  When they arrived, I hooked it all up and tried it out, seemed to work ok as gates.  Then I took three of the tubes (6 gates) and made a D-FF and that seemed to work.  IIRC, clocked around 100kHz (ahem kcps).

Meanwhile I wondered how common are the 6J6s?  Looking on the Internet and eventually ebay of course,...

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