Troubleshooting Tube Sockets.

AFAIK Loctals were primarily developed  for rugged conditions where the tubes could fall out of their sockets. (Military, aircraft, boats, etc.)

Many pieces of surplus gear had them in it.  Zenith and Philco used them in large quantities, as did some of the early TV mfgr's.
Then Motorola and others used them in commercial 2 way radio gear.

They were cheap after WW2 and surplus were bought up and used in conjunction with Octal and miniature tubes. Someplace here I actually have a table radio that uses Octals, Loctals, and miniature tubes. I would think it's pretty rare as a collectors item.

One HUGE drawback to Loctals was if the set got really damp or wet, often the Loctals  center post and pins would rust or corrode themselves into the sockets, and could be a real PITA to get out. Sometimes breaking the el cheapo Bakelite socket in the process.  To note that many "standard" tube types were produced in a Loctal format, and sometimes had different pinouts.

OTOH, One may be able to test Loctals in a tube tester without a Loctal test socket. I have a tube tester with the following instructions: One sets up the Loctal per the test chart but places it in the octal test socket and rotates the  Loctal tube until it contacts the pin sockets in the Octal socket, then holding it in place, test it. Total bullwinkie as a way to test them, but I did try it and it worked.

FrankB wrote:

Now on Loctal sockets, if they don't clean up, the only reliable  remedy I ever found was replacing the socket.

+1 - I sometimes think that Loctals were an all-around bad idea.

What? Troubleshooting tube sockets? Geez , they either have dirty pins or need the pins tightened, that's it, right?

Not exactly. There are numerous faults that happen with tube sockets. Thus I speak from experience.

Pin socket can be dirty. Clean and it may work Ok. Don't necessarily depend on it continue to work though.

Pin socket can be corroded and cause intermittent connections to the tube. Typically it's best to replace the socket. Corrosion can literally disintegrate the tube pin socket contact.

Pin socket becomes "loose" contacting the tube pin.  Two schools of thought on this. Re-tension it. Sometimes this works, sometimes you mess up the pin socket. I prefer replacement.
 I would NEVER try re-tensioning rectifier tube pins. Too much heat from current flow. Just replace pins or socket.

Wafer sockets have a nasty habit of the Bakelite breaking. Replace.

Now comes the "truly fun " one. The tube works when the tube is wiggled or pressed down. Tube tests OK. You turn the chassis over and the voltages are perfect on the tube socket pins. So you clean the socket, re-tension it, and the condition persists. Bad tube? Nope. The tail of the tube pin socket is BROKEN off INSIDE the socket. It looks just fine. Seems to be firmly attached in the socket. But in reality the top of the pin socket that grabs the tube pin is broken off from the solder tail of the tube pin socket. (You can imagine how long it took to figure this out. I finally replaced the socket and dissected it to find the pin broken into two pieces). 

One more tip. When scrapping out equipment you may find unused pins on the tube socket. If I am going to dispose of the chassis & old sockets, I try to carefully remove the unused pins and put them into a drawer. There several types of tube pin sockets and many use different types of tube pin sockets. This has served me well in restoring radios, and several times the tail of the tube pin socket has broken off while unsoldering parts from it. Annoying? Darned tootin' Pard. The thought of spending a large amount of time replacing an entire tube socket sucks. So I check out the type of socket pin that broke off. More often than not, I can use one of my salvaged spare pins to replace the broken one. What a time saver . Total win!  If you are truly blessed by the "Electron Gods", the socket may have an unused pin you can move to where the broken off pin socket was. Literally under 5 minutes to fix, instead of about an hour to replace the entire socket, and risk getting drill shavings into the set, etc.

Now on Loctal sockets, if they don't clean up, the only reliable  remedy I ever found was replacing the socket. On Bakelite wafer sockets, unless they are the very, very early ones where the pin socket is held in by a screw, replacement is needed. One chap I met claimed he had good luck cleaning the pin holes on Loctal sockets with the tiny wire brushes used to clean welding tips.

Do NOT put tubes with corroded pins into sockets. Clean them off with a wire brush and recheck to see the pin is solid. I have seen where corroded tube pins have broken off the tube and either stuck themselves into the tube socket or the heat from the arc of the pin breaking off has welded the pin into the socket.

One final work of caution. I have seen this numerous times. A tube socket had wires resoldered onto it and the "tech" (Idiot) heated the pin on the socket and used so much solder, that solder ran into the pin socket and soldered the tube pin into the socket. (Insert VERY choice words here). You try to remove the tube, not knowing it is soldered into the socket, and either break the tube of you can rip the socket out of the chassis.(Note- This can also happen when a rectifier tube does a " Nuclear meltdown" / massive failure, and the solder in the tube pins runs into the socket and "welds" the pin to the socket)  This happens easily on the miniature Bakelite ones in the AA5 radios and some TV sets. If you have difficulty removing a tube, check the bottom of the socket to see if this has happened. 9Or just remove the tube before soldering to the socket pin).
    The time spent checking will save you an hour or more replacing a socket. Especially considering not all sockets have the same mounting ear spacing, or physical size, plus top of chassis mounting or bottom of chassis mounting.  (Somewhere I bought a box of tubes and 2 of the tubes had the tube socket pin still soldered to the tube pin!). In this day and age, trying to find a tube socket with identical size, mounting spacing, etc, is really difficult.