Did I just cook my preamp?

This conversation is going a little nuts. Testing with multiple devices for this is nonsense, except maybe as a learning exercise for new techs.

You should let a tech do all of this instead but if you must, wear insulated shoes and avoid any other points of contact after the device is plugged into the wall. Touch the device ONLY with your meter. Same for the EGC. If you are holding the EGC when you touch an active AC voltage you can be electrocuted. Set up your test area so that you can reach everything, and the EGC is readily accessible for your probe before you plug anything in. If you are squirming and contorting to connect your meter you are likely to make mistakes. Use alligator leads so you can leave the ground attached without moving it.

First make sure you have a good outlet. Use a tester like this one to make sure your ground AND neutral are good. It will also measure your N-E which is important in this case. Yes, an experienced tech can do this all with a good meter but this removes a lot of guesswork.

Plug in one piece of gear at a time, connecting nothing but the power cord. Measure voltage to the EGC using LoZ. Do this from the chassis to EGC and the outer ring of the RCA connectors to the EGC. You should measure practically zero.

If you find a high voltage use a cheater plug and measure the CURRENT from the same points to the EGC. Again, current should be nearly zero, but small leakages around 10 mA are OK. This will tell you the magnitude of the short. I suspect you have a relatively high voltage with medium current (2A or so).

Please note that using a cheater plug is dangerous as it may elevate the chassis voltage. Take extra care after attaching it.

If none of these yield answers, check the voltage on the INSIDE of the RCA. Take care as it may be high especially with tube gear. It’s worth sacrificing a cheap RCA cable to do this easilly. Cut an end off and expose the inner wire for your probe.

If you suspect a tube pre/amp, you may have DC on the inner wire.  That's bad too, but also will need you to select DC on your meter.

 as it turns out as others have pointed out, no ground pin was removed and the amp is by design ungrounded. I agree it’s dangerous. Could you elaborate on what an outlet tester would tell me?

A basic one would tell you if you if the wiring to the outlet was correct and reasonably functional.  For instance, the ground wire may be missing or broken, in which case a ground pin on your gear would be useless.

Now that I think about it, get a more advanced one so you can see the N-E (neutral to earth) voltage.  This will help you see also that the neutral wire is near ground (should be 2V or less). 

How the preamp became so suddenly hazardous is beyond me.

This is exactly why I tell people not to ever remove a ground pin. What happens is, as an example, the amp shorts the chassis to a high voltage, anywhere from imperfect to solidly shorted. That voltage is now on the chassis AND on the signal ground, to some degree. In other words the outer ring of the RCA cable. That current and voltage is now flowing from amp, through IC to other equipment, causing (potentially) a fire on the IC if the current is high enough and damage to other connected equipment.

The problem is complicated by the relatively high resistance of the interconnect ground. They both heat up and prevent the ground pin from forcing a breaker fault, as they should.

What should have happened, normally with a ground pin, is that the amp should have shorted the chassis/ground voltage via the thick ground pin to your outlet, and if the short is strong enough force a breaker or fuse to pop letting you know something happened without an electrocution or fire.

While you are at it, I do recommend you get one of those cheapy ($12) outlet testers to make sure your outlet is also in good shape.

Hey OP,

I just ran upstairs to answer you. I’m not usually an alarmist but it sounds like you have a potentially dangerous situation here.

It seems like 2 things have occurred:

1. Equipment has had a malfunction shorting the power supply to the chassis
2. The equipment’s chassis ground is not functioning either by the deliberate removal of the ground pin or a bad outlet.

I strongly encourage you to take this seriously and resolve this. Potential side effects are electrocution or fire.

What may have happened is that the power supply has become shorted to the chassis, hence the burn you received while attaching interconnects. That should _never_ happen. This happens when a short has developed and the normal path to ground (the ground AC pin) is not working correctly. In addition to potentially lethal shocks a fire can also happen as the current is flowing through your interconnects, which are absolutely not designed to do this.

If you have previously removed a ground pin or used a cheater plug, immediately remove the cheater plug and/or get a properly functioning power cord with the ground pin working on both ends.

Otherwise, find a qualified technician to examine your equipment and/or an electrician to inspect your outlets. You may also want to use a cheap AC outlet tester like this one.

Be extremely cautious and use the breakers to disconnect power at the slightest hint of a shock. Also, of course, keep liquids and wet hands away from your equipment and wear insulated shoes while touching it while plugged in.