Did I just kill my rectifier?

[pompeiisneaks]

I was doing testing to troubleshoot issues with my AC30 build and was trying to test how many amps were going through different points, I touched the A point turret and it sparked, and then things just seemed to stop working. 

I can test the PT input into the Rectifier, and I get 290V on both poles of the red wires, the yellow wires seem to show 5.3 V I think on one and next to nothing on the other? (Odd).  From there, the only real voltages I can read are the heaters, which go direct from the Transformer, so I'm at a loss, is there something else I could have just burnt up by touching my positive test lead on my Multimeter to point A?

To be clear it is Point A where one side of the choke connects and the first 22uF cap goes. 

/sigh, probably a massive noob mistake :(

How can I test the rectifier to see if it is working?  Other than swap it out?

[eleventeen]

You say you can test for VOLTS after this incident and things seem normal. That's right. A typical power transformer can withstand a serious overload for a brief period of time.

Was your meter set on AMPS when you touched point "A"?

One has to be very careful when testing for amps. To test for amps, the circuit connection has to be broken (unless you have a clamp-on meter or probe that measures amps using magnetic flux and I am assuming you do not) so that the true electron flow can flow through the meter. The meter under that condition (trying to measure amps) is essentially a short. Traditionally, with analog VOMs, this very very often  resulted in meter destruction. Or, a violent meter-needle deflection that bent the needle against the stop. So the point is, you could have applied a near dead short across your power supply. What would blow? Amp fuse, possibly, hopefully. Rectifier tube or the diodes? Sure could. 

Old timers like myself are very resistant to (directly) measuring amps with a meter, even a modern "everything" DVM. Yes, the range is there. Move the probe, twist the knob, go for it. But at root we remember the bad old days when measuring amps was a meter-killer and we just hate doing it.


If your heaters are working and the tranny is still putting out HVAC to the rectifier (nothing surprising about that) but you have no DCV on the rectifier output, then yes, probably you scorched those diodes and they are toast = cheap lesson.
 

[pompeiisneaks]

Yes I was testing Amps, and that is when it went berserk.  It is working fine, now though, and I'm pretty sure everything in the amp is okay, I think what went wonky was my meter, I went and got a new 5AR4 and tested it and was still getting wonky readings, I have a second Multimeter, and started testing voltages, and everything there was fine.  So I was really confused.  So I pulled the fuses, checked them (on the one being weird) and they're fine, so I pulled the 9V battery out reseated it, and it is working fine now????

Maybe just because its a cheapie that caused it to be so wonky but it is working fine now.  I do need to put the old rectifier back in and see if that was just the meter.

Thanks,  I'll be much more careful doing amp testing now, I think I need to read up on safe ampere testing methods.

~Phil

[pompeiisneaks]

Nope I actually did kill it, I swapped it out and tried the old one and its giving me nothing.  The MM though is definitely having an issue, but I found it.  The leads I have use banana connectors to swap out tips.  The alligator clip on the ground wasn't well connected causing all kinds of odd/bad readings.  Once I made extra sure it was connected I've had no issues.  Sadly I think my reading of Amps on that MM is what killed the rectifier.   Thanks for the help. 

Any tips on safely measuring amps? 

[John]

safe ampere testing methods.

Safe method is measure voltage across the first resistor in the B+ supply chain, say the one between your first 2 filter caps. Apply Ohms Law. Probably not as exact as some other methods (that I don't know about lol) but close enough.


I've re-used and re-purposed some old PT's from reel to reels. If they don't get too hot, I figure I'm not drawing too much current. 

[HotBluePlates]

... One has to be very careful when testing for amps. ... The meter under that condition (trying to measure amps) is essentially a short. ... Old timers like myself are very resistant to (directly) measuring amps with a meter ... we remember the bad old days when measuring amps was a meter-killer ...

+3 to all that. We've all learned the hard way that while probing an circuit for voltage measurement is generally forgiving (unless you short 2 points with a single meter probe), the amperes setting is just waiting to kill your circuit or your meter.

[eleventeen]

"Any tips on safely measuring amps? "


Tip #1: NEVER use the "amps" range on the meter! Really, just pretend it does not exist. At least until you get quite a bit more conversant with things. Even then. My meter is a Fluke 77 which I have probably had for 25 years. I've maybe measured amps using it 5 times in that time....in puny little transistor circuits..... and those times, I have cringed and checked and rechecked my thinking as to the connection to measure what I am wanting to measure....but also thinking "is there ANY OTHER way to get the info I am looking for?" Please?! 


Tip #2: You really have to get it straight in your head that measuring amps requires breaking the circuit under test. The meter is then inserted where you cut the connection and THE METER becomes the connection. That implies: THE METER becomes the FUSE. THE METER GROUND WIRE IS NOT NECESSARILY CONNECTED TO GROUND! If it *DOES* remain connected to gnd because you forgot to remove it from gnd because that's where the balck lead lives most of the time, then you are shorting out the ckt under test and while there are situations where that's exactly what you want to do (eg; biasing a tube IF this was your method of biasing the tube) under all other conditions it is probably, almost certainly wrong. And not just "wrong", but dangerous to your meter or circuit or both wrong.


Tip #3: Always try to figure out a way to indirectly measure current. Go ahead and break the circuit but instead of measuring AMPS, insert a small value resistor of known size (eg; ohms) and measure VOLTS across said resistor. Most tube circuits will be completely oblivious to a 1 ohm resistor. They will not know the resistor is there. Measure volts across such a resistor and your current is the voltage reading in amps thanks to Ohms law. That is, .027 volts = .027 amps = 27 milliamps. This is how we measure output tube bias, for the most part, even though arguably we are ignoring screen grid current which we assume is small, thus negligable. We (1) have a strong preference to placing the meter in the cathode portion of the circuit where volts are very small vs in the B+ where you find your +350, +450 volts...AND (2) we have a strong preference to measure volts using the the VOLTS setting of our meter and NOT have current pass thru our meter using the AMPS range.

[sluckey]

As stated, you must break the circuit and insert an ammeter in series with the circuit to measure current. Since you must break the circuit, just put a 1Ω resistor in the circuit and measure the voltage drop across that resistor. This works just as well in the B+ line as it does in the cathode of a power tube.

If your amp has a standby switch to break the B+, then you have a very convenient way to check the total B+ current. Just tack solder a 1Ω resistor across the standby switch terminals and measure the voltage dropped across the switch while the switch contacts are open.

In your specific case involving an AC-30, you should know that the lion's share of the current will flow through the 4 output tubes. That same current must also flow through that 50Ω cathode resistor. Just measure the voltage across that resistor, divide by the value of the resistor, and the results is the current flowing through that resistor. We're talking dollars here. The rest of the amp runs on pennies.

Generally speaking, knowing that when you set your meter up as an ammeter, it acts like a dead short. You would never knowingly put a dead short between chassis and your B+ supply while the amp is on. Think, think, think when you twist that knob to amps! Maybe even put a dummy plug in the amp hole on your meter as a reminder.

[HotBluePlates]

I also said this on another thread, but don't remember where...

That last problem with the amperes setting of a DMM is that where a user almost always turns the selector dial back to volts after making a current measurement (probably because they killed something the last time they used the amperes setting), it is very easy to forget you have to move the probe from the A jack back to the V jack before making their next voltage measurement. Maybe there are some meters that anticipate this error and have protection, but my experience is you'll pop some fuses inside the meter (at least), and your meter will be dead until you swap those.

The moral of all these stories is you better have a good reason to use the amperes setting that absolutely cannot be addressed by using the volts setting and measuring indirectly.

[pompeiisneaks]

Thanks guys, that makes sense, and it cost me 20ish bucks for a rectifier :)  luckily that is all.  I appreciate the help.

~Phil

[DummyLoad]

https://www.youtube.com/watch?v=YYeWdukr3Ik

--pete

 

[pompeiisneaks]

I probably laughed harder than I should have there :)