How to measure voltages?

[calis]

Hi guys.

First, my apologies if this is in the wrong section. Second, my apologies for being such a newb. And, third, my apologies if this is a FAQ and I just haven't found the right info. I've gone looking but haven't really found the answers I need.

What I wanna know is how to measure voltages. I've built two amps and I'm working on a third built now: a 5F1, a 5E3, and the new one is an 18 watter (tremolo version). The first two turned out fine and sound great. No complaints. But I always see people talk about the problems they encounter and they measure the voltages and list them here on the forums.

How do I do that?

I'm aware of the high voltage risks. I know how to discharge the caps. And I've had no problems plugging in and playing on an amp, then turning it off, unplugging it, discharging caps, and going to work on the amp again. Pretty comfortable with that. I'm just a newb with the voltages thing.

So, how do I do it? Do I clip the black lead of my DMM to ground and then put (or clip) the red lead to each of the pins? I mean, I really have no idea what the correct procedure is. Please help me out.

Thanks in advance guys.

[zhyla]

So, yes it's a good idea to clip one probe to ground, that way you only have one hand poking around in your amp. And when I say "good idea" I mean it could save your life.

Is that what you're asking? You've got a DMM so I'm guessing you know how to measure voltage in general.

[MWJB]

When reading DC voltages, yes, black probe to ground, red to the part to the circuit you want to measure. Whilst you're still getting familiar, you might want to have probes with clips/wire grabbers, so that you can make connections when powered down, power up & read the voltage, power down & move onto the next component. This way you just hit the standby/off switch if anything goes awry.

Some parts, like preamp plates can make a disturbingly loud noise when probing, so the clip method saves you panicking and maybe shorting something else.

It's often a good idea to clip to the component on the board that is connected to the tube pin, rather than the pin itself...easy to short preamp pins, but also it doesn't take much carelessness to short a power tube plate to a heater...which can be spectacular (but not in a good way)!

E.g. pin one of a 12AX7, then clip to the plate resistor/coupling cap junction, pins 3 & 8 clip to the cathode resistor/bypass cap.

Octal sockets are a bit beefier & easier to probe direct, but if access is restricted you can sometime probe at the board...unless there are resistors actually on the tube socket.

Reading heater voltages, & assuming that you have a 2 wire twisted pair, it's usually easy enough to probe at the power tube socket, maybe the pilot light if access is good, sometimes the very first preamp tube gives good access, less crowded, especially on BF/SF chassis. Heater AC voltage is read from pin 2 to pin 7 of a EL34/6L6/6V6, or accross the 2 terminals of the pilot, or from pins 4 & 5 (joined) to pin 9 of a 12A#7 preamp tube.

Some old amps just have a single wire "daisy chain" on the heaters, these can be read to ground.

It's a good idea to record dc voltages at pins 3 (plate), 4 (screen), 5 (grid) & 8 (cathode) of a 6V6/6L6/EL34, and pins 1, 6 (plates), 3, 8 (cathodes) of 12A#7 tubes as often these voltages will give a heads up if something isn't working. Cathode biased amps won't have any dc at power tube pin 5 (grid) for 6L6/6V6/EL34, fixed bias amps won't have significant voltage at pin 8 (cathode - just a few tens of mV if a 1ohm cathode resistor is installed). EL84 have a different pin out, which I can never remember, so double check these and any other tubes that you're not familiar with, to see which pin does what.

Start off with the highest scale on the meter.

Buy some bias probes too, they'll pay for themselves in a few uses and save you blowing the mA fuses in your DMM.

[calis]

Thanks to both of you guys! And MWJB. Your post was perfect. Thank you so much.