• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Testing a tube amp

Hi guys. Trying to chase down a distortion and lack of oomph in a right channel. Switched cables and tubes around so it seems like it’s something in the circuitry. It’s a TU8600 from Elekit. They have places to measure voltage on the circuit board. But uhhhh. When I tried to do that the voltage would just drop and drop slowly to zero. So I’m assuming that I’m making a rookie mistake and the amp should be powered on but I figured I’d ask because that would mean the transformers would be running and uncovered and I don’t want to electrocute myself.
 
So I’m assuming that I’m making a rookie mistake and the amp should be powered on but I figured I’d ask because that would mean the transformers would be running and uncovered and I don’t want to electrocute myself.

Yes, it's pretty difficult to measure the voltages if they aren't present.;)

Only one hand near the amp if it's on. Put your other hand in your back pocket. Use a clip on ground, and test with the pos. lead from your DMM. That way voltage can't travel across your chest, and stop your heart.

jeff
 
On my valve amp designs I always add resistor and LED across main power cap so I can see when amp is fully discharged.


That sounds like a great idea, till one day the LED fails or a connection is loose, and you see the LED isn't lit so assume the cap is discharged when it isn't.



Why not just always discharge with a probe while monitoring the voltage - shortcutting a safety check is asking for a mistake I think.
 
That sounds like a great idea, till one day the LED fails or a connection is loose, and you see the LED isn't lit so assume the cap is discharged when it isn't.

This seems like a reasonably fail safe check since the absence of an LED to start with is also an error condition. So to make it fail safe, always switch on the amplifier, turn off, monitor the LED, then the unit can be assumed discharged.

The risk with individually discharging capaicitors is that it is also intrusive, so has risk.

Another consideration is 'Recovery voltage phenomenon', where high voltage capacitors could theoretically become dangerous again even after having been shorted.
Recovery voltage phenomenon

The safest solution would seem to be to take an unknown unit, add the bleeder resistors (so that is becomes a known unit), calulate the time to bleed to a safe voltage, then monitor the voltage after that time whenever turning off.

I picked up a guitar amplifier that had not been powered up recently, with no fuses and no valves, and gave myself a nasty shock since the tip of my finger just caught the end of a fuse holder underneath the chassis. You have to respect the danger, like it was a circular saw or a nail gun.
 
One thing I recommend is getting a few different meters. Especially when testing an amp for the first time, you can clip all the leads on before powering it up. That way you can power it up from behind a lexan shield.

I tend to do this whenever I need to monitor a lot of voltages at the same time or when the power supply in question scares the S*** out of me. For example, if the rail voltage is 900V with 2800uF of filtering, I don't want to be right next to that thing.
 
That sounds like a great idea, till one day the LED fails or a connection is loose, and you see the LED isn't lit so assume the cap is discharged when it isn't.



Why not just always discharge with a probe while monitoring the voltage - shortcutting a safety check is asking for a mistake I think.

You do have to use some common sense.
Unless you use a silly value resistor the LED should pretty much last forever.
If you turn it off and LED isnt lit then clearly there is a fault with it.
Even with LED its going to be quicker to discharge the caps rather than wait for LED to discharge.

I once discharged the main cap on a valve amp and then noticed LED was still on. I had forgotten to turn it off. In this case it could be the LED saved my life !