Bulb v.s. Variac

[Andy S.]

Enzo, in a recent thread you mentioned something about a variac instead of the a bulb.

If you don't have $100 for a variac, the bulb is a nice tool, but I will take a variac and ammeter every day over the bulb. Without an ammeter, the variac is only half a tool. No, it does not limit current, but I can advance the mains up to 10 or 15 volts, and if current is already ramping up, I can back off and nothing was ever stressed. The bulb way means at least the bulb current was flowing through the amp. And a 200 watt bulb on UK mains means a whole ampere was flowing through the unit under test.

Long ago, when in Columbus and working full time audio, we had a rig set up with a light bulb and an AC dual socket. We could plug the amp in the socket
and when the amp was turned on, the lamp would start to glow a bit. If it was drawing too much current, it would get bright.

We also had a variac and ammeter as you mentioned. As you pointed out, if the current started going high at low volumes, you knew you still had problems.

Was this what you were referring to?


If so, well, that was a long time ago and if possible, could you refresh my memory and show us how the wiring would be so I can set that rig up again until I can get a variac?

If that's NOT what you were referring to, I will try to locate my old boss and see if he has the layout.

Thanks,

Andy

[Enzo]

That's exactly what I was talking about. I certainly can't take credit for the idea, varaics have been a staple on the repair bench forever.

The bulb? SImple, mount a standard light bulb socket somewhere and wire it in SERIES with the mains hot wire to an outlet. Adding a switch too gives you more control. That's all there is to it.

You could also wire a socketed bulb to a pair of clip wires, and you can then clip it across a fuse holder with the fuse removed. Just another way to insert it into the circuit. But I like the wired outlet version better if I have to use the bulb method.

And the variac is just a variac, add an ammeter in series with the output adn run that to an outlet.

[trevorus]

As for the ammeter, if you were going for a permanent install, I would use a panel mount setup. If it's bench testing, I found an old Micronta clip on volt/ammeter for 8 bucks. It came with a plug socket that ran the hot through a set of loops. It had a single loop for 1x measurement, and a 10x loop that had some windings to it to give you an easier way to measure small amp draw.

[Alex R]

After that post of Enzo's I got a nice 8A variac and it works fine for me.

However I'm still not quite sure how to deal with those relays in big ss amps that don't turn the amp on till there's about 80% of the mains voltage through the variac. As you bring the variac up there's a kind of hum, then when the vac get high enough, a click, and the amp starts to draw current. For speaker protection are they? Why/how do they interact with the variac that way? What do you do to get around it, short them out?

All these questions, I'm beginning to sound like walters.

[Enzo]

First off, if you are bringing it up on the variac, it seems to me you are not yet confident it works, so there should be no load on it anyway. That makes the speaker relay irrelevant - for now. Once the amp no longer draws current AND there is no DC on the output bus, THEN put a load on it and see how it sounds.

Speaker relays are there to delay connection of the speakers so there is not aloud turn-on thump. When power is applied, the transistor that runs the relay has a cap in the circuit that has to charge up, thus providing the delay. When you slowly ramp up the mains with the variac, the thing doesn't get to charge. Now if the amp starts to draw heavy current when the relay snaps on, that means to me you have a load and DC on the output. Untill the relay closes, the amp can have DC on its output bus without drawing current. But connect the load, not that DC has to do work, and current results.

I work without a load. Follow the discussion of Bobs Carver M500t elsewhere around here. We discussed such troubleshooting. In a nutshell, I scope the output bus on the amp side of the relay. That way I can see what is going on whether the relay closes or not. A convenient spot to clip on is the leads of one of the output transistor ballast resistors.

When you bring up an amp, many of them will not balance out well at very low voltages, so if there is a load connected, you will hear power supply ripple and other stuff until you have cleared a minimum voltage level for that circuit.

[Alex R]

Mm, I know what you mean, I do follow that procedure - up on the variac checking current, then look for DC on the output, then up again with a load.
But it seems to me that this phenomenon (relays only clicking in when vac reaches a certain level on the variac) happens whether or not a load is connected. There's a hum from the amp, then as you go up more it disappears with a click when the relay fires. These jbl/urei amps I've been having fun with do it, and I seem to recall it happening with another amp also. I'll attach a schematic. Am I misreading the signs somehow (again)?

[Enzo]

Well the relay can't pull in if the voltage is too low. The timing cap has to charge, and so on. The thermistor R95 has to heat up.

But you mentioned current spiking when it did, and I can't see any reason for that happening without a load.

SOme amps will make a noise when ramping up since the junction drops of all the stages do not necessarily turn on at the same time. Once you know the amp will be OK above that point, don't turn it slowly anymore. Just whip past the unstable point quickly. If that and the relay happen about the same time, I'd be thinking coincidence. I mean all the relay does is connect the actual output terminals on the rear panel.

[Alex R]

- yes, of course the relay shouldn't make any difference without a load. With that in mind I will take another look, when I get around to putting right all the damage I have done to this amp with my shaky understanding of transistor bias and current draw.