Light bulb current limiter behavior

[rockman627]

Most of the time when I plug an amp into mine, if it's good, the light is pretty dim. But sometimes it is a little brighter but not totally bright. What does that signify?

[Jon Snell]

The brighter the lamp the higher the current flow through your equipment.
But, there is a problem ... the brighter the lamp, the more voltage across the lamd which means the lower the voltage on the equipment and that causes all sorts of odd annomolies.

[rockman627]

Is it normal for some amps to make the bulb brighter? Or does it mean trouble is brewing?

[Helmholtz]

Amps differ in current draw at idle. As amp and LBL are wired in series, their current always must be the same.
So more amp current means more lamp current. More current makes the LBL glow brighter.
Even hotter bias can make some difference.

[stokes]

A 5 watt Champ will glow dimmer than a 100 watt Marshall even tho both may be normal/ no shorts.

[Axtman]

Tube amps have filaments that draw current so the bulb will glow when the amp is on.

It is hard to know if there are problems without knowing the amp and the wattage of the light bulb.

[Justin Thomas]

Yes, different amps can cause different brightness. I usually use a 40W bulb in my LBL; my Rivera Concert lights it up more than my little 2x6V6 homebrew, which barely lights up a 100W bulb at all.

When I'm done working on an amp, I'll usually put it on the limiter with no tubes in. The bulb should barely light at all like that, because transformers don't draw current at idle with no tubes. Then I'll put the tubes in & the bulb will glow brightly for a second or two & gradually fade. If it gets bright & stays there, it's an issue, & we check the amp over again.

Jusrin

[Mick Bailey]

It's also worth remembering that a bulb limiter can cause a fault condition to occur with some transistor amps. Some will swing the output to one supply rail, placing a high DC voltage on the speaker output and drawing excessive current. It makes the amp look like it has a shorted output transistor when there's actually no fault at all.

[Helmholtz]

It's also worth remembering that a bulb limiter can cause a fault condition to occur with some transistor amps. Some will swing the output to one supply rail, placing a high DC voltage on the speaker output and drawing excessive current. It makes the amp look like it has a shorted output transistor when there's actually no fault at all.

Can this also happen with variac at lowered voltage?

[The Dude]

Can this also happen with variac at lowered voltage?

Yes it can. I see it often. It's why I never use a variac when a load is connected. It's also why I wouldn't use a LBL with a load connected.

EDIT: For clarity, I'm talking about a load on the amp output- not the variac.

[Helmholtz]

Fortunately tube amps don't latch.
..and can't have DCV on the output .

[Mick Bailey]

As per The Dude's reply, I've also only seen the latching when a load is connected and with a variac as well as LBL. Not all amps do this, but it can be a trap for anyone doing repairs or powering up a solid state amp for cap reforming.

[Mick Bailey]

Fortunately tube amps don't latch.
..and can't have DCV on the output .

Unless its a directly coupled OTL design using +/- rails. Unlikely in the guitar amp world, though I really would like to build one just to hear how one would sound compared to a transformer output. Maybe for an acoustic guitar amp.