Current limiter

[Bugman3183]

Hello all.  I recently had an amp blow a power tube and was attempting to use my lightbulb limiter to check things out before just replacing tube and moving on.   Amp in question is a 30 watt cathode biased 4 6v6 amp.  I checked all the resistors on the power tubes and in that  vicinity and all check ok.  It's been such a long time since I've used the lightbulb limiter I decided I'd head to YouTube and do a little research.  Unfortunately, I'm getting conflicting results.   Some videos say the lightbulb shouldn't light up at all if all is good, and some say lightbulb should be dim.  My results are that the lightbulb is extremely dim when I turn on power, and when I turn on standby switch it gets brighter gradually but nerver really gets fully bright.  In my opinion, I feel all is well, but I'm super paranoid because I'm no expert and the only 6v6's I have on hand are some really cool vintage ones and would hate to destroy them.   Any insight would be greatly appreciated. The lightbulb i have is 60 watts btw.  Thanks.

[92Volts]

Are tubes installed in the amp? Even preamp tubes?

Without tubes the bulb shouldn't glow at all, except a brief rush of power to charge the high-voltage supply while switching from standby to play mode.

Regulators or safety discharge resistors can waste a small amount of power whenever high voltage is present, but I'd be surprised if this was enough to light the bulb.

With tubes some power will be drawn for filaments at the very least. When high voltage is applied the amp will draw even more power as the amp is more-or-less operational now. This will definitely be visible.

[Bugman3183]

I apologize for the the lack of information.   With all tubes installed, I cut on standby switch, flashes bright, gets super dim, gradually gets brighter but never fully bright.

[92Volts]

Sounds fine. The amp is "trying" to operate normally. 30 watts of output could require more than 60 watts from the wall... trying to draw that much power through a 60 watt bulb would definitely light it. Not too bright though, since the presence of the bulb limits it from drawing full power.

If there was a major short circuit, the bulb would be full brightness as the amp drew every bit of power available to it!


It sounds like you measured the resistance of each cathode resistor and found they are correct? There are a few more things to check. Measure voltage across each cathode resistor while the amp is operating (limited or otherwise) and make sure they're similar. Other factors than the cathode resistor could influence how much power each tube draws.

Either check screen voltage or (without the amp running) check the value of screen resistors... screen resistors are rarely causes but are often victims of failing tubes.

Overall there's a good chance the tube died of "natural causes" and if it didn't fry its screen resistor, you won't end up finding any problems.

[Bugman3183]

Thanks for the advice.   Cathode and screen resistors check ok.  Next step is to check the bias.   Wanted to be sure it was safe to do so. I really appreciate it.

[HotBluePlates]

...   Some videos say the lightbulb shouldn't light up at all if all is good, and some say lightbulb should be dim.  My results are that the lightbulb is extremely dim when I turn on power, and when I turn on standby switch it gets brighter gradually but nerver really gets fully bright.  In my opinion, I feel all is well ...

I agree.

If you had the power tubes out of the amp (or even just the one socket empty), put the tubes back in and power up with the lightbulb limiter.  If it continues to only glow dimly/less-than-full-brightness, you're likely fine to power up normally.

The lightbulb should glow at least dimly if there's any power getting to the amp.  The circuit is Wall Outlet +  ->  Lightbulb +  ->  Amp (fuse/switch/power transformer)  ->  Lightbulb -  -> Wall Outlet -.  The only way the lightbulb should ever not light is if the amp power switch is Off/open, the amp's fuse is blown, or if the lightbulb burns out.  If you're in a well-lit room, it may be hard to see that the bulb is glowing.

Excessive current draw in the amp will tend to light the bulb full-brightness.

[sluckey]

The only way the lightbulb should ever not light is if the amp power switch is Off/open, the amp's fuse is blown, or if the lightbulb burns out.

There is one more way. If you use a 300W lightbulb on a 15W amp as Uncle Doug (not Hoffman) suggests then you will likely not see any glow unless there is a major current draw. Still got the protection, but I want my lightbulb to act in an analog manner, not digital... off is good, on is bad. I prefer to match the light bulb to the amp. I just like seeing the bulb jump up in brightness due to cold filament current and then drop back to a warm glow . And then slightly increase in brightness as the tubes warm up and begin conducting. And you still got the very bright light to indicate a problem.

[HotBluePlates]

The only way the lightbulb should ever not light is if the amp power switch is Off/open, the amp's fuse is blown, or if the lightbulb burns out.

There is one more way. If you use a 300W lightbulb on a 15W amp as Uncle Doug (not Hoffman) suggests then you will likely not see any glow unless there is a major current draw. ...

Good point!  Yeah, don't over-do it on the bulb rating...

[pompeiisneaks]

The only way the lightbulb should ever not light is if the amp power switch is Off/open, the amp's fuse is blown, or if the lightbulb burns out.

There is one more way. If you use a 300W lightbulb on a 15W amp as Uncle Doug (not Hoffman) suggests then you will likely not see any glow unless there is a major current draw. Still got the protection, but I want my lightbulb to act in an analog manner, not digital... off is good, on is bad. I prefer to match the light bulb to the amp. I just like seeing the bulb jump up in brightness due to cold filament current and then drop back to a warm glow . And then slightly increase in brightness as the tubes warm up and begin conducting. And you still got the very bright light to indicate a problem.

Wouldn't it also somewhat act as a fuse if you really draw too much current and blow the bulb :D 

or maybe it doesn't get hot enough on the bulb to do that? not sure.

~Phil

[sluckey]

Wouldn't it also somewhat act as a fuse if you really draw too much current and blow the bulb :D

How will you ever blow the bulb? The bulb is designed to work on 120VAC. Worst case scenario is a dead short across the primary circuit in an amp. That's like turning on the wall switch for a lightbulb. Think about it.

 

[PRR]

> Next step is to check the bias.

The bias won't be right when running through the limiter.

It may be good to look for four tubes running "equally lame". If you expect 30mA normally, and observe 15 15 15 and 60mA, or 15 15 15 and 0, one of those tubes has a problem. I would not panic for +/-30% differences on lame/limited supply; it may even-out when you go full voltage.