Am I being fooled by my light bulb limiter?

[BrianS]

So, I'm working on an amp...see this thread if you're interested: http://el34world.com/Forum/index.php?topic=17023.0

I did find and fix a bunch of issues with the amp, but figure I've still got a problem because my bulb limiter keeps lighting up.  Not super bright, but definitely lighting up.  The limiter has a 200W incandescent bulb in it.

As I was measuring some voltages, I decided to leave the amp on and see what happens with the bias current.  Well, after a short time, it levels out and I am able to adjust it to the recommended amount.  I decide to plug a guitar in and see what happens...

...Guitar sounds good.  I mess with volume, tone, drive switches, etc...everything sounds good.  All the while the light bulb limiter is glowing.  It gets a little brighter as I play louder, or hit big chords.  I'm watching my bias current fluctuate as I play, too.  So far so good, I think.

So I decide to plug my known working Traynor Reverb Master (50W tube amp) into the limiter.  Same deal...bulb lights up and stays glowing a bit.  I also plug my Peavey Bandit into the limiter...that one doesn't light the bulb until I really crank it and play hard, then I see some faint glowing in the bulb.

I also had recently repaired a solid state Traynor BLOC 50 bass amp.  That thing really lit the bulb up when played through...

I also decided to buy several different bulbs today and try them:  60W lit up the most; 100W lit pretty good; 150W seemed the same as the 200W.

What's going on?  Should I post pics of the glowing bulbs so you can see how much they glow?  I'm still a little afraid to plug the amp I'm working on directly into the wall, but all signs (other than the bulb limiter) are pointing to the amp being ready to rock.

[sluckey]

I'm still a little afraid to plug the amp I'm working on directly into the wall, but all signs (other than the bulb limiter) are pointing to the amp being ready to rock.

Well it's time to plug the amp straight to the wall.

The light bulb limiter has only one purpose... to tell you if you have a bad short or other problem that would cause your amp to draw excessive current. If the bulb glows as bright as it would if screwed into your overhead fixture, you got a serious problem. But. it has already proved that's not the case. Time to put the limiter under the bench.

[BrianS]

Ha!  I did just that a few minutes ago...voltages are all right on the nuts, bias is good and the amp sounds great.

[PRR]

If the amp power input is SHORTED, the lamp will light FULL brightness.

If the amp draws much less than 200 Watts, the bulb will glow but pretty dim. Time to skip the lamp and plug-in.

You can't usefully measure voltages while on a dim bulb. It may be slitting your 120V as 100V to the amp 20V to the lamp, or 80V to amp 40V to lamp... you just don't know the split. Or if you did measure the split, and try to correct the internal voltages by that ratio, you'd be wasting your time. The internal voltages do not go down in proportion to the plug voltage because the cool heater/cathode makes the tubes lame.

It may be worth checking that you have -negative- bias voltage and enough to predict a proper bias when fully-volted.

[Jack1962]

I had this problem when I 1st started working on amps too , I use a 60 watt bulb and it works fine anything above 100watts draws to much and will effect the voltage readings to the point they are useless and may indicate problems that don't exist .