Current limiter

[NotSure]

Finally built a current limiter, even though its too late to die young now. Working on a Filmosound 385 and I'm seeing a faint glow in my 300 watt bulb when I fire it up. I've only had the amp on for a few minutes and played through both an 8 ohm and a 16 ohm speaker. Amp seems good sounding especially through my Div by 13 cab. But I'm shutting it down after a few minutes cause I'm scared of the orange glow in my bulb.


Is a little glow ok or is there something shorting in the amp?


Thanks to all contributing to this excellent resource.


Dave

[shooter]

how many watts is a

Filmosound 385

?
the amp will draw current, so the light probably will glow.

anything seem "too" hot?
anything smell too funky?
put a 100W n repeat, will probably glow brighter, but full bright would be bad

[John]

If the schematic I found is the right one, that's a 15/20 watt amp. It's not pulling enough current to light up that 300W light bub. FWIW, I only put a 60W in mine, and it works equally well for testing against dumb mistakes (mine) on 5W to 30W builds.

[NotSure]

Amp shows 18 Watts, so yea between 15 and 20. This thing is loud, loud, loud, and sounds great through the 16 ohm Celestion 12". I have not experienced a hot power transformer, or overly hot tubes. They are hot after being on for 5 or 10 minutes but you can put your fingers on them for a moment or two. There is no hum that is significant but there is a static-sounding noise I've not heard in any guitar amp. Definitely no bad or bunt smell,  no smoke. The amp did seem to loose tone and power after being on today for about 20 minutes. But low power mostly. Trying to break in the new speaker (another 16ohm 12" Celestion)


Dave

[bnwitt]

It's hard to believe it doesn't sound crappy going thru the limiter.  You will typically get a real bright bulb on start up then the lamp brightness drops to a low glow if everything is good.  It's only when it stays bright you may have a problem.  Of course the bulb wattage should be chosen properly for the amp being tested. I usually use a 60-75 Watt bulb for low wattage amps like yours.

[NotSure]

I saw on an Uncle Doug video that he suggested high-watt light bulbs, 250 to 300 watts so that that there was as little resistance as possible. His logic for this, if I understand correctly, is that with the lower resistance you can play through the amp and here it pretty accurately while plugged in the current limiter. And, of course, if there is a problem the high-watt bulb will still glow accordingly.


Decided to plug some other amps I have into the limiter, and no glow at all from a Champ-type amp I built, or a factory-made amp from about 1972.



So I guess I can assume there must be a short somewhere. Or is it not a equivalent comparison from a single ended 5 watt to an 18 Watt in push/pull?


Thanks

[bnwitt]

I've been using 75 watt lamps in my limiter for 20 years and they work fine for most amps.  I wouldn't use that wattage testing a Fender PS400 though.  I saw Uncle Doug's video on the higher wattage lamps he uses.  You aren't going to get much glow in a 300 watt lamp from a champ but you should get some even if it is fine.  It the amp has a dead short you'll get a constant bright lamp after an in rush period.  With a 75 watt bulb and a typical 20 to 40 watt amp I typically get a quick very bright glow during the in rush current period and when the amp is ok it drops down to a low glow.  That is normal.  I think you could use just about any wattage you want for testing for shorts but for sure when I leave a high wattage amp on the limiter with a 75 watter it doesn't get enough current to play.  Below is a link to how a limiter works:

http://www.electronicsrepairmadeasy.com/2015/10/how-series-bulb-current-limiter-works.html

Barry

[John]

So I guess I can assume there must be a short somewhere.

Nope, if there was a short that bulb would be glowing brightly. The more current (amperage) drawn, the brighter the bulb glows.

[SoundmasterG]

I usually use a 100 watt bulb with my current limiter for most amps. After a repair or if an amp has been sitting for a long time, I'll bring it up on the current limiter and check for function and voltages. Voltages will be low but I can at least see if any major problems exist. Once I've determined that there aren't any major problems that will cause parts to fail, then I take it off the limiter and play it. I don't play it on the limiter as the point of the current limiter is to give you a little time to see if there is a problem before parts blow. I suppose you could play it if you used a higher wattage bulb, but that seems pointless to me as the higher wattage bulb won't restrict the current enough to stop possible circuit problems from occurring and won't give a usable glow to determine easily if there is a problem, so you end up needing the lower wattage bulb anyway. Why not just use a 100 watt bulb from the beginning and play the amp off the limiter once you have determined there is no problem?

Greg

[NotSure]

Thanks all for chiming in regarding the Flimosound 385 on the  current limiter. As you all pointed out, the low glow from the light bulb doesn't seem to indicate a problem. Played the unit for about an hour. No bad smells, tube glow was typical, and no super hot tubes or transformer. So I guess, for something built in 1955, all is ok.


Oh and thanks for the link on the current limiter.


Dave