Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: PRR on December 04, 2014, 09:15:15 pm
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"Fuse Saver", Popular Electronics, 1960
AKA "Lamp Limiter".
Sluckey has posted this often, but this shows it isn't a new idea.
Notes for modern readers:
The lamp MUST be incandescent. Simple hot wire in glass. NOT fluorescent (curly-tube) or LED.
Incandescents are going out of style, true; but they are not illegal and some types are readily available.
"Halogen" IS incandescent, just made better. This works fine.
Always wire a GROUND connection. Grounded outlets were rare in 1960, but the improved safety (and audio hum reduction) makes a Ground mandatory today. I have made some green marks on the article, but there are better diagrams out there. .
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I used a push button switch on mine. I like to get a quick memory refresh of what a full bright bulb looks like.
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One of the skilled guys that are here around
can post a table with W lamp values in relation to the apparatus under test, read as 5W amp that W lamp, 10W amp that W lamp and so on ...
on the article that PRR has linked there is something about that but referred to other than guitar amps
Also one question, on 230v line instead 120v line, the W of the lamp is to be assumed as 1/2 of the correspondent 120v lamp
as example if a 50W lamp is to be used in 120v apparatus, the same kind of apparatus that is operated at 220v requires a 25W lamp ? Is this correct ?
Franco
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The article *says* what Watts to use.
50W-100W for small amplifiers.
150W-200W for large amplifiers.
The lamp does not care if the amplifier is plugged into a guitar or a tuner or a tape.
"Small and Large" is obviously relative. I'd try a 60W-100W on a Champ and a 150W-200W on a four-6L6 amp. This will (if all is well) run them on part-power-- enough to check for smoke and get some sound through, NOT enough to work "right"(*). That's why there is a switch to bypass the lamp *after* you smoke-check.
> on 230v line instead 120v line, the W of the lamp is to be assumed as 1/2 of the correspondent 120v lamp
No! We want the lamp watts to be somewhat bigger than the amplifier maximum normal Watts from the power line. With tube amps, the wall-power watts is 3 to 10 times the "output watts".
Look at the difference, 115V and 230V gear. Your 230V power transformer has twice as many turns of thinner wire. Your 230V lamp has a longer thinner filament. Keep the same proportion. That means lamp Watts somewhat larger than amplifier input watts.
(*) I do not get much warm air in the back bedroom. I added a fan to that duct. The fan is loud and maybe too much air. I added a series lamp. I get less air and less noise. In this case, a 72 Watt lamp in series with a 35 Watt fan gives a large reduction in sound and air. For a fan that was "too much", this is now "about right".
But note that I have studied this fan. Not all wall-power motors can be speed-controlled so simply. In another case I blew-up an attic fan, BIG stink.)
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I am unwilling to tempt Murphy's Law. The "convenience" of SW1, raises the very real possibility of defeating the purpose of the lamp limiter. My philosophy: if the lamp limiter is in the circuit, then the lamp limiter is in the circuit. I do use a momentary SW to verify the that the light bulb works. My schematic is below.