lightbulbs as attenuators?

[rutledj]

I was checking out this clip http://www.steviesnacks.com/my-gear/sam ... r-amp.html that discusses using a series of light bulbs to reduce the output power. I'm curious how this might be implemented. Are they in parallel with the speaker? If so, looks like this would screw with the output impedance as you change the number of bulbs in the ckt.

Rut

[Phil_S]

Bulbs act as current limiting devices. I'm not sure it's a good idea.

[Andy Le Blanc]

there's a hot and a cold resistance, in series They'll turn the P into heat at a
certain point. Its pretty common as a protection device in PA speakers.
I'm not so certain its a good Idea either, until you know how it will behave,
if the the application is for guitar.

[Structo]

Interesting.
I wonder what happens if the lamp burns out?

V.A.C. uses lamps (light bulbs) to limit voltage and current flow into the power tubes. This balanced circuit changes the power rating of the amp and allows voltage to sag the optimum amount. Power tubes run cooler with V.A.C. and all of the output power reaches the speaker.

All models have several output power presets and use readily available lamps: 15 watt exit sign, 7 watt "night light", and 4 watt "night light". It is possible to increase lamp values and change the power level of presets using the higher wattage lamp. One can take advantage of the "feel" and touch sensitivity that coincides with the volume compression and output limiting of power amp clipping.

[Andy Le Blanc]

It uses a light bulb to act as constant current device.

I've read something about a SS amp utilizing the effect.

The current falls very conveniently into a range to do this,

its like using series filaments of pre amp tube to bias, just happens

to be the right range, put various bulbs just after the rectifier in

the PS, maybe on the B+ to the OPT, Don't think he'll be able to patent,

but should be able to get the VAC as a proprietary, stupid simple.

DIY VVR, got to love it.

[rutledj]

I guess you have to carry a supply of lightbulbs also unless he has a built in bypass to handle when one burns out.

[Andy Le Blanc]

I bet the math probably works out fine, 7W at 120V AC is most likely more
punishing on a light bulb filament that was designed for 3000 HRS of it
than say 100 ma. of 420V DC over the same hot resistance of the filament.
Its probably in series too, so I bet its pretty robust electrically.
It might have issue's with the extra glass in a combo, light bulbs can be cheap.
Not the same thickness as a tubes envelope.

[CapnCrunch]

THD amps use a bulb as an attenuator on their Univalve. It is in addition to the "hot plate" attenuator built into the amp. The bulb has the stated purpose of tone shaping. It attenuates the high frequencies. I had one of these amps for a while and when using the hot plate I never used the bulb. When I was not using the hot plate, however, I used the bulb to quiet hiss and take a little of the sizzle off the top end.

[Structo]

That is what I was wondering about.
If they are in series, then if one burnt out, the whole chain would be dead.

Sort of like the old time Christmas lights where you would have to go through the entire string before finding the one that was bad.....

[azrael]

Back when modding Valve Juniors was the hot ticket, I remember reading an article where one guy had a bulb as an attenuator, I think.

[bigsmitty]

NOS bulbs sound the best