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I recently tried what has been called by some here as the light bulb trick. That is, wiring a standard light bulb in series with a tube amp, to drop the voltage. I tested my line voltage and it appeared to range between 121 and 123 volts, depending on time of day, with no load on the circuit.I recently purchased a stock Dynaco ST-70 that was working when I picked it up, but I wanted to see if it would pass the light bulb test for shorts, and also planned to run this amp with the light bulb in the circuit to drop the voltage closer to 117VAC instead of where started out, closer to 125VAC.
I cut into the hot lead of a six foot two conductor extension cord and wired a plastic light bulb base (unswitched) with a 40 watt bulb into the circuit. I tested this wiring circuit and it measured the same 122VAC with no load connected to the extension cord except the meter probes. The bulb did not light up.
Next I connected the ST-70 amp to the light bulb circuit. The ST-70 was also connected to two 4 ohm speakers and also to my PAS preamp. The preamp was on a different circuit, volume off.
I powered up the PAS, then after a minute I turned on the ST-70 amp and the light bulb lit up. It did not appear to be glowing at the full brightness. I waited a minute for the amp to warm up, but it never did. I could detect the faintest glow in the tubes, but no audio was heard when I raised the volume on the PAS. I measured the voltage at one of the other outlets of this extension cord and it registered approx. 27 volts. I turned off the amp and unplugged the light bulb circuit and plugged the amp back into the usual surge protector. The amp worked normally.
I put away the light bulb wiring arrangement for the day, and although the amp was working and sounded fine, I was concerned that maybe I had a short in the power supply cap or in the power switch of the ST-70.
I have another tube amp, a Fisher 400 receiver. This evening I decided I would try plugging the Fisher into the same extension cord with the light bulb wired into the circuit. The light bulb illuminated, but not at full brightness. I measured the voltage available at the AC outlets at the rear of the Fisher and got a reading of 37 volts. The dial lamps in the Fisher were glowing weakly, but no audio was heard.
In both cases I had the light bulb/extension cord wiring plugged into a surge protector. Would this make any difference? Before I proceed with any further experimentation with this light bulb circuit, I was hoping someone here could provide me with an explanation of what happened and what I can do to get the desired result. I hope to be able to run the ST-70 with the light bulb in the circuit at least until I can get a suitable variac.
Follow Ups:
The 40W bulb was dropping the voltage too much due to the resistance it added to the line. When bringing up an unknown amp with a light bulb, it's customary to start with a small-watatage bulb and then move to higher wattage bulbs once one is sure that things are okay at the lower voltages provided by the small bulb.Try a 60 or 75 watt bulb, or even 100 watts, checking the voltage at the plug of your strip with the amp turned on.
YOu would need a MUCH larger bulb than that! To drop the voltage at the amp from 122V to 117V, you only want about another 3 ohms in series with it (since the ST70 draws about 180W at idle, it approximates a 75 ohm load in the power line). A light bulb has a resistance of V^2/P, where P is the rated power and V the nominal line voltage. For example with V=120V, P = 100W, you get a resistance of 144 ohms. So you would need 48 100W bulbs in PARALLEL, then connected in series with the amp.The light bulb trick is for forming up old amps slowly, through very low voltages. To slightly lower the line voltage, you really need a Variac--or maybe just a very long and not very heavy-gauge extension cord!
The dimmer the light bulb gets, the *lower* the resistance across the filament. When not glowing, the resistance drops down to several ohms.When powering up the amp, that's when the bulb should glow its brightest. And afterward, it should get dim to the point where it's barely glowing. A single 100-watt bulb generally does the trick with moderately-powered tube amps.
You may also want to try a 220-volt bulb for such an application (110-volt circuit).
Agreed--I know how conduction in metals (the filament) works. I just wanted to point out that one common (up to 100W) bulb in series will drop the voltage way too much. Great for slowly reforming the filter caps, but not too practical for achiving just a 5% voltage reduction.
Thanks everyone for your informative replys. I picked up some additional light bulbs at Home Depot on my way home. I tried a 75, 100, and 150 watt bulb in the circuit and watched the bulb and voltage on my meter as I powered up the ST-70.In each case, the bulb started off fairly bright, then became dim. The voltage rose steadily to a maximum of perhaps 91 volts (with the 150 watt bulb installed), then the voltage actually started dropping a bit to around 74 volts before stabilizing.
I unplugged the amp from the light bulb circuit and powered it up at the standard 120 volts AC line voltage. There was a semimetallic twang as the power hit the amp, but otherwise all was well. There is a slight hum that can be heard coming from the chassis of this amp after a while, but from what I have read this type of chassis hum does not indicate a problem with the amp. I listened to the amp for about 90 minutes and it performed well without any problems.
I have a variac on the way, so I will try that next to see if I can get the amp to run cooler and sound at least as good at a lower operating voltage. The variac I purchased is rated at 5 amps. I thought this would be adequate as the ST-70 line fuse is 3 amps.
So I guess I have a working light bulb wiring hookup for soft powering up of vintage tube equipment. But I'm still puzzled about an old post I read here where the author claimed he used a blue party bulb in this type of circuit to keep his voltage at around 110 volts. I can't see how this could be unless he was running his equipment at way under 90 or even 80 volts.
You wrote 'There was a semimetallic twang as the power hit the amp'.I am getting this same sound from my Fisher 800c when I power it on. I assume this is the voltage making its initial ramp up in the power transformer. Does anyone know if this is normal or harmful?
This is a phenomenon that occurs with most power amps or audio equipment that have large capacitor banks.You will also notice that this "twang" does *not* always occur at power up. It depends where on the 50/60 Hz AC signal the power switch makes contact. If it's near the peak of the signal (positive or negative), you will hear the "twang," if it's near the zero-crossing part of the signal, you will *not* hear it.
That's right, I do not hear it every time. What is causing it (where does the sound come from - the xformer or caps? Interesting. Why does it occur at the peak?
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