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Light bulb trick questions re: ST-70 amp

Posted by mkmelt (A) on November 4, 2002 at 21:52:08

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.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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