thd and using a light bulb to cancel hum

[funkmeblue]

how does thd use a light bulb to cancel hum in his univalve amp?

[drz400]

Does it say that he does somewhere?
Why should he need to cancel hum produced in an amp? There shouldnt be ANY hum.

[Clyde]

The one I saw a few years ago had no bulb, used dc heaters with much filtering. Maybe bulbs have unknown capacitive properties we've all been overlooking. Perhaps someone more in the loop can shed some "light"
on this.

[jride100]

A light bulb is placed in parallel with the speaker jack. The bulbs cold resistance is very low and hot resistance is high. The thought is that at low volume levels (if you find the right bulb, this only includes hum and hiss) all current from the transformers secondary passes through the bulb and never makes it to the speaker. At higher current levels, the bulb filiment heats up making it a much higher resistance so the current passes through the speaker preferentially. It's an old trick that can be found in the RDH. A bulb in parallel with the speaker load will act as an expander and in series with the speaker will act as an comressor. With the series compressor bulb you run the risk of the bulb filiment burning out and the OT operating w/o a load. There was a discussion on ampage or ax84 quite some time ago if you want to look into it further. Search for light bulb expander