Quote:
Originally Posted by 1100 man
that dropper sure does belch out some heat  It dissipates 30 W for the heaters and another 15W for the HT. So 45W in total. |
I once had a similar problem with an AC/DC set and the heat dissipation from the heater dropper resistor was extreme. I took the bold step of modifying it by adding a series silicon power rectifier to run the heaters off half wave (which has an rms voltage of about 170V for a 240V supply). It was critically important that with the now lower value of dropper resistor that it had a good Brimistor (large NTC resistor) to limit the inrush current with cold heaters because the overall series resistance was now lower. I think I had to change it to another type.
The mains supply has a very low internal resistance and the heaters when cold have a very low resistance, the small sized ones heat up very fast and explode in a flash of light with high inrush current without the Brimistor.
There was no evidence of added hum in any of the circuits half waving the heaters and while its not ideal to draw current off half the mains waveform, I think I had it so the HT was supplied by the + half cycles and the heater by the - which balanced it up better than it was before. Obviously if a set was modified like this, one wouldn't want the added rectifiers failing, probably best to use two 1000V 5A or 6A rated silicon rectifiers in series as extra protection in case one failed.
It is interesting when a transformer runs the heaters in sets with parallel heaters. When the transformer has about the right power ratings to run them, when they are hot, at turn on the low resistance load causes the transformer output to collapse so the heaters get a nice soft start. So in this sense the larger tubes protect the smaller ones, unlike in a series chain. One trap to fall into is, with a large multi-valve set, is to plug in a single cold small signal valve after all the other heaters are up and running, this causes a large peak current, that the transformer can support for a single cold tube and the heater in the small tube can fail just like in a series heater set.
One other method to eliminate some heat could be to add a small autotransformer to the heater supply and drop some voltage there rather than in the dropper resistor.
I guess though the manufacturers made it to handle the heat, but it does sound like a toaster oven in there.