Tweeker said:
Actually one seems to be talking then of thermisters on both primary and h.t. secondary? I have never tried this, but slower heater warm-up makes sense. Especially with global NFB there are still large signal transitions if the h.t. is switched in suddenly, depending on the different decoupling time constants reaching operating voltages. There could be output stage trouble with this if the h.t. is of the order of >450V. In this sense the slow heating up of a tube rectifier eases matters significantly, though I am not suggesting getting back to that.
One gets thermisters that can be heat-sinked for slower reaction time, for the h.t. Again one has a problem of availability here in South Africa, but I would like to try that because of simplicity.
Anybody with experience of using thermisters in this way?
Regards
Hi Tweeker,
The thermister on the primary is there to cause a more gradual heater warm-up; as someone said, to limit the initial peak heater current. This will cause prolonged heater life. Actually that should be in the heater chain, but I don't know that milli-ohm thermisters are available. The h.t. thermister must work after the heaters have warmed up, so to speak. That is to cause a slow rise in h.t., obviating large voltage excursions especially in NFB circuits. Ideally the h.t. should be switched in after the tubes' heaters are at operating temperatures, but then also not suddenly as explained.
But to get the ratio right might be a problem as you say. One might need to start the h.t. (with thermister) by relay after some 20 seconds.
The thermister on the primary is there to cause a more gradual heater warm-up; as someone said, to limit the initial peak heater current. This will cause prolonged heater life. Actually that should be in the heater chain, but I don't know that milli-ohm thermisters are available. The h.t. thermister must work after the heaters have warmed up, so to speak. That is to cause a slow rise in h.t., obviating large voltage excursions especially in NFB circuits. Ideally the h.t. should be switched in after the tubes' heaters are at operating temperatures, but then also not suddenly as explained.
But to get the ratio right might be a problem as you say. One might need to start the h.t. (with thermister) by relay after some 20 seconds.
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