Need Some Help With a DC Heater Supply Rectifier Current Rating.

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Need some help with a DC heater supply. Running 2x 6SN7's (0.6A/fil, 1.2A total). Started with a Rectron 4A bridge rectifier. Got a little too warm, and the voltage started to drop after a couple min. Tried 4x OnSemi 3A diodes, they got too warm after 5min or so, and voltage started to drop yet again...

How high of a current rating do I need to keep the voltage from sagging over time?

Supply is setup as so: Bridge rectifier > 10,000uF + .1uf film to ground > 0.4ohm 10W resistor to drop voltage to proper ~6.3V > another 10,000uf and .1uf to ground > filaments. "Ground" of the supply is referenced to 100VDC, for elevation. The 6.3 is floating.

Thoughts?
 

PRR

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Dropping over minutes suggests transformer copper getting hot, not the tiny Silicon.

What do you have to lose? If not much, leave it run (in a fire-resisting setting) until it stabilizes or smokes.

The rectifier may be commercially acceptable (not many failures under warranty) but will run HOT. In DIY I think a 10A part makes sense.
 
When I used to use a transformer for heaters I used a 25A bridge bolted to the case.

As DF96 said though, make sure your transformer is up to the job. Also you could use one LM7806 per tube to "clip off" the noise. You would need about 8V input for this. 10uf per tube after the 7806 works fine to eliminate heater related hum.

Have you considered using an SMPS instead? Smaller, more efficient and cheaper.
 
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PRR

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DC-elevating the heater supply is probably be more effective at reducing hum than rectifying/regulating it.

FWIW: 100V elevation was mentioned.

These address different causes of hum.

Thermionic emission happens heater-cathode, worse on some tubes. Making heater positive reverse-biases the effect and reduces the need to select tubes.

Running 6V of low audio next to milliVolt signals is just begging for trouble from electric fields between heater and signal points. DC heat logically makes wiring much less critical. Paradoxically, poorly-filtered "DC" with buzz harmonics far into the midrange may be worse than pure clean AC heat.

DC supplies need fat diodes, fat caps, and much more transformer VA than simple AC heat.
 

PRR

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Am I imagining things, or has something switched the font used in threads to a sans-serif a few minutes ago? The font used in post 9 now appears everywhere. Did JeffYoung switch something on but forget to switch it off again?

Font looks same as ever. See attached. I think it has always been a sans-serif??

Font is probably set by a Style Sheet. Sometimes the server forgets to send it to you. When all style sheets are dropped this forum looks VERY different. However a partial loss of style info might give a munged font.
 

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PRR said:
Font looks same as ever. See attached. I think it has always been a sans-serif??
All looks normal again today. Still not sure what happened, but what I seem to recall is that post 9 had a different font (slightly larger?) which then seemed to appear on all threads I then looked at - not only on here but also another forum which uses the same software. Maybe I just had a brain fart?

Going back to the thread topic, in most cases decent AC wiring is less trouble for newbies than DC heaters - but some of them are never convinced.
 
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