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Need Some Help With a DC Heater Supply Rectifier Current Rating.
Need Some Help With a DC Heater Supply Rectifier Current Rating.
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Old 5th November 2017, 07:50 PM   #1
satyrnine is offline satyrnine  United States
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Default Need Some Help With a DC Heater Supply Rectifier Current Rating.

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?
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Old 6th November 2017, 03:14 AM   #2
PRR is offline PRR  United States
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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.
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Old 6th November 2017, 04:09 AM   #3
dotneck335 is offline dotneck335  United States
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Put a heat sink on the rectifier.
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Old 6th November 2017, 04:12 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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I would try it with AC heaters to start with.

The last time I tried DC heaters it didn't kill the hum, the problem was elsewhere.
Since then I just stick to AC heaters. Its a lot less hassle.
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Old 6th November 2017, 10:36 AM   #5
DF96 is online now DF96  England
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For 1.2A of DC to the heaters you do realise that the secondary has to be rated for 2-3A AC current?

To make life easier for your rectifiers (and the transformer), use a smaller reservoir cap. Heaters are designed to run on AC so they do not need the sort of pure DC that a preamp needs.
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Old 6th November 2017, 02:30 PM   #6
kodabmx is offline kodabmx  Canada
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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.

Last edited by kodabmx; 6th November 2017 at 02:33 PM.
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Old 7th November 2017, 01:46 PM   #7
dotneck335 is offline dotneck335  United States
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DC-elevating the heater supply is probably be more effective at reducing hum than rectifying/regulating it.
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Old 7th November 2017, 05:40 PM   #8
kodabmx is offline kodabmx  Canada
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In the first post he said it was elevated to 100V...
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Old 7th November 2017, 06:49 PM   #9
JeffYoung is offline JeffYoung  Ireland
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Need Some Help With a DC Heater Supply Rectifier Current Rating.
I did a quick mock-up of your supply in PSUD2, and the current draw on the transformer is ~ 2.4A.

Reducing your CRC filter to just RC drops the load to ~ 2A.

Keeping a CRC but dropping to 4,700uF each side doesn't help as much: ~ 2.3A.
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Old 7th November 2017, 08:35 PM   #10
DF96 is online now DF96  England
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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?
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