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Old 20th September 2010, 09:23 AM   #1
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Default 5V AC maining for filaments

Hello,

Is there any trouble maining 6.3V filaments with 5V AC?

Tanks
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Old 20th September 2010, 10:38 AM   #2
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yes. 5 V is too low , the valves will not work well. you need 6.1 to 6.3 Volts, AC or DC.
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Old 20th September 2010, 03:49 PM   #3
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Plenty of 4.7-5 V tubes you could use, costing pretty much peanuts... 5V6, 5AQ5, 5CZ5 as power tubes, 5U8, 5GH8, 5AN8, 5BQ7 for preamp - 5HA7 or 5HC7 if you can find 12-pin sockets.
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Old 20th September 2010, 04:04 PM   #4
SY is offline SY  United States
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6V transformers are even cheaper...
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Old 20th September 2010, 04:15 PM   #5
TheGimp is offline TheGimp  United States
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Schottky diodes and a big cap and you can run them off DC.
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Old 20th September 2010, 04:43 PM   #6
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how?...
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Old 20th September 2010, 04:57 PM   #7
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A Bridge Rectifier using Schottky diodes followed by a big 20,000 microfarad cap should work.
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Old 20th September 2010, 05:37 PM   #8
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Ah,yes...ok, but the question was not how to power DC voltage, but if is possible to power with lower AC voltage...
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Old 20th September 2010, 05:42 PM   #9
TheGimp is offline TheGimp  United States
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Schottky diodes are necessary as they have a very low voltage drop, around 0.3-0.5V per diode. If you use a diode bridge at less than half it's rated current you can hold the voltage drop to around 0.8V. So a 5 V transformer would provide (5*1.414)-0.8=6.27V to the heaters.

If you run the heaters at less than nominal-5%, the tubes don't work well.
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Old 20th September 2010, 05:45 PM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by mosfetmaster View Post
Ah,yes...ok, but the question was not how to power DC voltage, but if is possible to power with lower AC voltage...
Yes you can, at least with preamp valves, but the characteristics of the valves will not match the data sheet. (lower gm, higher internal resistance).

Reduced heater voltage is not recommended for rectifier valves or power valves.
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