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Old 26th January 2013, 07:18 PM   #1
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Default Heater supply

A schematic I plan to use shows 12AU7 input valves heated by 6.3 volts AC that floats on (approximately) 90v DC.
If I was to use 12.6 volts DC for the heaters should it still float on the 90 volt DC as above or simply be connected straight to the heaters.
The 12AU7 cathode voltage is shown as 4.4 volts so heater/cathode difference is not large.
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Old 26th January 2013, 07:22 PM   #2
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The same d.c. voltage will apply.
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Old 26th January 2013, 07:32 PM   #3
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by dudleyperkins View Post
A schematic I plan to use shows 12AU7 input valves heated by 6.3 volts AC that floats on (approximately) 90v DC.
If I was to use 12.6 volts DC for the heaters should it still float on the 90 volt DC as above or simply be connected straight to the heaters.
The 12AU7 cathode voltage is shown as 4.4 volts so heater/cathode difference is not large.
Probably you mean 'straight to the ground'
On first sight with the cathode only 4.4V ,yes.But perhaps there is a reason for the 90V.The 4.4V could be only the steady-state but with a large voltage swing superimposed.Without a schematic,hard to say.
Mona
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Old 26th January 2013, 08:15 PM   #4
DF96 is offline DF96  England
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90V seems a high voltage to elevate a heater supply for a valve with its cathode at only 4.4V. Does the supply also feed another valve with a higher cathode voltage, such as a cathode follower? In most cases there is no reason to use a DC supply for heaters, as AC is perfectly good if wired correctly.
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Old 26th January 2013, 08:58 PM   #5
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This heater circuit supplies two 12AU7 valves, one per channel.
I probably should keep the AC supply, it's simpler than making a DC supply.
I had presumed the DC would produce less hum.
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Old 27th January 2013, 01:22 AM   #6
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Historically 45 to 50 volts for 12a..7 is good to reduce noise . 90 volts are a bit high but still below the max.
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Old 27th January 2013, 03:06 AM   #7
roline is offline roline  United States
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I'm floating the 12VDC heater supply at 70 volts for a SRPP based RIAA amp... Quiet as a church mouse....
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Old 27th January 2013, 04:34 PM   #8
DF96 is offline DF96  England
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Yes, but an SRPP has one valve sitting at a high voltage so the heater supply may have to sit somewhere between the two cathodes. Normally, 40V is the recommended DC offset for a heater supply. Most decent NOS valves with good heater-cathode insulation are happy with a grounded heater supply.
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