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Old 27th September 2015, 01:38 AM   #1
jjbunn is offline jjbunn  United States
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Default Nuvistor heater current inrush question

I have six 7586 Nuvistor tubes, arranged in three pairs so that the heaters in each pair are in series, and each pair is paralleled across the heater supply, 12 VDC from a Eurorack system for modular synths, rated at 1000mA maximum.

Using a 12VDC bench supply, I measured the current draw of the heaters from turn on to quiescent running. At quiescent running the aggregate current draw from all tubes is around 450mA, which will be fine.

However, on turn on, the current jumps to over 1000mA, for a fraction of second, before falling down to the 450mA value.

I'm worried that this may damage the Eurorack PSU, and wonder whether I should add some sort of current limiter in series with the 12VDC supply to the heaters.

The PSU is this one: http://www.tiptopaudio.com/manuals/uzeusps.pdf which itself is powered from a 15V 1000mA wall wart.
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Old 27th September 2015, 02:17 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by jjbunn View Post
I'm worried that this may damage the Eurorack PSU, and wonder whether I should add some sort of current limiter
in series with the 12VDC supply to the heaters.
Looks like it uses a standard 3 terminal regulator, which normally has built-in current limiting. This will be probably be ok.
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Old 27th September 2015, 02:31 AM   #3
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Or sequence the turn on so they don't all surge at once.
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Old 27th September 2015, 04:51 PM   #4
DF96 is offline DF96  England
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The initial current may be greater than 1A. This is quite normal for a valve. Some DC supplies can be caught out if the initial short-lived surge causes an overcurrent trip. Unlikely to damage a PSU as it only lasts for a few seconds, but it might stop the valves from warming up if the PSU stays tripped.
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Old 27th September 2015, 09:48 PM   #5
jjbunn is offline jjbunn  United States
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Thanks for the advice. I'll give it a try and hope for the best!
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Old 27th September 2015, 10:39 PM   #6
ArtG is offline ArtG  United States
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Your heater current issues are the same that I recently encountered. I wrote a short article about a fairly simple solution:

Unique Tube Heater Supply
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Old 28th September 2015, 01:12 AM   #7
jjbunn is offline jjbunn  United States
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Quote:
Originally Posted by ArtG View Post
Your heater current issues are the same that I recently encountered. I wrote a short article about a fairly simple solution:

Unique Tube Heater Supply
Very nice! ... I will try this if I observe issues using the PSU I have.
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Old 28th September 2015, 03:13 AM   #8
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This is why I prefer AC heaters.
The start up current is limited by the transformer.
So long as the transformer isn't way over sized things will be fine.

Quite a few people use DC heaters to get around hum problems.
In my experience AC heaters rarely cause hum if heater circuit is implemented properly.
I always offset heater AC voltage to about 50 volts DC.
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