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Old 24th July 2012, 05:21 PM   #1
Magz is offline Magz  United States
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Default Separate Filament and B+ Switches

Hello all,

In the process of building a tube rectified 2-box 26 preamp and I'm planning to have two switches on the power supply box, one for filament power and one for B+. There are separate filament transformers for each 26 tube, while the rectifier tube gets its filament supply from a winding on the power transformer.

The question: Since the rectifier tube is needed to supply the B+, does its filament need to be switched with the other filaments, or should its filament be switched on as the B+ is switched on (as the main power transformer is powered up)? I can think of arguments for both.

If the rectifier filament needs to be switched with the others, I'd need to buy another filament transformer, whereas the other alternative is already covered.

Any help is greatly appreciated.
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Old 24th July 2012, 05:32 PM   #2
DF96 is offline DF96  England
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Why not have one switch? If you need sequencing (which I doubt) then this should be automatic, otherwise you will sometimes get it wrong.
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Old 24th July 2012, 05:39 PM   #3
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I believe that give power to the plates of the rectifiers, with the filter caps empty and the cathode warm, may be dangerous for the rectifier itself. I prefer to give the filament temperature, so slowly charge the electrolytic caps. Itīs only my opinion.
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Old 24th July 2012, 05:42 PM   #4
Magz is offline Magz  United States
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I could go with just one switch, but I thought it might be useful to be able to warm up the filaments before applying B+, especially when using DH rectifiers. I plan to evaluate a wide gamut of rectifiers, both DH and IDH. I will be the only person ever turning the preamp on (in my lifetime!) and the switches will be labeled and in order from left to right.

Philosophy aside, if I want to have two switches, should the rectifier filament be switched with the other filaments or with the B+?

I've been planning this preamp for months, and my initial thoughts were to switch the rectifier filaments with the B+ because the rectifier supplies the B+, after all...but months of thinking about every detail made me wonder if that was actually correct, So I thought I'd ask to be sure.


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Last edited by Magz; 24th July 2012 at 06:00 PM.
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Old 24th July 2012, 05:45 PM   #5
Magz is offline Magz  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
I believe that give power to the plates of the rectifiers, with the filter caps empty and the cathode warm, may be dangerous for the rectifier itself. I prefer to give the filament temperature, so slowly charge the electrolytic caps. Itīs only my opinion.
By the way, if it matters, the PS is choke input, with only PP film caps.

So Osvaldo, you're saying it would be safer to switch the rectifier filament at the same time as the B+, correct?
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Old 24th July 2012, 05:57 PM   #6
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Quote:
Originally Posted by Magz View Post
So Osvaldo, you're saying it would be safer to switch the rectifier filament at the same time as the B+, correct?
Definitively yes. Yes, for me, obviously.

Although some reserves can be made. In my project, (search in this forum), if the rectifier tube has fast hot time (like 5U4), the initial current surge I believe may be dangerous. My project include a 3DG4 that has a longer hotting time, and also 12Hy choke input. The filaments of all set but that of this rectifier are powered simultaneously, and independent of the +B. 3DG4 filament is taken from +B trafo, so only it warms when +B is switched on, before normal amp heating. All trafos are toroidal hand wired.
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Last edited by Osvaldo de Banfield; 24th July 2012 at 06:14 PM.
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Old 24th July 2012, 06:15 PM   #7
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Hi,

unless you're using a mercury vapour rectifier which requires pre-heating for about one minute or so, you can turn on heater and B+ supplies at once.

Cheers!
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Old 24th July 2012, 06:29 PM   #8
Magz is offline Magz  United States
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Originally Posted by Kay Pirinha View Post
Hi,

unless you're using a mercury vapour rectifier which requires pre-heating for about one minute or so, you can turn on heater and B+ supplies at once.

Cheers!
Thanks for the reply! So the rectifier filament will get switched on with the B+, as originally planned.

Sometimes having too much time to think just muddles the mind...
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Old 24th July 2012, 06:32 PM   #9
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Quote:
Originally Posted by Kay Pirinha View Post
Hi,

unless you're using a mercury vapour rectifier which requires pre-heating for about one minute or so, you can turn on heater and B+ supplies at once.

Cheers!
Hi Kay. You possibly are right. My opinion was only this, a opinion. I prefer to start heater and +B at the same time.
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Old 24th July 2012, 06:40 PM   #10
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