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Old 4th February 2013, 06:00 PM   #1
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Talking High voltage winding no center tap?

I just picked up several Identical power trannies from tube tv's for nothing. 3 of them are the same. 5v winding 6.3v winding and a high power 322vac winding. Problem is that the 322v winding has no center tap. I was wondering if anyone has experience dealing with this problem. The trannies are RCA branded, and they all ohm out the same. Could I use the 320 from one and the 320 from another and use one rectifier for 2 trannies? Can I use the winding without a center tap? Does not seem like a complete circuit... I'm hoping to make some high power tube monoblock amps but I am just getting started, and I figure the first step is to design a heavy duty power supply. any advice would be appreciated.
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Old 4th February 2013, 06:21 PM   #2
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Use a solid state full wave bridge. Don't get drawn into the "No sand allowed trap".
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Old 4th February 2013, 06:25 PM   #3
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If you go for solid-state bridge rectification, you may go without the center tap - this would give roughly 320*1.41=450V DC with a cap-input supply....

Depends on the DC voltage you need, in principle you could wire the 320V secondaries of both transformers in series (mind the phase!) and use the connection as center tap and then use full-wave rectification with a vacuum rectifier. Only one of the secondaries is used at a time then, allowing for a higher current draw than in the bridge rectifier. The drawback: Only 450V (as before, using cap input supply) out of in principle 640VAC...

Of course, you may also wire them in series, use solid-state bridge rectification and get 900V DC with cap-input supply... But this is *not* a B+ range I would suggest for a first build! High-voltage power supplies can kill, and they will do if you're not careful! Keep that in mind, and read the safety thread at the top of the tube forum!

First, find out what you're going to build and what B+ you will need!

Greetings,
Andreas

Last edited by Rundmaus; 4th February 2013 at 06:32 PM. Reason: Added safety note.
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Old 4th February 2013, 06:28 PM   #4
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Quote:
Originally Posted by BZed View Post
Don't get drawn into the "No sand allowed trap".


Yes, it may be a trap, if you're doing it because 'it *must* sound better without sand'.

Following the vacuum tube rectification path in my own current project, but simply because I liked the idea of trying to make the whole amp using only vacuum devices - not for the sake of (questionable) superior sound quality, but simply for the engineering challenge...

Greetings,
Andreas
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Old 4th February 2013, 06:56 PM   #5
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If you want the slow ramp up of the B+, use 4 UF4007's as a full wave bridge and then follow with a 5AR4. Use the 5 volt winding to power the 5AR4. Those RCA transformers were for color TV's and are good for at least 250ma of B+ and some are good to 500ma.
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Old 4th February 2013, 07:14 PM   #6
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Hi!

As has been mentioned above, these transformers are meant to be used with a full wave bridge rectifier. This can be all solid state, but also all tube, as described in this article:

VinylSavor: Tube of the Month: The 6AX4

Or tube/solid state hybrid as in the PSU of this amp:

VinylSavor: Making of a SE 6CB5A amplifier: circuit


Quote:
Originally Posted by Midnightmayhem View Post
Could I use the 320 from one and the 320 from another and use one rectifier for 2 trannies?
Problem with this is that the current would always been drawn in one direction in the individual secondaries, which could cause core saturation. This can cause the transformer to mechnically buzz unless you only draw very little current.

Best regards

Thomas
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Old 4th February 2013, 07:28 PM   #7
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Quote:
Originally Posted by Vinylsavor View Post
Problem with this is that the current would always been drawn in one direction in the individual secondaries, which could cause core saturation.
There is no DC component and the maximum AC excursion is not higher than in any other rectification scheme - the core should be up to that or the transformer is badly designed and will hum in *any* application.

Greetings,
Andreas

EDIT: What you probably mean ist the following: If the current in the secondary flows in only one direction, it could in principle magnetize the core, if the core is not perfectly 'soft' in a magnetic sense and exhibits significant remanence. But even in this case I would doubt any measurable effect, as the field of the primary still switches core magnetization with the mains frequency...

Last edited by Rundmaus; 4th February 2013 at 07:36 PM.
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Old 4th February 2013, 07:36 PM   #8
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Hi Andreas,

If two windings of two separate transformers are wired in series to get a center tap, as the OP asked, current will flow in one direction only in each of them if they are used with a FW rectifier using 2 diodes. This will cause core saturation.

Thomas
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Old 4th February 2013, 07:37 PM   #9
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I think I will try solid state rectification. I just got the end caps off of one and there is no center tap hidden inside. Sounds like i'll be ok without it. I can just tie up the 5v rectifier winding and add a standby switch to protect output tubes on startup. Sounds like i need to order some diodes...
Thanks for your help!
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Old 4th February 2013, 07:43 PM   #10
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Quote:
Originally Posted by BZed View Post
Don't get drawn into the "No sand allowed trap".
Then you won't be able to listen to music when the nukes fall !
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