• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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.
 

Rundmaus

Member
2005-08-21 10:46 pm
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
 
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Rundmaus

Member
2005-08-21 10:46 pm
Don't get drawn into the "No sand allowed trap".

:D

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
 
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


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
 

Rundmaus

Member
2005-08-21 10:46 pm
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...
 
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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!
 

Rundmaus

Member
2005-08-21 10:46 pm
Hi there,

some Thread-Jacking going on^^:

I found a lot of web sites claiming that half-wave rectification causes core saturation.

I wouldn't think so but I am eager to learn.

So anyone out there who can explain (and prove) why this should happen?

Simplified thoughts on my side, and probably missing the point:

a) Saturation occurs when the flux created in a transformer core by by a winding exceeds the maximum allowable flux (determined by the core material).

b) A DC current in a transformer secondary winding (pre)magnetizes the core in one direction, so it is closer to a saturation situation in one field direction.

c) The primary winding of a mains transformer is designed to magnetize the core close to the saturation limit. It will magnetize the core close to saturation alternatingly in both directions

d) If the core is premagnetized in one direction, it will saturate in the corresponding half-cycle of the mains.

Up to here, it seems I gave the answer myself. But:

e) In our case, the 'DC current' in the secondary consists of sinusoidal half-waves which look just the same as in the full-wave rectification case. Onlye the negative half-waves are missing.

f) As the law of induction tells us, the induced current has a direction that counteracts the cause of the induction (primary current / magnetic flux). So if secondary current flows only in half-cycles, it should *lower* the total flux in the core by a certain amount. So why is there saturation?

Where do I go wrong?

Rundmaus, scratching head intensely....
 

CarlyBoy

Member
2006-08-08 9:13 pm
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....

While true in theory (and, of course, it also depends how big that first cap is), I have found the multiplier to be closer to 1.8 in practice ..at least for no-load DC voltage. So in this case, for example, I'd expect the no load voltage to be 320 * 1.8 = ~575 VDC.
 

CarlyBoy

Member
2006-08-08 9:13 pm
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....

While true in theory (and, of course, it also depends how big that first cap is), I have found the multiplier to be closer to 1.8 in practice ..at least for no-load DC voltage. So in this case, for example, I'd expect the no load voltage to be 320 * 1.8 = ~575 VDC.
 

Rundmaus

Member
2005-08-21 10:46 pm
To be honest, I never experienced a factor of 1.8 in any power supply. The 1.41 is simply the peak amplitude of a sinusoid AC voltage. With an AC voltage source of x volts (eff), the peak excursion is 1.41x. You can't charge the filter cap higher than to the peak voltage.

The only effect I can imagine is transformer regulation - a secondary winding specified to deliver y volts AC at z amperes may well give you 1.1y to 1.15y without a load. Making you read a higher DC voltage than expected from the specified winding voltage.

But even then, U_DC <= U_peak holds.

Greetings,
Andreas