• 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.

Series connecting output Transformers.

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In my pursuit of high end hi-fi on the cheap I was trying to work out if it was possible to get higher impedance matching of transformers by connecting two in series. This would be in a PP amp so the primaries would have to be cross coupled to get current balance in the two series connected transformers. All the secondaries of the two transformers would be connected in parallel.
The reason that this would become useful for me is because it would allow me to develope a 5K transformer out of two 6V mains toroidal transformers.

What do you think.

Shoog
 
Hey Shoog, I love all of the work you're doing with 'off the shelf' toroids. Keep it up. The idea of trying to balance the PP currents in a toroid scares the hell out of me, but if you make it work then more power to you!

Anyway, series connecting (identical) transformers will double the inductance. Another way to look at it is that the AC voltage across each will be half of a single unit.

-- Dave
 
Heres my idea.

Hey Shoog, I love all of the work you're doing with 'off the shelf' toroids. Keep it up. The idea of trying to balance the PP currents in a toroid scares the hell out of me, but if you make it work then more power to you!

With single toroidals getting the DC standing current balanced is simply a matter of putting matched CCS's in the cathodes of the output tubes. Since this will improve the performance of standard PP transformers it seems stupid not to use it everywhere possible.

Shoog
 

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Hi Shoog,

I'm new to valves and also like to keep costs as low as possible (http://www.diyaudio.com/forums/showthread.php?postid=1097170#post1097170)

I have a rescued working Rogers Cadet amp (see link above) and am using it to learn about valve amplification. I was thinking about using the ECL86 output valves to try a new output stage (probably a baby huey - http://www.diyaudio.com/forums/showthread.php?threadid=72536) I was thinking about output transformers and wondered about PS toroids as per the Zeus amplifier or just trying to wind my own (multifilar windings) and I just discovered your posts. You seem to have a lot of experience in using toroids.

As I'm also from Ireland, I wondered what toroid would work as a substitute output transformer for these ECL86 valves - current OPT in Cadet is 8-9kohm. Anything from Radionics that would suit? Would this require two connected in series?

Any help greatly appreciated
John
 
Thanks mississippi - can I ask where you got this from - I wish to learn, looked up plate resistance (I think this is what it's called?) but didn't find this formula or was too stupid to notice it!

How does dual secondaries change this formula - if connected in series/parallel? Also based on the first post by Shoog - he wants to develop 5K OPT by connecting two 6V trannies in series - according to my calcs he would get 11.7k from a single 230V/6V trannie - what am I missing?

This means the Pri Impedanve changes with the speaker load? My speakers are Rogers LS3/5as nominal impedance 15ohm but this obviously varies over the audio range.

Am I right in thinking that the ECL86 tube working in PP configuration works best against a Pri impedance of 9Kohm?

So many questions - just a noob

John
 
This formula is the normal, standard formula from transformers and its impedance. You should had it on school!
And yes when the secondary impedance is changing the primairy impedance is changing too. But that is no problem, the power what is given to the speakers is than a bit less or a bit more. That no problem for the tubes. may be it's 10 or 15 % and 3 DB more or less in power you cannot hear.
 
Jkeny,
Your overall situation is a lot better with regards to using toroidals as outputs, as you are using 15ohm speakers. This makes the range of transformers availble to you much greater.
The thing you have to bare in mind is that toroidals will not tolerate any DC current. This means that if your circuit is SE then you will need to convert it to parafeed. This may or may not be easy. Microwave oven transformers make food parafeed plate chokes.
If the design is PP then things are considerably easier.

Feed me some more info and I will try to help.

Shoog
 
Thanks Shoog,

As I said I have 15ohm LS3/5as (& Jordan Jx92S full range 8ohm?). I am redesigning a Rogers Cadet III integrated amplifier which uses 4 ECL86 tubes in push-pull config in the amplifier stage.

I thought about reusing this Cadet for the chassis, PSU transformer & front end preamp & redesigning the amplifier section & upgrading the PS section. One of the limitations of this amp is the output transformer & I reckoned on replacing with a toroid. Existing Caedte trafo is 9K.

I have two PS toroids 230V 25-0-25V 160VA - based on my calcs I would get 1.2k Pri impedance running 15ohm speakers from one secondary of this trafo. How does dual sec come into the picture? Do I need to unwind about half the secondary to get it down to about 8V to get 9K impedance?

Hope this is sufficient info?

John
 
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