Leak TL 25 plus OPT

[Mars]

Hi All

I have had request to look into the Leak TL25+ output transformer design, and clone or redesign it. (I am not trying to reinvent the wheel, or improve the work of the illustrious Leak Company, but only to fill in the gaps of information, where information is wanting)

Here is the schematic:



From the voltages noted on the schematic, I can calculate the expected current running through each output tube, and by inference through each section of the OPT.

So 30V/440ohms=68mA DC

There is more info on this page http://www.44bx.com/leak/transformers.html

Here I see the transformer is called the 3925 and the primary impedance is 7000 ohms with UL taps at 49%. The DC resistance from anode to anode is 102 ohms.

If I punch all this data into OPT design assistant I get this design:



Completely do-able on a 60mm stack of EI96 laminations.

Now the formula for power and primary impedance is

(Anode-to-Cathode-Voltage)/Power-Output

Thus from the schematic

420 X 420 / 25 = 7056

which is spot on.

Now the request is to design a beefier OPT to handle the higher currents of KT88's and 6550. These tubes would idle 100mA DC. Another suggestion was made to design for 35 watts output (to have a bit of headroom).

Let us explore this. Jacking up the current, does not need more iron. The thickness of the copper and the resultant heat is the issue as can be see from this screenshot. I need to keep copper losses not more than 10%. This will only happen when the current gets to 180mA - well above the KT88 margins.



To get 35 watt though the trafo gets bigger:

Now we need to use EI114 laminations:



You need a voltage of  sqr-root(7000 x 35) = 495 volts to realise this. It is not so difficult to do this. One can pull the tube rectifier and fit a two diodes and a resistor to get the voltages up. You need to check you filter caps max voltage though.

Otherwise to get 35 watts out of this same amp with 420V on the plates, you'll need a 5000 primary impedance which leads to this:



But now the trick is to design an OPT that can run on EL34's and KT88! To put taps on the Anode and Screen grid taps is impractical. Rather tap the secondary:



This turns out to be a 16 ohm tap! Now it is unwise to tap a secondary in a parallel arrangement. So if we change our secondary design to a 2 sections in series and both in parallel, we have full secondary layers connected in 5000 and 7000 ohm pri imp configurations.

We have gone the full circle and are back to a OPT with a 16 ohm tap! The final design I'll post next.

[Mars]

To continue with the proposed design for a TL25 OPT that can do EL34's as well as KT88's in the same amp. To be able to take advantage of the extra power that KT88's afford, we are proposing to tap the secondary. That will allow 25watts with EL34's/KT66/6L6 and 35watts with KT88 by just adjusting the cathode resistors and switching the speakers to another binding post. (The assumption is that you have 8 ohm speakers)

To tap the secondary, makes it necessary to consider another way of arranging the secondary windings. Whereas the above designs use parallel secondaries, it will be better to do an arrangement of 2 pairs in series the whole lot paralleled.

So let us use the 35 watt design with the 5000 ohm primary impedance, and make a tap in the secondary that makes the reflected primary impedance 7000 ohms.

[Mars]

This is what it will look like:



The fact that secondaries are in series, means that thicker copper should be used - thus the 1mm Cu.

[Mars]

Now for the same transformer to be a 7000 ohm primary impedance into 8ohm for the same amount of primary windings (2372 windings) the secondary windings needs to be reduced to:



80 windings. (Compared to the 94 windings of the 5K version)

So when we roll the transformer, we'll put taps on the second and fourth secondary winding 14 windings before the end.

This is a rather large transformer: 60mm stack of 114mm laminations. It is probably as large, if not larger than the Power Transformer. That's what you get if you want to go as low as 20Hz at 35watts!

What do you think?

[GearSlave]

What do you think?

I'm loving this thread, that's what I think!!!!

[Timber_MG]

An almost 50% UL Tap... how would one get that to a choice between 25% or 43%?

This is absolutely awesome, followed

[Mars]

Well, the easiest is the 50% UL tap. We have 4 equal sections in the design above. The tap is where the primary section joins the next. This is so close to 43%, I would leave it at that.

There is a total of 8 layers in each primary section. 25% of that is 2 layers. So after two layers one can fit a 25% tap. So there will be quite a bunch of wires connected to the primary if one allow both 50% and 25% options.

Since all these numbers are even, it means that these taps will all protrude on the same side of the bobbin.

[Mars]

Here is the proposed winding arrangement:

[charles]

Dear Mars,

All the best with the project. As a collector of Leak valve amplifiers it is interesting to hear that somebody wants to clone the Leak's OPT.

A very scare OTF, is that of the TL12 point one. Many attempts were made to clone it to its original electrical specifications and form, but most were unsuccessful.

Kind Regards

Charles





     

[Mars]

Thanks for the encouragement Charles. It is not strictly a clone of the transformer, since I do not have an original in hand to strip, nor any information on how the original was assembled. The idea is to do a transformer that would do the circuit justice for a DIY project. If anyone could give some suggestions on how to improve the design, please post here.

I could get this design of the 8615 OPT here http://www.44bx.com/leak/8615tx.html



If anyone can point us anywhere with more info, it might be useful.

I just follow regular design practice (still learning every day). I prefer the 4 section primary and 4 section secondary with one section split on the outer for better high frequency response.

So the my design is pretty straight forward. Maybe Mr Leak used parallel primary windings of very thin wire to lower distortion like in the A470? Please do not ask me to wind something like this! (yet)

[charles]

Dear Mars,

In the book " Firsts in High FIidelity" (history of Leak) by Stephen Spicer there is some information regarding the winding methods of the said transformers. If I am correct, the later transformers (excluding TL12 point one), were done with the interleaving method.

Kind Regards

Charles


 

[Timber_MG]

I am reading all these threads with great interrested. I am currently building a modiffication to my CNC router's Z-axis to fit a rotary axis to wind some OPTs. Some of these qires are already small and bifilar winding would make for some very fine work.

[Yves]

Hello Mars,
Here are some idea on how to make multiple impedances secondaries:

http://www.dissident-audio.com/AutoIndex/index.php?dir=OPT_da/Papers/&file=4-8-16.pdf

http://www.dissident-audio.com/AutoIndex/index.php?dir=Transfos/Dissident/Fiches/&file=DA172.pdf

Have fun !

Yves.
psst ! you should simplify your interleaving scheme 

[Ampdog]

Karel,

Very good! Due respect to Yves, but if by interleaving it is meant interconnecting of sections, I would stay with the way it is. Whenever one goes slightly class-AB (as valve designs often do), it is important that primaries should also have low mutual inductance, as is the case with Karel's schematic.

Also to just remind that EL34s work lower distortion with a 1K resistor in series with the screen (see the data), on 43% taps (50% in this case - the difference is a <10% lower max. output). KT88s are a different animal, no series G2-resistors.

Yves,
I could not open those references; display shows an error tab. Are they correct?

Off topic:
Charles,
The TL12 output transformers were quite normal (could in fact be improved). If you are the Charles I know I will chat to you on landline.

[charles]

Dear Ampdog,

I am the one.

Kind Regards

Charles