Help measuring a Luxman SQ38 signature Output Transformer

[fixproaudio]

*Adding for clarification - im intending on swapping out both transformers.

I am a bit new to transformer measurements, but I am working on an out of commission hifi amp (Luxman SQ38 signature) - Class AB ultra linear with 4 EL34s rated at 40W. 1 Transformer is good and I put in a known low voltage (~3V) and measured the secondary outputs - Got a turns ratio of about 26.7 and an impedance of 713.2 (turns ratio squared). At 8 ohms this comes out to about 5705.1 ohms primary side effective resistance. I found a made to order transformer with primary taps at 5.2k and 6.6k push pull rated at 35W - I would love to know what sort of performance difference I can expect from the original. Any insight?



Mainly im not sure if measuring at such a low test voltage (spec plate is 475V) would result in me getting a huge error in the turns ratio. I would imagine pumping 475V would heat the coils a bit more effectively changing the primary Side impedance.



There is little to no information on this amp and the company is not giving me any information as well. Best I have it a diagram from the manual.

If anybody has any good links to transformer I can purchase for this amp I would love to check them out.

much love,

Jon

 

[mad.ax]

Since it's a stereo amp, you want both channels to be as identical as possible. In order to preserve that symetry, I believe you have two options, either you find a Luxman original OT, either you replace both OTs.

Axel

 

[fixproaudio]

Hi Axel!

I definitely intend to change both transformers sorry if this wasn't clear. Editing original to reflect this.

 

[mad.ax]

All good then...
With so little info about this particular amp and its OT, we cannot predict "what sort of performance difference you can expect from the original"...
If the synoptic is to be believed, this is a simple push pull OT. No ultra linear taps, no fancy dedicated feedback winding, just one 8Ω output winding...
Of course you should trace at least the output stage to check those assumptions!
You could also measure the one channel that still works to determine the bias current (seems to be fixed bias).

From there, I guess chosing a replacement will be mostly a mater of size, availability, and budget!

Or if you really want to know more about the Luxman OT, maybe you can send it to CJ (our resident traffo guru) so he'd give it the hacksaw treatment and analyse its guts

Edit: If size match, those might be a good option:
Luxman OY14 5K

Axel

 

[fixproaudio]

Thank you Axel.

The link you sent me seems like it could be a viable option.

The outputs on the secondary go directly to the speaker outs

The primary side is split between 2 coils and from what I can measure the 2 centermost ends of those coils, of both transformers, are joined (and from the diagram, which I've attached) and sent to a part of the secondary side of the power transformer for some sort of Ultralinear? The screens are attached to this point, but are first put through an inductor (choke?). Im a bit of a noob when it comes to ultra linear. But as far as the output tran is concerned its 4 wire push pull I would guess.

 

[mad.ax]

Output transformer is a classic push-pull. 4 wires are represented, but it could as well be 3 wires since the two inner ones are joigned to B1 that goes to the first tap of the power supply. From there a second LC stage of filtering feeds the screen grids through point B2.
It's a pretty classic configuration. The Marshall JMP for instance is done the same way, but of course the iron is different

Axel

 

[fixproaudio]

Thanks Axel,

I have the transformers out of the unit and I can confirm that it is a 4 wire primary side. They are joined at the other end post connector.

Appreciate your insight.

Jon

 

[mad.ax]

So this confirms that your SQ38 Signature is NOT ultralinear as that would imply at least 5 wires on the primary side of the OT.
Actually I couldn't find any reference to ultralinear for the Signature edition of that amp. There's a version named SQ38U but despite using EL34s too that is a different animal.
Attached is the SQ38U diagram and you can clearly see how the grids are tied to the ultralinear taps of the primary (5 wires).
Note that the phase splitter is different too althought this is not related to the ultralinear topology.

Anyways, now that you got the transformers out, you can take precise measurements to check if the OY14 would fit.
Are there any references or indications on your transformers?

 

[abbey road d enfer]

The screens are attached to this point, but are first put through an inductor (choke?).

These are the usual screen resistors. Typically 470-1000 ohms 1-5W.
They are connected to a slightly lower voltage, after the choke, because being grids, they could amplify ripple.

 

[abbey road d enfer]

I would love to know what sort of performance difference I can expect from the original. Any insight?

As Axel mentioned, any significant difference will not result from a mere impedance difference. Tube amps are more versatile than is generally believed. Just consider how the loudspeaker impedance (and thus the impedance reflected at the primary varies, typically a 1:10+ ratio.
The dominant factors are the operating induction and core permittivity (how far from saturation), leakage inductance (governs HF response), nominal inductance (governs LF response), resistive losses, iron losses, and whatnot. All these parameters are generally undivulged.
Some hints, though: core size (usually the bigger the better) and construction (heavily sandwiched construction results in better coupling between primary and secondary, hence reduced leakage).

 

[fixproaudio]

Thank you both for your responses.

Axel,
Not sure where I saw where it was defined as ultra linear, so my mistake then. That OY14 is looking better by the minute - main diffe. Only information at all on the original transformer was on a tag underneath:

TL00341A01C
74GT

Abbey Road,
This is great information thank you.

 

[fixproaudio]

Just a heads up - I purchased and installed the OY14 and the unit sounded absolutely stunning. Thank you all again.

 

[erickPH]

Japanese Teardown of OY-15-5 output transformer.

http://musenan.blogspot.com/2011/08/lux-sq38fd.html?m=1

 

[volker]

Japanese Teardown of OY-15-5 output transformer.

http://musenan.blogspot.com/2011/08/lux-sq38fd.html?m=1

Holy mother of splices. Not for the faint of heart.

 

[erickPH]

Holy mother of splices. Not for the faint of heart.

Hi Volker,

I allready try the winding sequence on my DIY output transformer on original design they made of 16 layers primary .19mm wire and 8 layers secondary .44mm wires,

i'll send the photo once i found on my PC.

thanks

 

[erickPH]

 

[erickPH]

An audio Transformer teardown from a japanese blogger, i only make my own layout base on my understanding of how the winding of a luxman OY5k.
I didn't open an actual Luxman Audio transformer,