#124 Output Transformer

[v8pete]

Hi Guys - can anyone enlighten me on the following?

How come the photos of #124 show a sticker on the side of the OT (which is the standard Fender part) which have quite different values (ie. reflected impedance) than the standard part. The standard part reflects 2k (using the hammond data as a reference) whereas the sticker in the #124 photo refers to 1350 Ohms, ie a significantly lower turns ratio, and hence a rather differnt plate load? Am I missing something here? I'm no expert, but the sticker is presumably in Dr D's own hand?
Cheers,
Pete

[Structo]

This one?
I think it actually says 1750 ohms.

We know that HAD was obsessive about measuring everything.
He tended to label most pots and things that he could with actual measurements.

So that value is not that far afield of 2K and as we know impedance is a moving target depending on frequency among other things.

[jelle]

This is the impedance at 1kHz and, honestly, I would not call it 'obsessive' to measure things before you use them in a multi-part build, just good engineering practice, IMHO.

[v8pete]

Thakns guys for the replies. Yes it is rather hard to tell if that's a "7" or a "3" from the pic. Now you mention it, it does look quite like a "7", but it also has a curly bit on the bottom so could poss be a "3"? The bit I still have a problem with however is that the turns ratio is clearly written on the label as 18.3:1 - now taking a 4 Ohm secondary load, this would reflect back 1340 Ohms to the primary. A "standard" Fender transformer is specified as presenting a 2k load, so in this case the turns ratio must be 22.4 .

Surely if HAD wanted to alter the plate load downwards, the easiest way to do this would be to start with a standard fender p/n transformer, and remove tunrs from the secondary (this could be very easy or quite tricky depending on individual transformer construction).

If it was a "7" in the pic, rather than a "3", implying 1750 Ohms, then the implied turns ratio with a 4 Ohm load would be 20.9, and not 18.3 as on the sticker.


I gues the way to resolve this is to measure the turns ratio of an original fender item and see if it's closer to 22.4 rather than 18.3?? Would be very interesting to find out what's going on here !!

The bottom line is that it's clearly a transformer which has a turns ratio of 18.3:1 - which is all we really need to know anyway. Whether this is the case for the standard Fender part is just another matter, but a rather important one if everyone is using the Fender part in their ODS clones!


Cheers,
Pete

[martin manning]

The inductance ratio says Zpri is 1333, and the turns ratio says its 1340. I'm betting the number written for Zpri is 1350.

MPM

[v8pete]

Yep, it also stacks up with the two inductance figures as well!

Another thing that occured to me is why on earth would anyone write the turns ratio on the side of a "standard" transformer, which is an absoultly rigid parameter to which the whole production run would be built, unless they had altered it?!

I'm shooting for HAD taking the standard part and stripping off some of the primary turns!

Pete

[martin manning]

There are several reproductions of this Fender PN out there. I'd have to assume that they have been matched to the original specs using existing examples by knowledgeable people, so the ~2k assumed for the "standard" primary is probably correct.

Removing primary turns might be difficult. I think they are usually the inner windings, and interleaved. Could be a rewind, though.

MPM

[Structo]

Here it is zoomed in.
It is hard to see the 7 but if you look at his other 3's it isn't the same so I think it is a 7.

I don't think 1750 ohms is too bad for a 2K OT.

[Sonny ReVerb]

That looks like he wrote over the top of previous numbers. The '7' looks like it's written on top of a '3'. You can definitely see the turns ratio has been changed. The '18' is darker, like he went over the numbers several times to make them clearer, and you can see a '2' under the '1' in '18'.

[martin manning]

Interesting that all the numbers have been written over except the Lpri. Turns ratio looks like it used to be 20-something, and the overwrites look like they could be consistent with going from 1750 to 1350. The 18.3 turns ratio is clearly the overwrite since it's so dark. Maybe the secondary was modified with a few more turns added?

MPM

[David Root]

Assuming the 1350 is correct, what does this tell us about power, tonal character and distortion components when used with the Mesa 415-STR tubes it was photographed with, which I believe are essentially 7581As?

The 7581A data sheet shows only two differences from a standard 6L6GC, the plate dissipation is 5W higher and the plate voltage is 500 max instead of 450.

Looking at the plate curve, and laying in a slope of Za=675 (half 1350 since the curve is for one pair not two) for 420 Vp/Vs, then 675 Za is well above the knee, where Vmin is about 175V.

[erwin_ve]

This is the impedance at 1kHz and, honestly, I would not call it 'obsessive' to measure things before you use them in a multi-part build, just good engineering practice, IMHO.

Hi Jelle,

Do you measure your Ot's?

[v8pete]

Anyone able to put up a plot of the two different load lines?

Pete

[martin manning]

Looking at the plate curve, and laying in a slope of Za=675 (half 1350 since the curve is for one pair not two) for 420 Vp/Vs, then 675 Za is well above the knee, where Vmin is about 175V.

It's so far off I wonder if this OT isn't configured to drive an 8-ohm speaker, or maybe to split the difference for the half-power setting?

MPM

[llemtt]

I'm shooting for HAD taking the standard part and stripping off some of the primary turns!

+1

tweaking the OT is the key...