Output transformers... which one...?

[oodi]

Hi,

I am looking at the various Edcor output transformers. I am considering the XSE or GSXE series. But I am somewhat confused as to which to choose.

Let's say I would like to use an Edcor instead of a Hammond, then a XSE10-8-5k should be a suitable alternative to the 125CSE, correct? The Hammond is rated at 8W and the Edcor at 10W.

When I look at the various amps people have build using EL34, I see the following transformers recommended: Hammond 125ESE, Edcor XSE15-8-5k, or Edcor GXSE10-8-5k. Now the same circuit has output transformers of a wide range of power ratings specified. My questions, if a GSXE10-8-5k is suitable why then the XSE15-8-5k and not the XSE10-8-5k?

I am building a SE amplifier using a ECL82. What size output transformer would be needed? Would a GXSE5-8-5k be sufficient or should I look at the GXSE10-8-5k?

I am sorry for all these silly questions. But I am really at a loss to properly size the OT. Mostly been looking at what others have done and trying to learn from those experiences. But the OT issue has me greatly confused.

Thanks,
Adi

[TheGimp]

I'm still on the steep incline of the learning curve on transformers so if I'm off base I expect someone else to chime in.

I would venture to guess that the greater iron content of the XSE15-8-5K compared to the GSE10-8-5K would allow more GNFB to be used to increase the low frequency response. This would be most noticable as you approach saturation of the core (extremes of the B/H curve). The EL34 is capible of 10W output and that puts it on the edge for some 10W rated transformers. Look at the weight of the 10W transformers and see if some are heavier than others.

A larger rated (Power) transformer would also produce lower distortion as you would not be approaching the more non-linear ends of the B/H curve.

With a PO rated at 3.5W for the ECL82, I would expect the GXSE5-8-5K to be sufficient, but you might get a little more base out of the GXSE10-8-5K with appropirate GNFB. I would not expect you to have saturation problems with either.

For a guitar amp, the smaller transformer might be desirable for added distortion.

[oodi]

I know that weight seems to be one aspect to value a transformer by, the heavier the better. But obviously there is also a point of being too heavy. Looking at the specifications of these transformers, I just now realized to compare weight and dimensions, I see:

XSE10-8-5k
1lb 2.3" x 1.9" x 1.7"

XSE15-8-5k
1.75lb 2.7" x 2.5" x 1.8"

GSXE5-8-5k
1lb 2.3" x 1.9" x 1.7"

GSXE10-8-5k
2lb 2.6" x 2.2 x 2.5"
this one is an enclosed style with end bells

125CSE
1.3lb ?? x 2.3" x 1.9"

125ESE
3lb ?? x 2.9" x 2.1"

I suppose I should have compared transformers all along like this, rather then just the electrical specs So the XSE10 and GSXE5 are physically very similar. The XSE10 is rated 70 Hz to 18 kHz whereas the GSXE is rated 40Hz to 18 kHz. It makes sense when comparing like rated transformers that the transformer with better low frequency response is the heavier transformer.

That still leaves the one question about which transformer is the more appropriate for the two designs I am playing with:

ECL82 SE - XSE10-8-5k or GSXE5-8-5k
EL34 - XSE15-8-5k or GSXE-10-8-5k

Does that sound reasonable...??

Adi

[TheGimp]

Smaller transformer, less iron, higher cutoff for low frequency response.

In a PP amp, the transformer is not so critical to the bias current of each tube as it is to balancing the current to prevent offset and saturation of the core. So in the case of a PP amp it is possible to use a 20W transformer to build a 8W amp without upsetting the output transformer.

In the case of the SE output, the bias current is taken care of by gapping the core to prevent saturation.

How does bias current effect operation, given that one does not exceed the max that the transformer is designed for? Hopefully someone else can answer this question.

Does running a transformer at half the specified bias current upset the transfer characteristics? If so it is best to stick to the transformer with the closest power rating to fit the design; GSXE-10-8-5K for the EL34 and GSXE5-8-5Kfor the ECL82.

If not, then I'd go with the better transformer and run the GSXE15-8-5K for the EL34 (You didn't list this one). I think I'd still stick to the GSXE5-8-5Kfor the ECL82.

[rknize]

I would say that Edcor's XSE series is a rough equivalent to Hammond's 125SE series and their CXSE equates more to Hammond's 1627-1642 series. The GXSE is somewhere in-between.

The best solution for you really depends on what you are driving and how much you are willing to spend. Get the best transformer you are willing to afford.