position of output transformer

[echo44]

Hi I am building a #124 Dumble does it matter if I move the output transformer a little farther away from the power supply transformer
this in affect brings the output transformer slightly closer to the preamp tubes by aprox 2-3 cm. Just wondering how critical transformer
position is.

[paddy]

Hi there,

It shouldn't be a problem at all.

cheers,
Paddy

[tubelectron]

The "Enemy" is the Power Transformer, as it generates the hum by induction via its laminations, mainly the core. That's a plague that no shielding can really - or at least simply - kill...

That's the reason why everything containing lamination must be as far as possible, or far enough and well orientated versus the Power Transformer, say :

- the reverb unit
- the reverb transformer
- the choke
- the output transformer
- coils of a Boogie-type Eq

Well orientated means that the laminations core axis musn't be coincident, to avoid induction transmission by field lines. You may have to do experiments before to find the best relative position, like I often do on protos.

Tubes are less sensitive to induction than transformers - or at least are much easier to shield correctly - as the have no laminations. Of course, an input stage must not stand close to the Pwr Xfo, but there will be no problem if it is as close as 2-3cm to the power trafo - if you make decent shielding of the circuit, indeed !

A+!

[Diablo1]

The "Enemy" is the Power Transformer, as it generates the hum by induction via its laminations, mainly the core. That's a plague that no shielding can really - or at least simply - kill...

A+!

I agree that correct transformer orientation and location should be your primary method to control the problem. But, it seems that you could add magnetic shielding to block the alternating flux coming from the PT. If you added a high permeability material, like a steel plate, between the PT and the other transformers, very little flux would go beyond the plate to reach the other transformers. A steel chassis would also provide similar magnetic shielding for the circuit inside.

[pablogt]

According to Morgan Jones, a steel chassis would make things much worse: "If an output transformer leaks flux into the aluminium
chassis of a power amplifier, it probably isn’t a problem
because aluminium doesn’t conduct magnetic flux. But a mains
transformer leaking flux into the steel chassis of a pre-amplifier
is a problem because the steel chassis passes the flux into
sensitive signal circuitry"

Pablo

[tubelectron]

But, it seems that you could add magnetic shielding to block the alternating flux coming from the PT. If you added a high permeability material, like a steel plate, between the PT and the other transformers, very little flux would go beyond the plate to reach the other transformers. A steel chassis would also provide similar magnetic shielding for the circuit inside.

Wrong, at least for any steel or non-magnetic metal : the reduction will usually not be significative, by experience... Usually, as there are indeed exception due to good luck...

The Mu-Metal, ImphyMetal may do something, as they are used to shield oscilloscope CRT, low level audio and measurement transformers and coils, etc... But it has to be tested by a case-by-case basis, and - most of all - the Mu-Metal enclosure must be closed to be "induction-tight" as far as possible - I mean : a screen is not enough. The enclosure must be thick (3mm or more) and the MuMetal is a pain to work... Not to speak about the price !

a steel chassis would make things much worse: "If an output transformer leaks flux into the aluminium
chassis of a power amplifier, it probably isn’t a problem
because aluminium doesn’t conduct magnetic flux. But a mains
transformer leaking flux into the steel chassis of a pre-amplifier
is a problem because the steel chassis passes the flux into
sensitive signal circuitry"

Right, particularly if the transformers are mounted flush on a steel chassis : it then acts as the last lamination common to all transformers, transmitting induction from PWR trafo to others.

A+!