From stephen.delf–(at)–elcom.gen.nz Tue Feb 20 13:19:32 CST 1996
Article: 6509 of rec.audio.tubes
Path: geraldo.cc.utexas.edu!arlut.utexas.edu!news.io.com!imci4!newsfeed.internetmci.com!howland.reston.ans.net!gatech!news.jsums.edu!news.uoregon.edu!waikato!comp.vuw.ac.nz!welcom!stephen.delft
From: stephen.delf–(at)–elcom.gen.nz (Stephen Delft)
Newsgroups: rec.audio.tubes
Subject: Transformer placement
Date: Fri, 16 Feb 1996 12:07:00 GMT
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In reply to: Peter Mosinskis Subject: Transformer placement

PM>I understand that the power and output transformer cores should be placed
PM>at right angles to one another (I forget why). Nevertheless, I need to
PM>know where to place the heater transformer. Should it be at right angles
PM>to the power transformer or not?
PM>And what about the reverb transformer? Or any general tips about

Hi Peter….this just from personal experience building guitar amps.
Others may have different opinions/methods.

I don’t think directional alignment or spacing between power
transformer and separate fil. trans is critical. As they are _both_
potential sources of unwanted induced hum in other transformers, I
suggest you fit them adjacent, at the same end of the chassis,
and with the centre of their coils pointing the same way.

I usually try to get power trans (and fil) aligned at 90 degrees to the
o/p transformer in as many planes as possible, and also spaced as far
apart as possible. So I often end up with a power transformer fitted
“dropped-through” a large hole in the chassis at one end….and the o/p
trans fitted “above” the chassis, and rotated, at the other end. On the
one occasion when I used a small “square” chassis, rather than a long
thin one with good spacing between transformers, I had to fiddle with
the positions of the wound components to get the residual hum level down
to an acceptable level. If you are using a single-chassis fender-style
layout, with output tubes at one end, and preamp tubes at the other, it
is _not_ a good idea to have the output transformer and its wiring very
close to the early preamp stages, so you will need to follow the same
compromise as Fender….putting the power transformer at one end of the
chassis, the output transformer roughly in the middle, and often the PS
choke somewhere between them.
________________________________________________________________
| ________________ ——–
| | *…………* || rev.tx | ___
| | . . . . | __ ——– / *
| | . . . . | /___|___ _|
| | . . . . | /…. / . .
| | . . . . | | . . | | …… |
| | . . . . | | …. | | . . |
| | *…………* | .__./ | …… |
| —————- __/ | . . |
| _. ___. /
| power tx choke | | output tx
_*_/

This is not to scale – it is hard enough to draw in ascii when it keeps
trying to “reformat” the picture – but it will give you the general
idea.

If the three components are not exactly in line, and not all the
same height, then you may get some further reduction in 60Hz coupling by
raising the lower ones, leaning the smaller ones over slightly (like
a sailing yacht with the wind at one side), or rotating them.
-particularly if the PS choke has to be fitted close to one *corner* of
the power transformer. On one recent amp, the hum was noticeably lower
with the choke as shown above, but rotated about 25 degrees clockwise.

BTW, The usual “inverted” chassis in a tube guitar combo amp can
get quite hot after one or two hours of use. If the power transformer is
mounted “through” the chassis, with one side of the lams firmly against
the chassis, then a combination of self-heating from its windings,
and some heat transferred from the chassis, may make the transformer
run hotter than intended.
So if you mount power transformers “through” an inverted guitar amp
chassis, then I would advise using four extra, oversize, flat nuts on
the transformer mounting bolts, to act as small spacers between the
transformer iron and the amp chassis. The resulting gap gives some extra
ventilation, and minimises heat transfer from the chassis.

I have seen some older tube _power amp_ chassies which had both power
and output transformers fitted “dropped through” at opposite ends of a
long narrow chassis: the coil axis of the power trans. is parallel to
the short side of the chassis, and the coil axis of the O/P trans. is
parallel to the long side of the chassis ( pointing towards the centre-
-side of the power trans coil). I don’t know how good this would be with
the two transformers closer together.

You should be cautious about running signal ground wires very close
to the power transformer – even if there is the thickness of a steel
chassis between them. It is posssible to get some significant hum added
to what should be a “clean” ground, simply because a ground wire passes
too close to the leakage 50/60 Hz field from the iron core. Don’t assume
that 1mm of steel chassis will keep this field away from signal wiring –
it may also need at least an inch of distance – possibly more.

If you get a reverb driver transformer from New Sensor, or use the
similar Fender one, it should come fully enclosed in steel casing. This
is not perfect shielding, but certainly helps. I generally use
separate chassies for pre and power amps, but you could try the position
shown for an (enclosed) reverb transformer…or perhaps to the right of
the O/P trans.
It is _much more_ important that you keep the “output” end of
the reverb tank as far away from the power transformer as possible.
This is a common cause of troublesome reverb hum in a home-built amp.
The pickup at the end of the reverb spring is sensitive to stray hum
fields, and the returned reverb signal is very small. When you amplify
the reverb signal to a useable level, you also amplify any 60Hz hum
which may have leaked into the reverb pickup coil from a nearby
transformer. ( In theory, there could also be leakage from the _output_
transformer, or the PS choke, into the reverb pickup coil, but this is
only very rarely a problem.) Thick steel sheet shielding around the
“output” end of the reverb tank may help, and may be simpler than
rebuilding an existing amp layout, but _distance_ is the best fix.

Power transformers which have a wide copper strap around the windings
and the _outside_ of the iron (not through the coil window), usually
have smaller external field. Careful placing of other transformers and
chokes is still necessary, but not so critical.

This is the basics of layout – for a simple tube amp with a push-pull
output stage – I hope it helps.
If I have forgotten anything important, I am sure someone else here
will fill the gap. Cheers, Stephen.

 

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