From: stephen.delft@welcom.gen.nz (Stephen Delft) Subject: Transformer placement [The original question concerned placement of filament and power trans- formers. I found it helpful in general - Ed] 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 possible 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 usable 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. [Minor editing - NPS 2/24/96]