Buzzing power transformer fix

[Rhodesplyr]

It's not a guitar amp, but the same principles apply :-)

A client asked me to take a look at a late 50s Grommes 10PG-8 mono integrated amp: 2 x 12AX7, 2 x EL84, 11W output.

With three coupling cap replacements, it was working fine, but the power transformer had a loud and annoying buzz. It wasn't blowing fuses and wasn't drawing too much current. All secondary voltages were where they were supposed to be, and it didn't get hot. I tried tightening the machine screws holding the laminations and bell-ends together, but that didn't work.

Fortunately, the leads were long enough to allow me to examine the interior without desoldering everything, and I found loose pieces of insulating paper that looked like they'd originally been glued in place to the insulating paper surrounding the windings. I painted a little varnish on them, put them back in place, reassembled the transformer, and let it dry.

Result: a perfectly quiet power transformer. Those pieces of loose paper were able to produce a surprising amount of noise.

This amp has a "Flat" setting on the selector switch that makes a decent guitar input. With existing tone controls and 2 x EL84 outputs, it's not a bad little guitar amp.

The sad part is that it had a pair of shield-logo Mullard EL84s in it that initially looked to me like they might be good--little getter wear, etc..., but the leaky "Little Chief" coupling capacitors had ruined them. They died young.

[Jazz P Bass]

Nice fix on the "buzzing" transformer.

[R.G.]

Nota Bene:
All transformers vibrate with the AC power line frequency or twice that. The magnetic field causes magnetostriction, a physical change in size and shape of the core material, to a greater or lesser degree in all core materials. If an iron lamination is loose, it buzzes like crazy because the magnetic field mechanically moves it and whams it against the rest of the core. But even well-bound cores vibrate, enough in your case to vibrate the paper which had come loose. Coils also vibrate, because the magnetic field changes also cause mechanical forces in the wires themselves. Again, a loose binding can cause noise.

But you had the right solution: glue it down. This is why most transformers are impregnated with varnish, or at least one of the reasons. It glues everything together. In addition, the varnish fills up the air pockets inside; varnish is a better heat conductor than air, so it improves cooling.