[Amps] Buzzing transformer

[Steve Wright]

On 30/09/16 09:53, Jeff AC0C wrote:
> My PA76 buzzes a bit.  But I had another PA76 transformer from a guy 
> who swapped it for a Dahl and that one sung like crazy.  Used it for a 
> ton of key down power tests on an amp I was building and it never 
> showed any other side effect than rattling.

Often it is about how the transformer is designed ;

A low flux density (more overall windings on the core) will give a quiet 
transformer at idle with low standing loss, but worse regulation at full 
load.

A high flux density (LESS overall windings on the core) will give a 
humming transformer at idle with plenty of standing current flow, but 
very good regulation at full load.

It's about what you need, or what you're prepared to tolerate.  This is 
why power transformers on the side of the road hum loudly - they are 
less concerned about idle current or a noisy transformer, than they are 
about efficiency at full power.  Same principle applies in a HT PSU.  I 
rewound the secondary only on a 2KVA (cont rating) isolating transformer 
- and of course I ended up with a wicked boing on startup, a gnarly idle 
hum, and a standby current of about an Amp hehe..  We did hook up the 
rectifier incorrectly and destroyed a piece of 2.5mm copper wire that 
someone thought was clever to put in the fuse holder.  Many thanks to 
Manfred for the help on that particular adventure!

So if you want a balls-to-the-wall tube amp, the transformer is going to 
hum somewhere...  ;)


Steve