Strange Power Transformer Behaviour (heat related)

Mr G · Post by **Mr G** » Wed Sep 13, 2006 6:25 am

Gents,

Strange one this. Got a new build 100W 12XXX type amp - about 3 months old now. Has quality hand made output and power transformers in it from a well know supplier. It kept having current draw problems (i.e too much) until we added swamp resistors to the base of every tube instead of the usual 2 (inner/outer tube pairs). This cured that problem. The b+ settles at around 490-500v...but the PT always got very very very warm...almost to the point where i was worried about melting the head cabinet tolex/glue. Not your usual heat levels...with or without swamp n screen resistors...

The weirdest thing has just happened after I replaced (yet again) the T500ma fuse in the back of the amp (as it kept blowing after a few hours playing - with or without any attenuator/dummy load).

The PT now runs cool...evan after hours of playing at full whack...into and out of dummy loads/cabs etc. My question is this,

1) Has anyone else had any similar problems like this?

2) Is it possible that the PT has basically cooked itself better?

3) Is it possible that certain components in that part of the circuit are now bedded in and working to their desired spec? Whereas perhaps before they werent?

This may sound strange, but if there was a small short in the PT when first built/wound....can this rectify itself with a serious george foreman style grilling/cooking session?.

The amp now sounds better than it ever did, more controlled and stable...and the TX's are running cool (at wall voltage and well below when variac'd).

I have thought about running a voltage meter from the mains outlet...to see if i get big swings during the day, but i wanted to throw this question out to anyone who might have some reccommendations/answers for me (maybe bias drift into a cooler zone??who knows??)

But has anyone ever heard of a shorted PT that cured itself without intervention...or could it be all the new parts settling in to their correct values/Bias drift up and down????

Strange things these amps...