Bad redplating, PT Very hot!!

[Silvio55]

Hi guys, I just had a very bad redplating problem with my plexi 50w clone, wich Has been running without any problems for some years now. Some days ago I replaced the power tubes with JJ EL34, adjusted the bias at a little lower than 70%, and all was just fine, played it every day since, until now, I noticed a strange smell, like hot paint or varnish while playin in my living room, looked at the back of the amp and saw the power tubes plates glowing red. Inmediatly I switched it off, and after cooling the tubes with a fan y took the chassis off, and noticed that the power tranny was very hot, and the smell was coming from it.
I have no idea what could have coused it, what do you think I should check first?

[Jazz P Bass]

Check the bias voltage at the output tube socket.

If it is good, then you got a batch of bad tubes.

Red plating can only be 'caused' by bad tubes or a very low bias voltage.

The fact that the tubes where running flat out is most probably what caused the PT to get hot.

[Mick Bailey]

+1

My money would be on you losing your bias voltage, or it's not sufficiently negative - tell us what it reads. If it's missing, that's a straightforward fix. If the voltage measures fine, hook up your meter with some clip leads to a power tube socket pin 5/chassis and chopstick the bias circuit right the way through to see if the voltage changes or disappears. if that doesn't reveal anything, play some hot air on the components from a hairdryer. I always suspect bias pots - give it special attention - some are not wired or positioned to 'fail safe' if the wiper loses contact. Do the checks with the tubes removed.

Check your PT for visible signs of charring, oozing etc. Usually they're OK. Smells can be due to plasticisers, wax or other compounds evaporating off at a higher than normal temperature. What feels hot to the touch doesn't necessarily mean that it's outside the thermal tolerance of the materials and construction. Depends on the quality of the transformer.

[Silvio55]

Thanks! I checked the bias circuit and the problem was the first resistor, it went bad and there was no bias voltage. I replaced it and all seems ok now.