Preamp Transformer overheats

[MVZIK]

Hi,

Mine is an AI L1 that uses 6922. Lately I realized the power supply section is running very hot. too hot to touch. I thought it was due to aging tubes but things didn't change even changing to a pair of new EH6922. Can anyone tell me what are the possible causes of transformer temperature to rise?

So far there is no blown fuse and the amp is still working even the PS section gets very hot but I'd stopped using it now to avoid causing further damages.

Please help guys. Thanks .

 

[ericj]

Not familiar with the product but i went looking for pictures.

Huh. I have those exact blue axial 0.01uf caps.

Anyway - my general guess is that maybe one of those heatsinked transistors has died in a bad way and is causing an overload in the power supply. It is interesting that it still works because a responsible designer would employ a fuse or circuit breaker to prevent excess current from damaging the device or causing a fire, etc.

If you have or can get a non-contact (infrared) thermometer you should be able to narrow down which parts are excessively hot.

But it may be advisable to seek authorized, professional repair service.

 

[MVZIK]

Hi ericj,

Thanks for the reply, they are LM317T and LM337T. How to know if they are faulty?

 

[JonSnell Electronic]

If a regulator had failed there would be serious sound problems. To prove the transformer has not got shorted turns, disconnect the HT winding an insulate to make them safe, remove the valve and then run it. If the temperature increases with no load, replace the mains transformer. If you email your circuit to Majestic Transformers, Parkstone, Dorset, they may have the drawings still or at least make you a new one. Electrical Transformers Company, Poole, Dorset UK

 

[MVZIK]

What's HT ?

 

[MVZIK]

It's does have cracking sound and sound of crumbling of PP polybag(you know what I mean?). One side is more obvious than the other.

 

[Enzo]

HT means "high tension", another term for high voltage.

 

[DF96]

In the UK we call it HT. Some other places call it B+.

 

[MVZIK]

Got it. B+ is more commonly used in Asia.

 

[djn]

I had the same over heating with my pete millett low mu preamp. Turned out the 6.3A supply was for 5a and all the tubes added up to 8a. I installed a separate heater transformer.

 

[ericj]

Hi ericj,

Thanks for the reply, they are LM317T and LM337T. How to know if they are faulty?

i meant one of the devices in the preamp itself

 

[AndrewT]

I had the same over heating with my pete millett low mu preamp. Turned out the 6.3A supply was for 5a and all the tubes added up to 8a. I installed a separate heater transformer.

Maybe Mv could get a clue by measuring the heater winding voltage.
If well below rated voltage, then that would indicate overloading.

 

[MVZIK]

i meant one of the devices in the preamp itself

Ya there are two 14 pins 723. One on each channel.

and 712 with heatsink on each channel.

 

[DualTriode]

How hot?

Hot!
How hot?
It the old days with work callused hands I could hold a hand on 140 degrees F for a short time with no ill effect. Today my lily whites are not so tough. 140 degrees is not so hot for a transformer, however too hot for my hand. I use a Fluke IR thermometer.
DT

 

[AndrewT]

Hot!
How hot?
It the old days with work callused hands I could hold a hand on 140 degrees F for a short time with no ill effect. Today my lily whites are not so tough. 140 degrees is not so hot for a transformer, however too hot for my hand. I use a Fluke IR thermometer.
DT

why ?

 

[DualTriode]

AndrewT

It is an eye brain perception thing to me. Arial jumps off the page more easily than veranda or other founts do.

My preference is a carryover from the engineering office where I worked. Arial was the official fount. I kicked up the size of the fount for the notes I would teach from.

We talk a lot of the perception of sound. Arial is a visual perception thing.

That is my story.

DT

 

[JonSnell Electronic]

OK so when you disconnected the secondary windings from the transformer, ( HT B+ or whatever), what happens?
Does it stay cool?

 

[MVZIK]

It's simply too hot to touch but it's still playing. I try just plug it in on stand by and 20 mins later it heat up again. I'll send it to a friend who builds his own tube preamps to check. He suspects something shorted but the trans is nearing it's end too.