Power Transformer Issue
 

I have a 100 watt EL34 Push Pull amplifier here that has some strange symptoms. A friend of mine said his amp was dead quiet until the other day when there was a noise though the speakers, even in standby (typical standby that just opens the B+ to the amp but leaves the heaters on).

Ok so I asked him if he ever noticed his power transformer being so noisy and he said he isn't sure. Well to me the noise through the speaker is the same noise that power transformer is making, you can even feel the chassis to the amp vibrating.

So here is where I am at. TUBES ARE REMOVED AND NO HELP:rolleyes:

I disconnected the B+ to the standby switch taped it up and left it floating and no help. Next I disconnected the primary taps from the output transformer and that didn't help. Next I measured 500mA of AC current on the primary side of the power transformer. I disconnected the power transformer high voltage secondary taps, taped them up and left them floating. I measured about the same current on the primary of PT and the noise is still present. Next I disconnected the 6.3vac heater taps for the EL34's, taped them up and left them floating, I measured about the same current on primary side of PT and the noise is still present. Next I disconnected the 6.3vac taps for the preamp tubes, taped and left them floating, I measured about 400mA of current on primary of PT (the on bulb is on this circuit) and the noise is still present.

If I relocate the output transformer off the chassis the noise goes away through the speaker.

Ok so with nothing but the primary side of the power transformer connected I see almost a half amp of current. Is that normal? I would think not. Is this a power transformer or output transformer issue? I would think power.

It seems as though the noise is being injected from the power transformer into the output transformer and on to the speakers. :eek:

Turn the power or the output transformer (but not both) 90 degrees and see if the hum goes away.

Sounds like bad transformer to me!!!

If there is noise coming out of the speaker with all of those lines disconnected then there is possibility that the primary maybe shorted to the core.
BE CAREFUL IT MAY BE LIVE!!!!!!

Especially if the sound goes away by just removing it from the chassis.

500ma of Idle current seems kind of High to me if there is no load on it,I may be wrong as it has been a while since I have messed with a tube power supply transformer.

FWIW

jer :)

I tried to move the output transformer 90 degrees and no help. But if I move it off the chassis the noise goes away. The layout is good, the power transformer is mounted on the total opposite side of chassis a good foot apart. This is an old amp from the 60's and my buddy has owned it for 6 years or so and he said the noise was never there, and he is total sound nut and would have noticed.

For a 100W amplifier, that could be normal. The key point being that with the secondary disconnected, the current is almost 90 degrees out of phase with the voltage - it's just acting as an inductor rather than as a transformer in this case - and so the power lost in the transformer is very low.

As for the main problem, I am inclined to suspect magnetic coupling. What is the chassis made from?

"As for the main problem, I am inclined to suspect magnetic coupling. What is the chassis made from?"

It is made from steel.

Almost half amp of idle current without anything connected to secondary seems like 100V primary (maybe some old Japanese transformer?) or overheated and so shorted turns. If you have variac you can slowly turn up voltage and measure input current. If it goes up linear something is shorted. If you will notice some point where it will start to increase rapidly you will find out for which primary voltage was calculated. 0.5A is probably too high even for 2KW unit.
Cheers

Thinking some more, and re-reading, you could be right. I missed the bit in the original post about the noise coming from the mains transformer. That does begin to sound like an internal short somewhere.

I would split the chassis from the wall socket safety ground, and measure current chassis (transformer shell) to safety ground. Hammond Clock Co suggested you put a 10k 2 w resistor parallel to a .01 uf cap from chassis to safety ground, and measure AC voltage. More than 4 v they said your organ needed a new power transformer.Use clip leads and one hand at a time, don't touch the chassis while making this measurement.

I will try this out and post my readings:up::up::up: