Transformer hum?

[Emubitter]

I am amazed by the depth of knowledge I see in these forums.. please bear with my question since I am at a loss and need guidance...

I had a substantial external power supply with valve rectification built to drive my power amp which is a modded Audionote P2SE style 18 W single ended tetrode design.

The valve rectification in the supply was then deemed to be the source of an unacceptable level of hum through my 105db horns.

The power supply was then changed to SS rectification and the electrical noise dropped to an acceptable level. However there is now a mechanical hum from the transformer that is unbearable from the listening postion!

The technician who built the supply now recommends that the hum is because the heater coils for the rectifier are no longer in use and that we should try a new transformer without said 'spare' coils...

The frustration levels are rising and I feel I am throwing good money after bad... The fellow building this stuff is not an amateur and has been building and retailing high end gear for years but my faith is being tested!

Any thoughts much appreciated, unfortunately I do not have a schematic.


Thanks,

Ken

 

[AndrewT]

Hi,
Mechanical hum can be due to either loose components inside the transformer or to susceptibility to DC coming from the mains. (DC=asymmetric AC waveforms).
Have you tried reducing the hum by isolating the transformer from the chassis? I have seen some hanging from the support bolt and the strap they are attached to being of low torsional rigidity.
or
Try using a DC blocker on the mains AC feed to the transformer. Take care the DC blocker is at mains potential!!!!!

 

[Emubitter]

Thanks Andrew,

We have tried rubber isolation without much luck. I may be able to try the DC blocking....

I am interested in the hypothesis that not using the heater windings is making the difference between humming and not humming...

Does this make theoretical sense? Sound feasible?

Thanks,

Ken

 

[gareth]

Is this a toroidal transformer ? Is so you can rotate it to see if the hum goes.

If you are not using the said output windings from the transformer then your tech. man is right. Basically these windings need a load to drive.

Read up on transformer regulation. This may help.

Gareth

 

[dsavitsk]

I am interested in the hypothesis that not using the heater windings is making the difference between humming and not humming...

Does this make theoretical sense? Sound feasible?

No, but it is easy enough to test without resorting to a new transformer. Put a 2.5R 20W resistor across the 5V winding. See what happens.

 

[Emubitter]

No, but it is easy enough to test without resorting to a new transformer. Put a 2.5R 20W resistor across the 5V winding. See what happens.

If this works could it be practical to put a permanent load here to solve the problem or is there too much heat and power to dissipate?

Thanks,

ken

 

[AndrewT]

If this works could it be practical to put a permanent load here to solve the problem or is there too much heat and power to dissipate?

Let's hear if it helps, then you can experiment with how little current maintains that improvement, if any.

 

[john65b]

Emu,

I have acquired an amp that has similar topology as the AN P2SE and wonder if I can bother you for the schematics?

I have searched everywhere and cannot find it.

I believe the amp I have may be designed around the P2SE, since there are not many 6L6GC driven by 6SL7.....

I thank you in advance...

 

[Emubitter]

John. sorry no schematics for the power amp... good luck with yours...

Regards,