Bad OT?

[catalin gramada]

Hello everybody.
I have seen a weird phenomena regards the OT in one of my Plexi project looking for potential hum sources. Each time when I power on the HT ( with all tubes pulled off) I recorded a sort of short slight "thump" in speakers for second and progresively disapear after. I did a measurement with multimeter conected on secondary 16 ohm tap while I powered on and it recorded a momentary spike from 0 to 12mV AC and comes slightly to 0 in seconds. Seems it works exactly like a PT at momentary premagnetisation but I wonder why cause the primary side it is in open circuit without tubes.
Can someone enlight me what is going on , please ? It is a bad OT or a damaged one ? or could be in efect a EMI interference from PT maybe ?
Thank you
Best Regards
Catalin

PS: I didn't notice that noise when re-power on if capacitors remains charged.

 

[Cirrus]

The transformer blocks dc voltage, which is good because the transformer primary is referenced at ht voltage by the centre tap which connects to the first filter cap.

When the ht voltage is first applied the jump from 0v to ~400v on the primary looks like AC voltage to the output transformer (ie it's a changing signal) so voltage is inducted on the secondaries until the ht voltage stabilises. This is the thump you hear. It's also why you don't hear it if you turn it on when the caps are already charged.

 

[catalin gramada]

When the ht voltage is first applied the jump from 0v to ~400v on the primary looks like AC voltage to the output transformer (ie it's a changing signal) so voltage is inducted on the secondaries until the ht voltage stabilises.

Hi Cirrus.
thank you. that have pretty much sense. but the real think I cannot imagine is how this voltage is induced in secondary side as time the primary was in open circuit (power tubes was pulled off). just primary center tap of OT was conected. Thanks
Catalin