Oktava MKL-2500 hum

[andow]

We have a Oktava MKL-2500 in the studio that has been humming for some time. Recapping the power supply and changing the tube didn't help, so I looked at the grounding scheme since I read that these chinese power supplies (BP-2) are not the best in this regard. I don't have much experience with tube microphones but to me it really looked like a mess, so I rearranged the grounding to what I would expect to work (see the attached sketch). Unfortunately, the hum didn't change (maybe it even got worse a bit)...

So my question is how would the ground in such a microphone usually be laid out?
I don't have the schematic, but the regulation in the power supply is done with regulator on 6,3V and Zener diodes on 120V.

 

[Tubetec]

I have an MKL2500 with BP2 psu .
6.3v dc on the heater might be too high , mine says 4.5v 250ma on the enclosure ,although its been a while since I measured it .
The multipin xlr at the mic end isnt of the best quality and I remember having issues the odd time with pins making contact , the multicore cable was also garbage (thin unflexible grey wire)so I changed it out for Gotham 7 core tube mic cable . There is a few other posts about this mic on the forum somewhere which may have the PSU schematic attached .

I did a few extra mods on my supply , including changing the diodes to Shottky in the heater supply, bypassing the electrolytic caps with foils , repositioning the toroid into the corner of the enclosure and rotating it for minimum hum , I also took care in how the cables from the transformer to the pcb were routed . Another thing I did was permanently wire the mains transformer to the IEC input and turn the on/off switch into a standby , I not saying it was nessesary to have a standby ,however the mod did make for neater mains wiring inside the enclosure and hopefully less chance of mains interference making its way into the audio path .

 

[andow]

Thank you for your help and sorry for the late reply. We went on spontaneous holiday before the next lockdown kicks in...

It seems my assumption was right, the problem is the grounding in the power supply. Obviously the ground traces on the PCB itself are done really bad. After trying out different configurations I was able to reduce 100Hz hum about 20dB! Quiet enough...