lumophon: WD507 (WD 507); A large hum in supply circuit

Hi,

If someone could help me with the hum in supply circuit in wd 507?

Problem: After turning on the radio, great hum voltage appears on "Feld" coil. The hum voltage is about 20 - 30 Vpp. The supply voltage on cathod of az11 drops below 180 V. New tubes were applied, but the problem was the same. All passive elements in supply unit and around of el12 are ok. The hum is very loud in speaker.

We tried to replace capacitor 0,2 uF parallel to feld coil with 220 uF and the hum was gone and the supply voltage (cathode az11) was below 160 V (which is not enough to supply el12).

Question: What is the maximum hum voltage (peak to peak) allowed in supply circuit of wd 507?

Thank you,

Nikola

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Dear Nikola,

the hum and the voltage drop can be caused by excessive current drain of the audio amplifier. Please first check the grid bias of the EL12. You should find about -6V (minus 6V) on pin 2.

Regards, Emilio

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