Hohner Concertophon IV and Hum

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I have an old Concertophon IV that has some annoying hum. I bought this thing as not tested, and it was even working in the beginning.

I changed the filter caps and installed an elevated humdinger but it still has a lot of hum. So I tried swapping the tubes, but it didn't worked. After shorting the grids of the preamp tubes through a 100nF 400V cap I found the stage where the hum was getting into the signal. Right after the 12au7 before the PI. The other capacitors of the power and preamp tested good, only the reverb was a mess.

This is a three chassis amp, with chassis for power, preamp and reverb. Everything is connected using a multi-conductor cable, as the fender Excelsior, but without the metal jacket. I think that this cable is where the hum is entering the sinal path. Anyone had a similar problem? Or is it just poor heater wiring and ground loops. The amp as like 6 input jacks, and all of them are the metal kind, and at the same time connected to the circuit ground, It looks like a lot of ground loops to me.

and the schematic:

I'm testing the amp without the reverb (last picture) because it needs a total recap, and I'm still waiting for the capacitors to arrive.
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I managed to reduce the Hum changing the cable connecting the chassis, where now the core is the ground, and the heaters are at one side (initially the core was one of the heaters). The image helps to understand it>

Other small changes that improved the amp were>
Instead of connecting the ground coming from the power stage to the signal ground, as it was before, I bolted it to the preamp chassis. The filter caps of the preamp were also grounded at this point. The rest of the preamp ground was connected to the input jacks at one point, where the jack makes the connection to the chassis.

This way I managend to reduce some ground loop noises. Now, with much less hum I could finally identify some hiss, so back to work.

I hope this could be usefull to someone.
Cheers, Thomas
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.