1626 Darling monobloc - unexpectedly low B+ voltage when choke loaded

[mikw st]

My power supply for 300b mono-block se is 2uf (filmcap) 10h choke 100uf 220R 100uf @50ma current for 300b then rcrc filter @9ma for wonderful trioded ef184. The 10 henry@ 120ma choke I sourced out of china and in this configuration it works like a charm.
''I guess what I have to do is convert back to C input and lower the raised volts with more series resistance in the fiter stages after the choke - and see if that cures the hum.''-----this will be logical move pls do it and tell us the outcome.

Regards

Minhaj, I'm definitely going back to cap input with the new PT when I get it. If that doesn't get rid of the hum then I'll remove what grounding I have and put in a new gound bus.

 

[mikw st]

Hum can be from -
1 - mechanical noise - is the transformer itself actually humming? Does isolating the bolts from the chassis make a difference? Is the hum from the unit or the speakers?
2 - radiated, does moving the transformer away, rotating it or shielding it make a difference?
3 - insufficient filtering.
4 - ground loops.

1. The transformer is rubber mounted and doesn't feel like there's vibration. Hum comes from the speaker, not the chassis.

2. If I ground the built in shield or lift it there's still hum. I haven't removed it but I could unscrew and stand it on its end if the wiring permits.

3. I tried more filtering and it hasn't made a difference.

4. Possible, if it still hums when I put in the new PT then I'll redo all the grounding.

 

[mikw st]

I do have a 0-300v DC lab supply, so I might disconnect the choke from the 5y3 cathode and inject regulated HT DC from the lab supply instead. If there's still hum, then it's a grounding issue, right?

 

[mikw st]

Hadn't thought that through - the PT would still be supplying the heaters on the 12.6v secondary. I also have a variable LT supply too, so could DC heat the amplifier tubes with that...

 

[N101N]

N101N, you're telling me not to implement it?

Yes...While functionally and qualitatively superior to anything else, the practical drawbacks of the choke input filter cannot always be overcome.

Some (Hammond) transformers are innately prone to humming. (Some manufacturers have a hard time making decent products).