Congrats! Exactly this was my last assumption: If everything else proved to be unsuspicious, tha hum most probably must have been induced from the heater supply.
The remaining faint hum ist typical for radios with SE output stages. You may try to optimize by altering the values of R45 and R46.
Best regards!
The remaining faint hum ist typical for radios with SE output stages. You may try to optimize by altering the values of R45 and R46.
Best regards!
I tried lowering R45 and R46 to 1.8k but didn't hear any improvement. There were two 680ohm resistors in series when I got it but only on one channel as I said (I suspect the other two were on the channel that had blown the output tube and opened up). Could it be that at the factory instead of the 2k mentioned on the schematics they went for 1.36k to lower the hum level ? I don't have that value but I do have two 1.5K 3W resistors, might give it a try.
The optimum value of those resistors depend on the pentode's internal plate resistance and the turns ratio of both primary parts (the quotient of the part above the B+ tap and the part below it). Generally, the quotient of the plate resistance and the filter resistance should equal that quotient.
So, due to tube ageing, you might even get an improvement when you increase those resistors. But remember, I won't promise you anything
.
Best regards!
So, due to tube ageing, you might even get an improvement when you increase those resistors. But remember, I won't promise you anything
.Best regards!
One of the tubes is nos so values are probably a little better than the other one but there is no audible difference in hum between both channels. I actually reinstalled the 2K resistors and this seems to be the best value (going to 2.2k did increase the hum level ever so slightly on the channel with the new tube). Without the Hi-fi mode activated it's really not bad at all so I'm happy with the result for now.
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