A very interesting "remark" from Phillips...

[Alexo]

From their ECC83 datasheet:

"REMARK

With Vf applied to pins 9 and 4+5 and the centre tap of the heater transformer connected to earth, the triode section connected to pins 6, 7 and 8 is the more favourable section of the tube with respect to hum."

[rjgtr]

Which is why many companies use that section as the first stage, or favor that side for higher gaine stages. It probably has to do with the internal construction of the tube.

It is good info to know.

[Franky]

Since I'm using DC to heat the tubes I never got that 50/60 cycle hum again. However, I'm not using a 6.3 V transformer that is usually on tube PT (because of voltage drop on the diodes and the voltage controller). But I gonna try to use 6.3 V transformer to make DC heating. Wish me luck .

[rjgtr]

You can actually get 6.5 DC from a 6.5 filament tap if you use a bridge rectifier. It will result in about 8V which you can divide down. Make sure the transformer has plenty of extra current capacity.