Need Help Getting Bias Right For KT66 On An Original ST70

Both Seamus and Arremono is correct in a way.

True that an amp in idle don't need load as there is no signal across the primary, only DC
False that even an amp at idle may encounter disturbances ( large motors turned off in the neigborhood
or other types of mains-transported noise) or that the amp itself starts to oscillate or that one
mistakenly inject a signal by touching some connector)

To be safe : always connect a load, don't have to be 8 ohm but less then 100. If none available
then short the outputs.

arremondo wrote:SEAMUS: A quick search and I find that several sources disagree with you.  Tube amp are different than solid state amps.  You can damage the transformers, tubes or circuits if you run it without a load (speakers or a dummy load).  Solid state amps don't care if there is a load or not but tube amps do.  

I said:

Seamus wrote:
Hence there is no need to use a load resistor in place of the speakers.

You most certainly need a load with a transformer output, either speakers or load resistor.
Perhaps there is a language barrier? "in place of" is the same as "instead of".


It's also a good idea to load the highest impedance output tap with 20 to 40x to protect the transformer in the event of an inadvertently open speaker lead: blown speaker, kiddie fingers, missus cleaning, etc.  

As stated, a load resistor is a better idea with new tubes or build. As is an oscillator and an oscilloscope to determine if there is speaker destroying hum, motorboating or oscillation...

AND IMO, bias should be done with shorting plugs in the input.

AND IMO, bias should be done at 100 hours and every 500 hours thereafter.
±1% is close enough. For 500mV, that's 495mV to 505mV.
Too much knobdicking on the pots in the same spot wears the traces.
Cermet pots have a limited number of cycles, about 200.

AND IMO, bias pots should be multiturn for better precision.
Bournes 3339 series