Bias question

[Mr.Death]

I was biasing my 100w EL34  Traynor YBA3 today and I noticed the colder I biased the amp the higher the plate voltage. At 28MA I had 578 plate voltage, 32MA 574V and 35MA got me 572V to the plates. Does this make sense? Thanks.

[pdf64]

Yes, no power supply is perfect, there’s always internal impedance / resistance. Hence as the current drawn from the supply increases, the supply voltage will decrease.
Try measuring HT voltage at the rectifier / reservoir cap with the amp cranking out full power into a dummy load.
HT sag is key piece of protection for valves, without it they’d tend to overdissipate with signal.

[jewishjay]

Yes, more current draw = lower voltage. That's always the case when adjusting bias. Perfectly normal.

[Mr.Death]

So if red plating is a result of high plate voltage wouldn't a hotter biased amp( less plate voltage) be less prone to red plating? Seems to be the case in theory or am I wrong? Thanks.

[sluckey]

So if red plating is a result of high plate voltage wouldn't a hotter biased amp( less plate voltage) be less prone to red plating? Seems to be the case in theory or am I wrong? Thanks.

Red plating is not caused by high plate voltage. It's caused by high plate current.

[PRR]

Redplating is Voltage TIMES Current.

Since power supply impedance is normally "low", the lower the current the cooler the tube. The small rise as the voltage un-sags is more than made up by lower current.

Another situation you rarely see today is PS impedance similar to amplifier load. Seen with old field-coil speakers. The raw B+ may be as high as 450V, the FC dropped 200V, the tube ate 250V. Different tube current gave different tube voltage, but for a -wide- range around nominal the tube dissipation hardly changed. (You could burn-up a field-coil if you went crazy.)

[Mr.Death]

Thanks again guys.