hot bias- actually hot?

[eharmstrong]

I keep reading that "hot bias" kills tubes faster because the valves will dissipate more quiescent power- but I don't see how this necessarily has to be the case.

The way I understand it, hot bias refers to setting the grid to cathode voltage more positive than center bias (closer to 0V) so that the tube is "more on" (more quiescent plate current) with no signal applied to it. Even though you have more quiescent plate current in hot bias, the power dissipation of the valve also depends on the plate to cathode voltage- which should actually be less for hot bias if you are pulling more current through the same plate resistor used in center or cold bias. With more plate current and less plate to cathode voltage, the quiescent power dissipation is not necessarily higher.

What am I missing?

Thanks all

 

[Mickeystan]

I believe most people refer to hot bias in reference to output stages which have minimal resistance between the output tube plates and the power supply. There is always some resistance in the output transformer windings and the power transformer windings. Maybe be a small amount too in the PS choke if there is one between the output transformer and the power transformer and rectifiers. With all that said, as bias is increased such that the output tubes draw more idle current, the power dissipated in them increases. The simple truth of course is in the math (plate volts, minus cathode volts times current through the tube). Your point may have a better chance of being true in a line amp stage with lots of plate load resistance. Mickeystan

 

[eharmstrong]

Yes, to clarify I was talking specifically about a high gain preamp stage with a 100k plate resistor. A tube would definitely dissipate more power at a higher quiescent plate current driving an inductive load in an output stage, where the real resistance is very low. That is probably what I am confused about- hot and cold bias are probably not the right terms to use when talking about low power preamp stages.

I was reading through this article http://www.valvewizard.co.uk/Common_Gain_Stage.pdf, and section 1.14 does mention higher power dissipation in a hot biased low power preamp configuration, but I don't think that is true because the plate voltage drops significantly with increasing plate current when there is a large plate resistance.

Anyways, thanks for the reply, I didn't think about the high power side of things- that at least explains the origin of hot and cold bias actually relating to thermal dissipation. For low power preamps I guess the terms are just shorthand for which way the bias is offset from center.

 

[PRR]

If there is substantial Resistance in series with the tube (any device!), maximum device Power happens at half supply voltage.

This power peak is very broad.

As we often bias voltage amplifiers roughly half, almost never near B+ or zero, the tube dissipation is essentially known from B+ and series resistor, and unlikely to run wild.

Say 300V and 100K. With plate at 150V we have 1.5mA and 0.225W in tube. If we force plate down to 100V, 2mA and 100V is 0.2W; at 200V 1mA and 200V is 0.2W.

Different in Power stages which have significant AC/Audio impedance with far lower DC resistance. These are quite easy to cook.

 

[artosalo]

I think that the original meaning of the term "high bias" refers to high negative grid voltage i.e. low idle current and therefore low power dissipation.
Am I right ?

 

[Sorento]

you are missing the point that, when you increase plate current (keeping load and B+ the same), plate voltage drops to a lower value, from 1/2 B+ say to 1/4 B+ which may be too low to be comfortable with (you are now on a different load line);
so you will almost inevitably tend to increase B+ such that plate voltage is up again where it was before; hence - higher plate dissipation ...