Laney VC30 -self biasing?

Voxman

Hi guys - is the Laney VC30 'self biasing' and if so what exactly does that mean.  If you install a matched full set of power-tubes I'm assuming biasing isn't necessary, but if you're only changing e.g. a single tube you have to replace it with the same 'grade' of tube.  And how do you know the grade?

Please educate me here!   

ICBM

Yes it is. Also called 'cathode biased'. It means that the bias voltage is generated by the valve current itself, passing through a resistor in the cathode connection - so it automatically compensates to a very large extent for different valve performance and you don't need to bias it.

Most of this type of amp use a single cathode resistor for all the power valves, so you need a matched set or some of the valves will run hotter than others because the bias voltage will be the average for all the power valves, not specific to each one. It's not easy to know the 'grade' of the valve without proper test equipment or if they came from the same vendor with the same test number marked on them. (You can't assume different testers' numbers will mean the valves are matched even if they are the same number.)

So really it isn't ideal to change just one valve - although in practice it doesn't usually cause too much trouble, it may shorten the life of one or more of the valves but you would generally only do this to get you through to the end of the 'set life' anyway, if one has blown.

Voxman

Thanks ICBM - very helpful and informative as always - have a wisdom young man!