MAMEWorld Forums - The Loony Bin - Re: Anyone play with Tube amps?

> > > The way I read a similar circuit layout the tube heater was wired in series with
> > the
> > > turntable motor, why?
> >
> > So you don't need a step-down transformer - saving money!
>
> Can that really be a reliable form of voltage control? No transients? Does the tube
> care?

Since the load on the motor is pretty doesn't vary much, and it's designed to run at a constant speed, you can expect it to draw fairly consistent current while operational, which should give fairly constant cathode heat. The current will be higher while accelerating, but this should only take a few seconds, during which time the tube will heat faster. Having the tube heater in series like that will also limit inrush current to the motor when it's switched on.

In a tube that has separate heater and cathode elements, the heater supply doesn't have to be particularly stable. The temperature of the cathode won't change much for transients as it has significant thermal mass.

With older tubes where the heating filament actually is the cathode, you need a regulated DC power supply or a battery to power the heaters - you can't use an AC heating supply or this will be coupled straight through to the output. But that's a different story.