Variac question

[FeloniousBishop]

Hi,

If you use a variac to lower the supply voltage to the amp this would scale back both the plate and grid bias voltages by the same scaling factor, right?

So, since the grid bias now decreases wouldn't that actually cause an increased standby current flow between cathode and plate i.e. cause the amp to be under-biased, even though the plate voltage decreased also?

thanks!

[phsyconoodler]

You would have to track it with a bias meter or measure the current,but it would depend on whether the amp had a dedicated bias tap or it has the marshall style taken off one leg of the primary.
If the plate voltage goes down the bias current can come up safely to a point.
There are other problems associated with variac style voltage reductions like heater voltage being too low.That is what damages tubes.

[FeloniousBishop]

Thanks psychonoodler. Do you have any idea at what heater voltage (or lowered mains voltage) you start to get cathode stripping?

[TED STEVENSON]

You should stay within 10% of the specs.