Heater Wiring (5): Variacs and Van Halen

Just wanted to add this info, so it isn't forgotten,
while we're discussing heater circuits:

"Some nice sound can be had by cutting the line voltage to ~90v from ~120v" - Eddy Van Halen

I've been told repeatedly that this was one of Van Halen's (off the cuff interview) pranks.

That is, he really didn't use a variac,

unless perhaps experimentally in studio once, and its not a part of his 'sound'.

Those running out to try this ought to take serious heed of the obvious warnings, such as:

(1) A variac on most amps would also drop the heater voltage,
and this directly contributes to tube death.

So much so that RCA did a long comprehensive study

and warned designers not to vary the heater voltage

from recommended values more than 4%:

LOWER VOLTAGES especially KILL TUBES QUICKLY.

With 85% of the rated voltage on a tube, it goes from a 5,000-hour+ tube to a 3-hour tube!.
Letting your tech experiment with a variac on your amp for a couple of hours will cost you a whole set of tubes within weeks of installation!

Don't let the heater voltage on your tube drop below 96% for any extended length of time:

12.6 v heater (signaltubes): greater than 12.1 volts or else! (Never use 12 volt regulators (7812) without resistor adjusts)

6.3 v heater (powertubes): greater than 6.05 volts or else! (Never use 6 volt regulators (7806) without resistor adjusts)


Thus rather than a Variac, for most amps, we really want an ANTI-Variac (surge/voltage regulator) that provides correct heater voltage regardless of MAINS or LINE INPUT.
To do the opposite is to destroy tubes, drastically shorten tube life, and is only sensible if you are an idiot millionaire.

"cutting the line voltage to ~90v from ~120v" may give you some 'nice sound',
but it would cut your heater voltages to about 75%,
and for any length of time, cut your tube-life expectancy to only a few hours!

You can get the same results without destroying your tubes several other ways.


The RCA study included currently manufactured power tubes, signal tubes, sweep tubes, TV tubes etc.
It presumably included all tubes with modern cathodes and filaments, and those with coated cathodes and anodes.

The large directly heated triodes of the previous era were probably not even under consideration. The reason would have been that RCA did not make them. I think for a while that Western, and Taylor continued to make the older tubes, thoriated tungsten, carbon anodes etc., and probably RCA continued to make its 813s and 814s for a while under army contract for transmitters and radar.

But the study refers to common tubes made and promoted in the 1960s and beyond.

Not 'Power vs Signal', but rather 'modern vs. pre-war'.