Dave Platt wrote:



"Note:
1. Apply the rated ac line Voltage to the set directly. Do not increase
the Voltage gradually by using a variable transformer or other such
instrument; this will cause a V-FET failure."


That feels to me more like a design flaw in the circuitry... something
which mis-biases the VFETs if the rail voltages don't come up and down
at the prescribed speeds. Possibly the gate voltages rise faster than
the drains, and the gate/source or gain/drain voltage limit is
exceeded?

** Take a look at the power stage schem he

http://www.angelfire.com/sd/paulkemble/sound8f.html

Note the unusual crosswise connection from the drive stage to the six V-FET followers.

The V-FETs used behave like J-FETs requiring reverse polarity gate bias voltage to turn them off, with zero bias they conduct heavily - the opposite of the case with transistors and MOSFETS.

So, when using a variac, a condition must exist where the needed reverse bias is missing but the V-FETs have enough DC supply available to fry themselves if held there.



..... Phil




















A power amplifier that will fry its output stage if the mains voltage
bounces around (say, during a "brownout" of commercial power) really
seems like a poor design to market.