[Amps] Henry Amp - Temp 2000

[Greg Hicks]

I just acquired a Henry Tempo 2000 amp.
 
When I power it on, It comes to life.
 
When exciting it, I get plat current, and grid current but no plate voltage.
 
I have observed that if I push a small button (switch) on the back, I get
plate voltage and hear the power supply increase in the hum.  Immediately
after releasing the button, the plate voltage falls back to zero.
 
I can't find any schematics yet, so kinda working blind..
 
Thanks for any help.
Greg
WM5D