[Amps] Alpha -91b Problem

[Brass Pounder]

Hey folks,

A friend of mine (W6AAN)  reported a problem with his Alpha-91b (2XGU-74B, 
made in Bulgaria): when attempting to send CW at speed more than 5 WPM at 
1500W output,  the protection kicks on. Nothing similar happens at output 
power less or about 700 W. The same RIG (TS870S) works just fine with a 
different amplifier. There are more clues:

1. On SSB mode - no problem, Alpha-91b delivers full output power.
2. Tried ALC "on" and "off" - the problem is still there.
3. This happens on ALL bands.
4. VSWR on all bands is less than 1.1:1
5. On CW mode, brick on the key - no problem, Alpha delivers 1500 W as long 
as the key is depressed.
As soon as sequence of dots from the rig is being sent to the AMP - the 
protection kicks on.

Any ideas?

73!
bp (wf3j)