[Amps] Triode amp. Problem - Low output

[Mike]

Hello,

Need help in trying to isolate the problem with a homemade GS-35B amp.

  The amp. worked okay 6/7 January in the ARRL RTTY Round-Up.  Today, 
the symptoms are:
    * SWR varies as the load capacitor (C2) is decreased when 
operating into an antenna (<1.4 : 1 SWR).  Into a dummy load, the 
output increases until a value of C2 is reach and then the output is 
steady as C2 is, further, decreased.  SWR does not change.  (SWR 
meter is a LP-100 Digital Vector RF Wattmeter)
    * The tune capacitor (C1) does not produce a noticeable increase 
in the grid current as its capacitance is increased or decreased 
after C2 reaches the point of minimum capacitance for maximum output.
    * The output is ~900 W for 3500 V at 750 mA.  Previously, this 
plate voltage & current would produce ~1400 W output.
    * Half as much drive needed for twice the plate current.  The 
input is ~35 W rather than the 80 W previously needed to reach the 
750 mA plate current.
    * Grid current is only 150 mA for the 750 mA plate 
current.  Previously, >250 mA of grid current would flow for 750 mA 
plate current.
Things tried so far and results:
    * Inspection found no burned or arcing marks on any parts in the amp.
    * Parts and connections are tight.
    * The tank circuit (PiL) was tuned to the previous tune/load 
values when the amp was working okay (~1 : 1 SWR, 50 Ohms) using 
MFJ259 on the amp. output, 2200 Ohm resistor from the tube plate to 
ground, and amp. transfer relay shorted.
Things to try:
    * Check filament voltage.
    * Take the bandswitch out of the amp. and thoroughly inspect.
         - Look for any corrosion that may be keeping the switch 
contacts from fully making contact (some corrosion is seen on the 
silver           plated part of the rotary)
         - Look for any between-the-terminals arc tracks

The other homemade GS-35B amp of the same design but using a 
different type of bandswitch and output transfer relay in the output 
tank circuit is working okay.

Any ideas on where to check will be appreciated.

         73,
         Mike, K4GMH