[Amps] MLA2500 issue

[Mike Smith VE9AA]

Hi All, 

 

I'm no amp expert, but have owned an MLA2500 for close to 3 decades.  I've
repaired it a few times (all new caps, timer tube replaced, PS board
replaced one time). Might have put in a new tube once.

I use it for contesting. I've used it on DXpeditions back in the day, so
it's sees LOTS of use.

 

When I am on CW, I always operate it in the lower plate voltage mode.

 

Recently in a contest I started experiencing sudden/rapid power output loss,
from its usual 750w CW to perhaps 300w for some minutes, then briefly 100w,
then within several minutes, no output at all. If I shut it off and restart
it 20-30 minutes later from cold(warm?) I usually sees perhaps 300-400w out
for a couple minutes, dropping to 100w within a minute,then 0 (or
essentially 0) watts out.

 

It's just getting worse.

 

When I took the cover off, it appears one of the 8874's is much darker than
the other. This might be nothing, as I have had various 8874's from
different sources over the years, but thought I'd mention it in case it
means something

 

.

Looked around and measured continuity with the ohmmeter here and there. Not
seeing anything obvious.

 

 

Is it a bad tube?

 

Where would I start troubleshooting?

 

Thanks

 

Mike VE9AA

 

Mike, Coreen & Corey

Keswick Ridge, NB