[Amps] dropping tube output

[Steve Bookout]

Hello all,

I have had plenty of amps in the past and currently own a bunch.

I have a question for the group because this situation hasn't come up 
before with me and amps.

I have recently bought an Acom 2000a at a fair price.  The tubes were 
described as 'full output', but I am seeing something different and it 
just got me wondering...

Turn am 'on' and increase drive to about 52 watts and get just under 
1500 out.  So far, so good.

I start this mornings CWT mini contest and almost immediately the power 
starts to slowly drop.  After several minutes it stabilizes at just 
under 1200 watts, about a 300 watt drop.  After the contest, I ran the 
power up to see how much it now took to get to 1500 watts.  I had to 
drive it with 70 watts to achieve 1500 watts. Come back later for the 
1900 Z CWT and the power was where I left it.  Now, after a several hour 
rest, 70 watts gives me almost 1800 watts.

It could be several things, but it reminds me of how the old sweep tube 
rigs acted when the tubes got 'soft'.  Was told tubes were 2 years old.  
He had 3 pairs of tubes and may have put an old pair back in due to 
mixup.  None of my other HB or commercial amps do this, so it's all news 
to me.

What would cause a tube(s) to behave like this?  Is it typical of tubes 
whose filaments were to high in voltage?... High or low plate voltage?  
High or low screen voltage?... Lack of cooling?... Over driven?...  Just 
age?...  two non-matched tubes?... etc.

I'll get it all worked out, but was wondering.

Comments pse.

73 de Steve, NR4M