[Heath] An "interesting" discovery...Cetron 572Bs and reactivation.

[Kenneth G. Gordon]

I have been working on reactivating some Cetron 572Bs (9 of them) I have here and made 
an interesting discovery this afternoon. I really have no explanation for it, and am hoping one 
of you here may have some ideas.

Here's the story:

One Cetron 572B I have been working with was obviously very heavily used: it is very dark all 
around the top, and for much of the center of the glass.

The getter still looks "decent" however.

I installed this tube all by itself in my SB-200, and with eveything loaded up normally on 40 
meters into my 600 watt dummy load, through my tuner, with 1:1 SWR, the maximum power 
output it would show was 75 watts and no grid current showing, even at 100 watts drive.

I felt I really didn't have anything to lose, so I gave it a second "reactivation".

After re-reactivating it, and re-installing it, I then held the key down to measure power output: 
as the tube heated up, power output rose from about 200 watts to beyond 300 watts, but the 
tube plate started getting very red, fairly quickly. I held the key down until I could no longer 
see a rise in power output, then un-keyed the rig and let the tube cool a bit. At that point, 
power output was 300 watts plus, and the top half or 2/3 of the tube's plate was very, very 
red: almost orange in fact. I didn't notice grid current at that point, but it certainly wasn't very 
high.

I then shut the rigs off and went to bed.

This afternoon, I thought I would take another look to see what that same tube's maximum 
power output might be under the identical conditions as yesterday, as I had not written it 
down, and power output, with that ONE tube, at normal drive level was now 500 watts plus 
(!!!), and grid current approached, but didn't quite reach 45 mA. The plate stayed dark. 

To say I was surprised would be an understatement.

I just installed a second old Cetron, one which showed a maximum power output of 110 watts 
under the same conditions as the first tube, and after reactivation, it showed close to 200 
watts, at first, rising to well over 300 watts output, again with almost no grid drive showing, 
and with the plate of the tube getting redder and redder, approaching orange, when the key 
was held down. Grid current was beginning to show, but was still very low.

I then started smelling something getting hot, and noticed smoke rising from the 40 meter 
grid coil on the back of the amp, so I unkeyed it. :-)

I had noticed, or rather THOUGHT I had noticed a few days ago ("suspected", might be a 
better word) that there was a significant change for the better in the way my reactivated 
Cetron tubes worked if I let them "sit" for a day or so, or even longer, after "operating on 
them". None of those tubes I suspected had gotten better over time had ever had the plate 
get red.  What I found today give my suspicions some basis in fact.

But....I cannot figure out WHY these two things happen.

Might anyone have a clue? Even one?

Yesterday, I also went through every one of those dozen Chinese-made tubes I had here. 
Although I found two that I had missed that were in very good condition and showed pretty 
decent power output (over 400 watts each) with good grid current, all the rest, 10 of them, 
were very weak, and all those I had attempted to reactivate were exactly as they were before 
the operation, or far worse. I tossed all ten of the bad ones, but kept the two "good" ones.

I also found one Cetron that although its remaining power output was 280 watts, only 1/2 the 
filament was working, so I tossed that one too. From this it is apparent that the two parts of 
the filaments are in parallel within the tube. I had one Chinese-made tube that was built the 
same way, but its power output was about 10 watts.

If, by letting the plates get so red,"re-gettering" the tubes is taking place, then I am going to 
have to find a different way to accomplish this, since my SB-200 grid coils are taking a real 
beating. Although I have some spares, I really don't much like the job of replacing them.

Ideas? Comments? Please?

Ken W7EKB