[Amps] Rehabbing that dusty old

Manfred Mornhinweg mmornhin at gmx.net
Sun Apr 22 22:41:16 EDT 2007

Hi Brian,

>  Thought I saw a faint blue glow after an extended keydown
> into the dummy load. The first CW QSO, however, into a resonant
> antenna brought a brighter blue flash in one tube, popped breakers on
> the primary, and the that familiar cooking something electronic smell
> ('The smell of progress!').

Sounds like you have a gassy tube.

> Any way other than destructive testing to figure out if the tube's
> going to arc again?

Applying some high voltage through some sort of current limiting device. 
But it's not simple. You need to apply at least twice the normal high 
voltage of the amp, no RF drive, current limited through a fast high 
voltage fuse, and then adjust bias to control the current... not very 

> Any other suggestions ( I have a couple of NEW 3-500zg's, but really
> want to see if these Eimacs can be put back into service )?

In my experience, when a 3-500Z starts flashing over, it's ripe for 
mounting on a nice wooden base and exhibiting in your living room, 
rather than in the amp. They make great decoration! But they are no 
longer good for practical use at high voltage. They will flash over 
again and again.

Some years ago I was asked to install new 3-500Z tubes in a Kenwood amp. 
No, I didn't need the instructions that have been circulating on this 
list lately! But the owner of the amp would have needed them, so he 
handed the job to me...

The original tubes were Eimac, and the guy had a set of new Eimac tubes 
on hand. I put them in, checked the whole beast, replaced a charred 
resistor, then test ran the amp a few hours, trying all bands. But after 
a few months, the guy was back, and again one of the tubes was flashing 
over as soon as about 500 watts were going into the antenna! He had no 
more spares. The non-arcing tubes of the two sets were too different to 
be used together. And the guy didn't want to spend a lot of money for a 
third try. So I suggested to buy a pair of those cheap Chinese 3-500Z 
tubes. He went for the try. They arrived, I put them in, and they worked 
quite well.

That was about 7 or 8 years ago, and the last news I have is that these 
tubes are still working fine. No arcing at all. Those Chinese cheapies 
have produced a DXCC with several add-on stickers, and are in the game 
for the 5-band DXCC!

This shows that the old adage of "you get what you pay for" is not 
always true! It seems the Chinese 3-500Z are somehow more resistant to 
developing that arcing problem, than the Eimac tubes are.


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