interesting econco tube failure

engineer · Post by **engineer** » Mon Jan 15, 2007 11:22 pm

I have a 4CX3500 econco rebuild that just died after 6 months. The transmitter dropped to 30% power with the screen at 400v, but you could drive the screen to around 900 volts and get 50% power. I had lower than usual grid voltage and absolutely no grid current. The solid state IPA was showing normal voltage at 30. The IPA current was down to 3.5 amps from 6. I suspect a grid wire failure inside of the tube. A grid short should have tripped the bias supply and folded back the IPA to nothing. I guess the RF drive was still picked up by the cathode emissions even though the grid wire would have been open. This may explain a match issue and low IPA current. Anyone ever experience this possible open grid condition?

Cameron · Post by **Cameron** » Tue Jan 16, 2007 8:40 am

It's sometimes hit-or-miss with rebuilt tubes.

I've had a similar problem with about 30% output and the screen grid (I) pinned negative. Another install had fluctuating readings on the screen grid and power coming back on the IPA.

Replacement tubes fixed the problem.

4CX15000As and all of their little brothers come in here new, when needed. I haven't had a used tube since 2000. My take on it is that one half the cost is not worth twice the headaches.

A malfunctioning screen grid could increase capacitance between your control and plate. The screen grid is kind of an electron gas-pedal - pushing the plate to a second emmission of electrons and creating, at a point, a dip in current.

Check your surrounding components (bypass cap, resistor to the high voltage supply, fingerstock, ect) since your nose is in that circuit and then call Econco for another tube. Hopefully that "about 6 months" is more like 4 months so you can be part of their 3,000 hour unconditional guaranteee.

engineer · Post by **engineer** » Tue Jan 16, 2007 5:18 pm

Yeah, an old noisy tube off the shelf went right up to 100%. Econco said the condition described is an open grid. I'm going to give then another shot at a rebuild. I use their rebuilds in several AM settings with no problems.

genlock · Post by **genlock** » Tue Jan 16, 2007 5:28 pm

It is really handy to have a working spare tube on the shelf.
Put all this info in your maintenance log. These symptoms may pop up again and it may save you or your successor some headaches.

Econco is now owned by Eimac and they both operate from the same facility.
Options are decreasing.