What causes grid shorts in good 4-400's?

[VE7 Kilohertz]

For the last week or so I would hit the go button on the Bauer 707 and it was no go. All the relays were clicking but no HV and no output. I could see the final grid drive jump and then drop right to zero. After spending about 4 hours checking relays, fuses, connectors and reflowing solder joints I finally found the source of my "no grid drive" problem on the Bauer 707. I could tap the front panel and get the drive to come up but it would drop right away. I finally thought to tap the 4-400's and shazam! It was one of the tubes. Had a scratchy shorted grid. I could tap it and hear it tinkle and the drive would come on and then drop off. Changed out the tube and the problem is gone.

So, this is my first 4-400 failure in the Bauer and it was a good tube putting out full power. What can cause a good Eimac to go bad like this, so I can avoid the conditions that may have lead to this? I wouldn't care if it was a chinese knock off but a classic 80's Eimac??

Thanks

Paul
VE7KHz

[2ZE]

Had a scratchy shorted grid

Shorted grid? Shorted to what, the filament, screen or plate?
First need to make some ohm meter readings on the grid pin to fils and screen pin. If there's conductivity, its shorted, hopefully not to the screen pin because that would place all of the screen voltage on the grid circuit!
Chances are its open or intermittent open. I would re-solder the grid pin, heating it up and flow in some new "slober". Also check the connections at the socket too.
If that don't do it, you may have a grid parasitic causing problems, but you would see other problems with the rig ( exciter acting flaky, screen and plate overloads without warning, etc...).
Also, you just have to accept the fact sometimes that these tubes will fail.Especailly 4-400's. The grid and filament structures inside are very delicate, and during the course of a move, these tubes may get knocked around, no matter how carefully packed. Also, the failure may not show up for sometime, then POOF!, dead tube.
Hope all goes well for ya, OM.

Mike, 2ZE

[VE7 Kilohertz]

HI Mike,

Good questions. I don't know which element it shorted to. As you suggest though, it may be open too. I will have to check with a meter. I did a complete cleaning of the sockets, no difference. This tube has been working 100% for the last 4 years and hasn't been moved although it may have been just about to fail for that 4 year period.

As for parasitics, not sure exactly how to check for those. I did a sweep of the output with a spectrum analyzer and didn't see anything major. Just the usual little harmonics. It's possible the 4-400 has a parasitic but is being filtered by the tank cct. Could I scope the 4-400 grid to see parasitics or maybe just loosely inductively coupled to the plate??  Should I be seeing the parasitic grid current on the grid meter? I am only seeing normal 20mA grid drive.

Thanks guys.

Paul
VE7KHz

[2ZE]

Yea, what Phil said.....
a parasitic suppressor would be the way to go. Pretty cheap insurance. Also Ohmite makes little suppressors called the Z series. I used Z-50's on my grid input and screen input when I ran my 4-250's rig. Also, check all of your bypasses. There maybe a cheapie plastic tv type doorknob used as a bypass, but I doubt it since its a commercially made rig. Just check all connections and make sure there aren't any broken connections.

Mike

[Tim WA1HnyLR]

 Hi Paul I have had 4-400 and other similar types develop intermittent grid to screen or gid to phillament shorts. Connect a set of robust clip leads between the grid and screen pins. connect 120VAC from a suicide cord with an electric heater as a current limiter and plug it in. Hold the tube in your hand and Bing, Bing ,Bing Bing , rappatap on the tube envelope with your middle finger. Things will arc and spark but you will blow away the short. The tube will have altered states It will either pull more plate current then the other tube if it is grid that gets blown away or less plate current if part of the screen gets blown away. Under normal full power parameters  a brightlly glowing zit or pimp may appear on the plate. I had a 4-250 that I treated I used it in my first big rig. I had no other replacements so I had to make it work. I got a number of years out of that tube. Good luck De Tim WA1HnyLR

[K1JJ]

Under normal full power parameters  a brightlly glowing zit or pimp may appear on the plate.  De Tim WA1HnyLR

Let the Tron "Pimp Your Tube", caw mawn.

[VE7 Kilohertz]

Hi Tim,

That sounds like a cool method to zorch out the junk. I may try a big light bulb as a current limiter. That way I will get a visual indication of the current draw. That's the setup I use for bringing up boatanchors on the bench. About a 200W bulb gives a good inrush limit and provides enough pass voltage to get the filaments and B+ going.

Will let you know how it works.

Thanks!!

cheers

Paul

[WA1GFZ]

does the middle finger get the best results............