Oddball 32V2 problem

[steve_qix]

I'm fixing Peter W1ZZZ's 32V2.  The initial problem was a shorted RF output DC blocking capacitor - and it put the full DC onto the ANTENNA OUTPUT!!  What a bad design - no RF choke across the output..... Someone could have actually been KILLED.

Anyway, I fixed that problem... then tried the transmitter - the grid current meter pins backwards.... did some tests, and determined the 4D32 must have a short from the control grid to (probably) the screen....although it tests ok with a meter (no resistance between the elements when the tube is removed).

When I remove the tube, and operate the transmitter, the grid current does not pin backwards, and the bias measured at the grid of the 4D32 does not disappear and go positive, as it does when the tube is in circuit.

So, I'm wondering if this is a common 4D32 problem.  I don't have any spares here - and Peter lives far enough away that's it's not exactly easy to just "drive over" and pick up a spare.  Al W1VTP has one, and we are supposed to meet up and I'll borrow his.

But, just how common is this?  I'm postulating the tube was probably damaged in rig transit - the rig was handled a number of times, including being put into another cabinet prior to transport.

I don't see any other possibilities as to the cause of the problem - everything points to the tube.

[WD5JKO]

Hi Steve,

   To my limited amount of understanding, grid emission is when the grid becomes an emitter of electrons. It has to get real hot for that to happen meaning something else is going on. With the 4D32, grid emission is common when flakes of the oxide cathode come off and get lodged in the G1 structure. Here as the flakes heat up, they get hot, and grid emission occurs. In a class C RF amp, the grid current (from the drive) on the grid current meter slowly drops, and then the RF output wanes as the tube plate current rises. soon the tube will turn red, and short out. This was common on 10m with the Viking 1, and an old 4D32. Switch to 80m and it was fine for a lot longer.

    With the 4D32, dropping the filament voltage a bit, say 6.9 to 5.9 sometimes eliminates the problem.

Also if you have cathode oxide specs getting loose, you might see them inside the glass envelope with proper light and magnification.

Jim
WD5JKO

[steve_qix]

Also if you have cathode oxide specs getting loose, you might see them inside the glass envelope with proper light and magnification.

Jim
WD5JKO

There's all sorts of white material rattling around inside the envelope.  Some of the pieces are pretty big (about 3/8 inch long!)

[N8ETQ]

Hey Steve,

   Not sure what that is but all my 4D32's have that. All are
functional to some extent. I think your right, Heavy Handeling
shorted something out in your tube. 32V's do have some
WW Resistors that are prone to corrosion on that terminal
board in the back. Swap those out if needed and find a diff.
jug. It will be OK. Old 4D32's loose high freq. preformance
like no other tube I've ever seen. 75m 100w,  40m 100w,
20m 100w, 15m 75w, 10m 10w. You need a "Megger" to see
that kind of short. Or the tube needs to be hot.

GL

/Dan

There's all sorts of white material rattling around inside the envelope.  Some of the pieces are pretty big (about 3/8 inch long!)

[KM1H]

Running the filament 2-3% below spec and reducing the screen voltage a bit does wonders for longevity. My Vik I has shown no deterioation for about 5 years and the 32V2 was given the same treatment but doesnt have many hours on the new tube it came with.

I think I'll replace that plate blocking cap pronto

Carl

[steve_qix]

Dropped in another 4D32 that I borrowed from Al W1VTP, and all worked properly    I suspect the jarring encountered during the moving of the transmitter damaged the tube, since it worked prior to the DC blocking capacitor shorting out, and there isn't any way THAT would have damaged the tube.

[Ralph W3GL]

Steve, I hope you put an RFC on the output of that tx...

[KM1H]

Dropped in another 4D32 that I borrowed from Al W1VTP, and all worked properly    I suspect the jarring encountered during the moving of the transmitter damaged the tube, since it worked prior to the DC blocking capacitor shorting out, and there isn't any way THAT would have damaged the tube.

Its possible the cap was already shorted or on the way out the last time someone tried loading it up and wound the screen current beyond molten. Since the 4D32 was used in airborn radar it is pretty shock resistant.

[steve_qix]

Dropped in another 4D32 that I borrowed from Al W1VTP, and all worked properly    I suspect the jarring encountered during the moving of the transmitter damaged the tube, since it worked prior to the DC blocking capacitor shorting out, and there isn't any way THAT would have damaged the tube.

Its possible the cap was already shorted or on the way out the last time someone tried loading it up and wound the screen current beyond molten. Since the 4D32 was used in airborn radar it is pretty shock resistant.

That was my initial thought, but there didn't seem to be any good way to get screen voltage without plate voltage.... and a plate voltage short to ground would have, at best, blown the fuse, and at worst, would have damaged something in the power supply.  But a melting screen could certainly give rise to a grid-to-screen short!!!

[KM1H]

I was thinking before the full failure. Just a few seconds of disturbed loading is all it takes as the cap is boiling.