LOOSE HEATER CONNECTION
So far - all that have examined the cavity have found loose heater connections. I found this true when I first received the cavity. (repair was made) and again after operation of the cavity. These are the connections that go to the cathode and filament through the plastic feed-thrus. The plastic has a very low temp melting point. When heat is applied to solder the heater lines the lugs actually melt into the plastic, causing the feed through to become loose. (as large wire is required to carry the necessary current - a large amount heat must be applied to the wire and thus the lugs). I have found that the lugs actually melt into the plastic on both ends of the plastic feed through, which will cause a loose connection. When heater voltage is applied the poor connection causes more heat and soon you really have a serious problem.

To prevent this problem I removed the screw and outside lugs from the feed through. Then attached the heater wires to the lugs and re-assembled the feed through. I used heavier lugs that were supplied with the cavity in hopes that I would get a better connection (at least on the outside). To do this I found it necessary to removed the plastic handle from the tuning rods and also remove the 5 small screws holding the bottom of the cathode compartment. Once removed the bottom of the cathode compartment may be slipped out over the tuning rods, thus giving ample working room.

INSERTING THE TUBE INTO CAVITY
When inserting the tube into the cavity - care must be taken not to push the screen tube away from the anode section of the cavity. The screen tube is held in place by 3 screws located just below the anode section of the cavity. These screws could be loose or in some cases by pushing hard to seat the tube the screen tube could slip or be pushed down. (See Notes below on Cavity Mods)

ALIGNMENT OF ANODE FINGER STOCK RING:
In some cases the anode ring which contains the anode finger stock has to been removed. (such as to replace the dielectric material) It should be noted that the 5 insulators are NOT self centering. There is considerable play that will let the anode ring move around and not properly center. When inserting the tube - leave the 5 screws loose and let the anode ring center its self when the tube is inserted. Then lightly tighten the five screws. There has been one case when the anode ring was not centered and when the tube was removed - it caused the finger stock in the anode ring to fly apart and break up to a point that it was unusable.

CAVITY MODIFICATIONS:
Modifications have had to be made at both K5JL and KD4LT before the Cavity could be placed in operation. In a nut-shell the tube could not be properly inserted into the cavity without the following modification. If you measure the distance center of anode ring to center of screen ring on the tube, the distance is 19mm.  With the 3 screws that hold the bottom of the cavity loose and that section of the cavity pushed all the way up into the anode section of the cavity it was found that the distance from the anode finger stock to the screen finger stock was in excess of 24 mm. This would let the anode ring on the tube slide past the anode finger stock and not even make contact with the anode finger stock. (To see if you have the same problem loosen the 3 screws just below the anode compartment of the cavity and push the input section of the cavity all the way up into the anode compartment). There is a notch milled out that the input connector wall slide up into. When you move this part of the cavity it moves the screen ring, grid ring, cathode ring, and heater ring all together. Push it up and measure the center to center distance from the anode finger stock ring to the screen finger stock ring. It will be about or in excess of 24 mm instead of the proper distance of 19 mm.

You will note that when the input section is pushed all the way up that the input connector will touch the milled out notch and there is about an additional 3 or 4 mm that may be ground or milled off of the ring on the bottom of the cavity. By milling off this additional metal until the notch for this input connector is flush with the bottom of the cavity the center to center distance of the anode and screen ring can be reduced to about 20 mm.

Yes, to make this mod the whole cavity must be taken apart and the input section removed from the bottom of the anode section of the cavity. Just clear a work space and go to it. Be sure to unsolder the yellow wire that connects to the grid ring. (It will re-align ok) This grid ring must also be removed (6 set screws)  You may or may not have to remove the cathode and heater tubes at this time. If you do remove the cathode and heater rings - be careful as you pull this out as the finger stock that slides against the screen tube as it will fly all over. In most cases it has been in two or more pieces. You will also find another piece of finger stock that the screen tube slides on. (This can be removed and discarded - not necessary).

While the cavity is apart - REPLACE THE ANODE BYPASS CAP - The cavity is shipped with either 5 mil Teflon or Kaplon. All five amps that have been fired up have blown a hole in the 5 mil. Use either 10 mil Teflon or 2 pieces of good 5 mil Teflon or Kaplon or even better us a piece of Ultem. If you blow this bypass you will also blow a hole in the cathode bypass (which is 2 mil Kaplon) and blow the 2 mil Kaplon in the input tuning . In addition to all of the above I created a short between my HV filter choke and ground. DON'T EVEN THINK ABOUT FIRING UP BEFORE A NEW ANODE BYPASS IS INSTALLED.

When the above mods are completed re-assemble the cavity. Carefully insert the tube into the cavity. The tube should bottom out on the screen ring and the anode ring of the tube should now be about 1mm below the top of the anode finger stock ring. When the tube bottoms out on the screen ring, it should also bottom out on the heater ring. So far this has not been the case and now is the time you must adjust the cathode and heater tubes to be sure you are also at the bottom of the heater ring with the tube at the bottom of the screen ring. The cathode tube is held in place by 3 counter sunk setscrews. To get to these set screws you must remove the 5 screws that hold the input compartment to the screen tube.(Input compartment hold the cathode bypass and is where the cathode, heater and grid connections go in) This may be pulled down about 1/4 to get to the 3 set screws holding the cathode ring. You will probably have to take the bottom off of the compartment and loosen the screws that hold the grid, heater, and cathode bypass caps or you will break their leads when you pull the compartment out to get to the set screws. You can actually cut out the 3 bypass caps (the blue ones) as they are not needed.

With the setscrews loose - push the cathode tube (heater tube is connected to it) all the way up so that the heater ring of the tube now hits bottom of heater contact on socket. Tighten the 3 setscrews and re-assemble.

I found that when the tube is not installed in the cavity - there is a short from grid to cathode, do to the cathode finger stock touching the grid finger stock. When the tube is installed this short goes away as the finger stock is properly positioned by the grid and cathode rings on the tube. By now like Scotty and I you will be able to take the cavity apart blind folded. You are ready for Goo....

FIRING UP
Once all mods have been make you are ready to light it off. This should go quite normal now. The input tuning rods will tune about one inch out. By setting the input tuning and with the proper placement of the input connector (about 3/16 inch from all the way in) you can obtain no swr. Play with both adjustments until you have no swr. (swr on the input will cause excess heat) This adjustment can be make without plate or screen voltage on the tube. Once you find this point no further adjustment on the input is necessary.

The output probe shows optimum very far out (nearly to wall of cavity) We think we know why this is and will follow up on it at a later time. Plate tuning is very sharp and tunes with about 34mm of the probe sticking out.

With 3kv on the plate, 480 volts on the screen and 75 volts of grid bias, my idling current is about 200 ma. 130 watts of drive will increase the plate current to 1.1 amps, 20 mils of screen current and just a few mills of grid current. Additional drive will saturate the tube at about 20 ma gird, increase screen current to about 30 ma's and plate will go to about 1.2 amps. Further drive at this point only saturates the tube. If you want more output you will have to raise the screen and grid voltage, but remember the limitations of the grid bypass cap.