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[AF6AU]

I suggest you visit a internet plastics supplier and get a 1/4 inch thick sheet and a 1" dia. rod (usually 6" long), of a 20%-30% glass fiber filled Teflon. It's white, is saw cuttable, drillable, is reasonably stiff, and really great for insulators, bushings, and making replacement broken ceramic pieces like this variable capacitor dog ear.

It will easily handle 500F heat, and is a great insulator.

Using 2 washers of this under your plate choke sandwiching the circuit board, would have reinforced the circuit board attachment, and guaranteed no arcing, and no more flexing failures. Making a copy of the Variable cap's ceramic piece would be a breeze with a hacksaw, vise, and a hand drill.

[W1QJ]

Fixing that capacitor is fairly easy if you are experienced at doing it.  I do it routinely.  I can tell you that cutting cardboard probably won’t help you.  Since I’ve done it umpteen times I can say it’s more of an art than a science.  Don’t try gluing or repairing the ceramic. You need a new ceramic piece.  To save you grief I’d suggest you either buy a new cap or mail it to me and I’ll fix it for you.  If you take it apart further it will be harder to fix so if you decide to send it leave as is. 

[K6AER]

Ameritron sell all of its components including the variable caps.

[HAMHOCK75]

These comments have been interesting. I hadn't really given much thought to creating a replacement part for the damaged ceramic piece.

It wouldn't be that hard since I have all the tools from laying ceramic tile in some remodeling work done awhile ago including a diamond bladed ceramic tile saw and a Dremel with a carbide cutting tip which can form almost any shape. Below is a photo of a white ceramic tile about 1/4" thick with shapes cut with the carbide cutter.



I think I will remove the two brass rivets holding the remaining piece of ceramic to the frame so all three pieces can be bonded with structural epoxy on a flat plate. Not sure why that would not be strong enough since there are really few forces on the ceramic piece but if it is not strong enough, then I can use it as a template to cut a new ceramic support from a tile.

I can always fall back on a new capacitor since as AER mentioned they are still available from Ameritron.

[HAMHOCK75]

Does anyone know how maximum plate dissipation is actually determined? In other words, is the tube convection cooled, the temperature of the plate measured as power is increased? If that is the test then what maximum plate temperature is typically allowed in the test? I suspect the temperature will depend on if the tube is glass or ceramic so there will not be one number.

[W1QJ]

Plate dissipation has nothing to do with temperature, if it did the units would  be in degrees.  Plate dissipation is the difference between input power and output power.  The maximum dissipation as it relates to temperature is based on how well the tube is cooled based on the cooling requirements for the tube.  Though there is a relationship with heat and dissipation, dissipation is not directly dependent on or cooling for changing its actual value.  Naturally the mathematical formula tells us what the variables are.  Simple math. Pd=Pi-Po. 

[HAMHOCK75]

Supposing I just designed a new tube. Your job is to measure its parameters for publishing in a manual. State the criteria and conditions by which the plate dissipation will be determined.

[W1QJ]

Supposing I just designed a new tube. Your job is to measure its parameters for publishing in a manual. State the criteria and conditions by which the plate dissipation will be determined.

I think you have your hands full designing and building a ceramic replacement part for the variable capacitor.  I would tackle that first before you move on to designing and building tubes and then asking someone else to set the specifications on it.

[HAMHOCK75]

If there is someone who has knowledge of the test criteria and the actual conditions under which plate dissipation is determined by manufacturers, that information would be appreciated.

[W1QJ]

If there is someone who has knowledge of the test criteria and the actual conditions under which plate dissipation is determined by manufacturers, that information would be appreciated.

Asked that way, I submit this as consideration to your question.

https://www.google.com/patents/US2053126

[HAMHOCK75]

Thank you for the interesting patent post.

I have to wait for the epoxy to fully cure before doing much more with the tuning capacitor.

Meanwhile a used 6LQ6 was ordered from eBay to replace the damaged one. It arrived today. The plate current was measured for the replacement tube. It was 12 mA. The four tubes then have bias currents of 10 mA, 10 mA, 12 mA, and 20 mA. The plate current was measured with a Fluke 87 digital multi-meter inserted between the ceramic plate cap and and the tube anode connection. All four tubes were inserted in the AL84. The temperatures of each tube was measured with an infrared thermometer in standby and operate modes. The results are shown in the image below.



The probe in the photo is a high voltage scope probe used to monitor the plate voltage just in case anything strange happened with the long leads from the Fluke 87. Fortunately all was stable.

Notice the fan is to the left of the tubes although there is nothing on the rear panel to prevent the fan from being directly behind all the tubes. It does has a pretty strong effect on the glass temperature of the tubes. The laser on the thermometer was aimed right at the center of the plate of the tubes. I understand the infrared thermometer cannot read past glass so it is measuring the glass temperature not that of the plates.

[HAMHOCK75]

Here is what the ceramic capacitor part looks like after one coat each of two different epoxies meant to work together. The first coat is a very low viscosity epoxy that flows almost like water and is able to penetrate well. The second epoxy is structural and designed to bond to the first high penetration epoxy and give structural strength. One more coat of the structural epoxy will be applied mostly to improve the cosmetics. After 24 hours the structural epoxy can still be easily carved with an X-Acto knife to the proper shape as shown in the photo below.

[KM1H]

What are the names of those epoxies?

[HAMHOCK75]

PC Rot-Terminator and PC Woody

[HAMHOCK75]

All the missing chunks of ceramic have now been filled in with the structural epoxy which will take 3-7 days to fully cure. There are no stresses on the ceramic since the part slipped right on with no forcing necessary. After the cure is complete, the final adjustment of the plates will be done but it does not appear many plates need adjusting.