My mistakes & lessons learned
Below some pictures of mistakes made and lessons learned. Maybe it will prevent you from making the same ones.....
Flash over
Flash over is a common problem when using high voltages. Especially in the area of the anode a combination of high rf current and high voltage is present. This gives quite some strain on the isolation used.
I did use Teflon which is strengthened with a fiber mesh. It provides superb mechanical properties, but apparently could not cope with the rough electrical conditions.
It was replaced by regular white Teflon sheet of 0.5 mm.
I tested this sheet under high voltage conditions. The maximum voltage my test set can provide is a bit over 30 kV. The Teflon did not show any leak currents what so ever at this voltage.
In the picture you see a flash over through the original Teflon. It could not withstand a voltage of more than 10 kV.
Insufficient spacing
At irregular intervals the tube would arc, at least that's what I thought. However it turned out that the spacing between the anode ring of the tube and the grounded plate of the lecher was not wide enough. A wider hole would give a mechanical weaker structure so a Teflon insulation ring was needed to improve the insulation..
The difficulty is caused by the fact that the tube has a kind of small ring on the anode ring itself, effectively lowering the safe distance to the grounded lecher plate.
Although I did take this small ring into account when laying out the mechanical construction it still caused problems.
A Telfon "collar" of 0.5 mm cured this problem.
The "collar" seen with the grounded part of the lecher in place.
Untidy workmanship
While constructing the PA you have lots of holes to drill and filing to do. It is inevitable that small particles of metal are loose somewhere, also in places where you certainly do not want them to be.
Although I cleaned everything as good as I could a little particle still found its way into the sandwich capacitor, used in the HT line. Due to the pressure on the plates of the capacitor the particle was slowly pushed into the Teflon.
At a certain moment in time (after many weeks of problem free operation) the particle was pushed so far into the Teflon that the remaining Teflon was not thick enough anymore to prevent a (small) arc. There was no short circuit, because the path for the current was very small. Only load cracks could be heard.
Investigation yielded a tiny hole in the Teflon though which arcing occurred.
For comparison : the small holes on the bottom left and right are 3 mm in diameter.
Miscalculated dissipation
There are lots of places where high currents and/or high voltages can be found. The Ug2 shunt regulator being one of them.
If you take a look at the schematics you will find a resistor of 3k9 and a capacitor of 80 pF in series, providing better loop regulation of the stabilization when using a capacitive load (screen grid de-coupling capacitor).
A low voltage capacitor was taken just out of the junk box, a mistake, given the fact that is carries at least 800V. A 1KV type should have been used here.
The same applies for the zeners. These got so hot that the solder melted!. They were replaced by 8 times 10V/5 W types in series.
Most resistors connected to the 800V should be 10 W, just to stay on the safe side.
It's wise to calculate the dissipation of each component before carrying out the various tests.
Safe viewing
Especially during start-up you want to keep an eye on things. No matter how well everything was tested in advance (and should work right away) I always like so see what's going on.
Testing a QBL amplifier with an open cabinet is not a very wise
thing to do. Lots of lethal high voltages are exposed and the rf-radiation is
not very healthy. High rf fields can even permanently damage your eyes when you
look into them!
To allow viewing into the cabinet during operation I use a Plexiglas cover with a grounded wire mesh on top of it.
It effectively stops most rf radiation and allows you to see what's going on.
If all is OK that's quite boring, because there should be nothing to see!!