572B Problems

TOF Tuning Aid

Stability problems and arcing problems are unrelated!

Arcing or Gassy 572B Tubes

The tube to the left is in my high voltage test fixture.

In normal operation the anode voltage swings between twice high voltage supply to a few hundred volts positive. If mistuned of if there is a load fault of some time that unloads the tank, or if the exciter has a ALC overshoot issue, peak anode voltage can easily reach 3 or more times steady DC anode voltage.

A good 572B should take about 10,000 volts from anode, and show no signs of color streaming or arcing inside the tube. Slight florescence in tube glass in operation is common and not harmful, but any glow in cold tube internals with application of high voltage is not acceptable.

At 7.5-8 kilovolts, this brand new 572B glows a reddish color deep inside the ceramic and along the upper surface of the ceramic.

This 572B has an obvious problem in the ceramic insulator used to support the anode. This is almost certainly impurities of contamination in the ceramic insulator.

In an amplifier, this tube runs for a few hours and then randomly arcs. The anode meter kicks upwards without drive, the grid meter backwards, and the amplifier randomly  "pops".

No amount of preheating or operation cures this tube. It probably should not be trusted even at 1500 volts.

Below is another 572, this one with even worse ceramic.

This is gas inside the tube:

Note the color and that the glow is outside the ceramic in the "vacuum". The plasma streamers originate from all sharp metal points connected to the grid and anode inside the tube.

Stability Problems

Stability problems have nothing to do with arcing inside tubes. Arcing comes from exceeding voltage breakdown. Sometimes a bandswitch or tuning capacitor will arc, the most common cause being improper power amplifier loading or tuning. Loading arcs are normally sizzling arcs, and have to sustained for several seconds before damage occurs. These arcs do not normally blow fuses or other components, they just damage the component that is arcing through heat from the arc plasma. These arcs are caused by operating with tube and tank energy not absorbed by the load.

Arcing inside tubes, which is typically the loudest noise (bang or pop), is almost always instantly destructive. Plasma arcs inside tubes blow fuses, diodes, transistors, and other power supply and bias components. The tube, in effect, shorts the HV line directly to the grid or cathode. The section deals with stability issues, not tube failures.

The 572B is a relatively unstable tube. It has needlessly long thin grid and anode connections.

Let's look at the inside of the 572B tube:

The grid-pin to grid connection has almost 2-1/2 inches of thin single conductor length.

This sets the tube up for HF instability or for VHF parasitics.

The anode is much better. It is a short distance path with only 1 inch of thick ribbon.

The grid could be substantially improved. For many years, I tried to convince manufacturers of 811 and 572 tubes to use an additional grid wire to the empty pin, from the opposite support rod of the grid. This single change would be a major improvement in RF performance of the 572B and 811A vacuum tube. The anode could be slightly improved.