Gettering tubes 3-500Z, 572B, and similar
AL572B AL80B
When I designed the AL80B and AL572 amplifiers, I included a high voltage reduction provision. The transformer in each amplifier has HV winding taps that provide 1/3, 2/3, and full voltage.
In the AL572 amp, the 2/3 tap can be used to run 811A tubes!!
When you buy new import tubes, it is a good idea to getter the tubes. You can getter the tubes, or perhaps recover tubes that arc, by following the process below.
| NEVER remove the cover with the
amplifier connected to power mains.
Never defeat the interlock. Always verify high voltage is zero volts with the front panel meter before touching anything. WAIT for the bleeders to work. If you ground the HV to discharge, do NOT use a dead short unless the voltage is nearly zero on the HV meter. Never run filaments below nominal voltage during break-in. |
For filament voltage, see link
1.) Remove the cord from the outlet
2.) Remove the cover
3.) Locate the step start fuse near the left rear corner and relay. Change the fuse to a 1-amp fast-blow fuse.
4.) Locate transformer HV wires. Normal connection is red/green to R/G, red/yellow to R/Y, and red to R (C21 negative)
5.) Swapping R/G and R/Y without moving red (red stays on C21) produces 2/3 normal high voltage
6.) The layout pictured below produces 1/3 high voltage. Provided you do not connect two wires to the same point, there is no disastrous combination
It is OK to tack the wires in place, just be sure they do not easily pull off.
7.) Install the tube(s) you want to getter or burn in
8.) Install the cover and connect the amplifier to your radio normally
9.) Plug the amplifier in. Be sure the cover is on and has several screws holding it on!
High voltage should read 1/3 of normal
If the tube arcs at this voltage, it is "junk". It will never recover.
10.) After confirming high voltage, set your radio and the amplifier to 40 meters and run the PLATE and LOAD controls nearly open.
The idea is to keep the PLATE far from resonance.
11.) Place your radio on FM or RTTY on 40 meters, and bring up drive until you see about 200-250 mA plate current
You should have no or very little RF output.
Heat in the anodes is Ip times HV. In this case HV is 900 volts, and plate current (Ip) is 250 mA.
Notice Grid current is about 1/3 of the Plate current. This ratio indicates a good 3-500Z
12.) Watch the color of tube anodes. Anodes should be a very faintly detectable red. Do not run the tubes too hot.
Look for the brightest part of the anode. In this case the anode is faintly red. Adjust drive power and give the tube time to readjust color, and try for barely red at the hottest place.
Run the tube like this 2 minutes RF drive on and 5 minutes RF drive off for 30 minutes. Do not turn filament off and on!
When finished remove RF drive and allow the fan to run for 10 minutes or until exit air is not warm.
Unplug the amplifier, remove the cover, and exchange the red and R/Y wires. This will place the amplifier at 2/3 voltage.
Operate the amplifier normally at normal maximum plate current for several hours of normal use. This should be about half to 2/3 of normal RF power output.
After this break-in period, if you have a recoverable tube, it should be good at full voltage.