This might be of interest to anyone who owns one of these ( Yaesu FL2100 linear amplifier ). One of these came my way for repair. Both the 572b's had failed, one had lost it's vacuum due to the envelope developing a crack all the way round its circumference near the anode top cap ( see attached pics ), the other one had no emission at all. A new pair was installed, only to find that on 15 and 10m, it would burst into wild oscillation causing both valves to pass extreme anode current ( the anode current meter would hit the end stop! ). This fault was traced to one of the anode parasitic resistors ( in parallel with the parasitic anode coil ) being open circuit, the other was fine. After much difficulty in sourcing suitable replacements ( the originals were 22 ohm 3W metal film - metal film are about the only non inductive resistors that are available cheaply today. Old timers will remember carbon composition, these would resemble a carbon rod if you broke one in half, sadly these are no longer available today. ), I used two 56 ohm 3W and a 100 ohm 1W all in parallel to bring to total resistance to 22 ohms @ 7W all three resistors are metal film. Both valve's parasitic resistors were replaced as both were of similar age ( probably identical - I think these were the original manufactures judging from the solder work ). After replacing these, all was well and the amp performed to spec. One other thing I noticed was the tank coil had been arching on 15m at some time. This was due to the tap point having a sharp protrusion in close proximitey to the adjacent turn. I filed this down as best as I could, but also widened the gap between the offending turns. At full tilt no arching was evident. Neutralization was set up, a very easy task with this amp unlike others I have come across.This is a nice amplifier, my only criticism is that the band change switch could do with being uprated, the contact surface area does seem small ( even though the contacts on the other side of the wafer are also utilised ) given the high rf voltages and circulating currents present, however, provided correct tune and load procedures are followed and no parasitic oscillations are present the switch should have a reasonable life.
Barry Zarucki M0DGQHOME