[Amps] 4XGU-81M amplifier

[Yueh Hsu]

Now the 3CX series vacuum tubes have become jewelry, and the price of the
3-500Z has become less affordable. With the reduction of the price of
solid-state devices and the increase of the output power of a single
device, the end of the vacuum tube has come. But personally I still prefer
vacuum tubes.
     Of the vacuum tubes currently available, I don't think there are any
better value than the Russian GU-81 and GU-5B. I don't know about the
situation in the USSR, but these vacuum tubes used to be widely used in SSB
transmitters designed and manufactured in China in the early 1960s. After
the 1970s, these ancient relics were replaced by metal-ceramic vacuum tubes
FU-250F (4CX250F) FU-100F (4CX1000A), FU-728F and FU-101F (4CX5000A)
replaced. However, the FU-81 (GU-81M) made in China is still in production
until now, and its purpose is single, that is, medical equipment. The newly
made FU-81 is now priced at $400. The Chinese-made GU-5B is named FU-5F in
China. It also has a water-cooled version FU-5S. The GU-5B was used in the
4KW SSB transmitter in the 1960s, but Many put out 5kW+ of power.
   A single GU-81 is used for 250-400W SSB transmitters, and 2XGU-81 is
used for 1kW SSB transmitters, but in fact one GU-81 can easily output
1000-1500W without significantly affecting the lifespan. And now the price
of GU-81 and GU-5B on ebay is still reasonable. Don't be prejudiced against
these tubes, their linearity is as good as modern cermet vacuum tubes. I
just completed a 2XGU-81 amplifier, and my practice has proved that the
power does not drop significantly between 21MHz-28MHz, but pay attention to
the excessive output capacitance. The design of the EX8A is correct.
    Inexpensive 4XGU-81grid-driven amplifier possibility. I know that many
people will suggest cathode-driven, which may be more stable, but the
driving power will be around 400-500W, and grid-driven will be directly
driven by a 100W transceiver. I know that a simple parallel connection of
four will definitely not work. Yes, my Grid-driven 2XGU-81 amp already
shows some kind of instability between 21-28MHz, when I increase the drive
from 30W, it will tend to some degree of instability (parasitic
oscillation), I've noticed that many Russians remove the aluminum tube
base, which may be to solve this problem (shortening the long lead). Our
Ukrainian friends suggested that I add ferrite collars to each pin. I think
a more perfect solution for 4XGU-81 may be to add a neutralization network.
Come to think of it, a 5kW+ amp, four $20 vacuum tubes instead of four
$2000 "jewels" is a lot of fun to think about. any suggestion will be
appreciated
      73!
                Hsu