[Boatanchors] The 8072 tetrode in a transceiver or small amplifier

[Mark K5AM]

To: Sandy, W5TVW

I saw your recent note about use of the 8072 in an amplifier.

The 8072 is one tough bottle! I use a total of eight of
these tetrodes. They were all obtained on the used
surplus market at very little cost; most are 25 to 30 years
old. With proper protection circuits, failures almost never
occur.

Photos and brief discription of the 8072 that I use on HF:

http://www.zianet.com/k5am/hfd.html

Screen current limiting

The crucially delicate part of the 8072 is the screen;
screen protection is absolutely mandatory. Unfortunately,
the Signal/One CX7 had no effective screen protection
circuit. Thus the failure rate of the 8072 in the CX7 was
quite high, earning the 8072 an undeserved notoriety.
The protection circuit used here monitors the screen
current and uses the ALC system to limit the drive level
to the PA, holding the screen current to a safe level. At
300 V, a preset limit of 20 mA keeps the screen
dissipation under 6 W, well below the manufacturer's
absolute maximum rating of 8 W. High screen current
occurs under improperly light loading conditions. When
the loading is set too lightly, or when a faulty antenna
fails to provide a proper load, the ALC voltage will rise
and the transmitter output will be low, but the screen
current will be held at the safe limit.

Grid current limiting

Grid current is limited by the ALC circuit. This is not so
much to protect the 8072 tube, which is also rated for
class C service and can take 50 mA of grid current, but
to ensure linearity. Lack of ALC in amplifiers is a major
cause of splatter on the ham bands. The 8072 requires
grid current of about 2 to 3 mA for full linear output; this
means that the tube operates slightly into the class AB2
region. The ALC circuit used here provides limiting at
about 2 mA of grid current and is very simple and
reliable.

Heater warm-up delay

The 8072 heater requires a 60 second warm-up period
before plate current may be safely drawn. The classic
glass vacuum time-delay tube is still available, at rather
inflated prices, and is only infrequently found at flea
markets at bargain prices. The easiest solution, as for
most timing problems, is an op-amp timer.

Heat sink fan timer

The 8072 PA heat sink does not require a fan, but in the
interest of long service life, it is best to keep equipment
as cool as possible. In high-duty-cycle operation, as on
AM or RTTY, the heat sink can become quite hot if a fan
is not used. A small inaudible 117 V ac muffin fan keeps
the heat sink fairly cool. A timer turns on the fan at the
start of each transmission, and keeps it running for
several minutes after the end of the transmission.
Another muffin fan, also inaudible, is installed directly
above the tube; it runs continuously whenever the heater
is lit. In receive mode, this exhaust fan removes heat
generated by the tube heater; in transmit, it helps cool
the anode.

Complete details:
"HF Circuits for a Homebrew Transceiver", QEX, Nov/Dec,
2001, 20-42.

The 8072 is also used here in a 2M amplifier; photo and
brief description:
http://www.zianet.com/k5am/amps/2m8072.html

Sandy, please keep us posted on your results with the 8072.

Best regards,

Mark, K5AM
www.zianet.com/k5am