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

[Ron.K3PID]

The 8072 is the final in the high end (for its time) Signal One CX7A and it proved to be an excellent choice.

Ron H
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  ----- Original Message ----- 
  From: Mark K5AM 
  To: boatanchors at mailman.qth.net ; Sandy 
  Sent: Friday, December 11, 2009 2:10 PM
  Subject: [Boatanchors] The 8072 tetrode in a transceiver or small amplifier




  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

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