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

Fri Dec 11 16:00:12 EST 2009

I agree, it is a nice tube and they are often available at hamfests for as 
little as $5 NIB.

The internals are exactly the same as the 8122 used in the NCL-2000, 
NCX-1000, and SR-2000. With a little bit of work it could substitute for 
one.

The early (at least) 4X150A and 250B anode cooler can be removed with a 
propane torch. The center hole can be built up with copper pipe. I havent 
had any dead Russian tubes to experiment with or later ones of the US built 
250 family.  An 8930 or Svetlana 4CX400A would be a perfect size for some 
real dissipation.

For homebrew and some horizontal airflow the cooler from a 2C39/3CX100 
family can be used, its held on with Allen head screws.

Ive run up to 600W output in a 6M converted SB-230 using a combination of a 
small exhaust fan on top of the cabinet and another on the heatsink. The 
anode cooler is from a 4X250 that Ive cut down to mount on the BeO insulator 
block.and the socket is the same as the original 8873.  A conduction cooled 
4CX250 can be used as a donor, they come in a few flavors. The plate current 
metering function is now screen current, plate current monitoring is not 
mandatory as long as some protection is included. I use a NCL-2000 plate 
overload relay with a different trigger sensor value. Screen overload 
protection is the same that Ive been using at home in my 6M NCL-2000 since 
around 1967; I finally pulled the never failed original tubes 2 years ago 
when the output went below 700W, its now at 1300W with some Burle pulls. 
Screen voltage is taken from the HV and is zener regulated. Not the best way 
but space dictated it. A new filament transformer was added and it does 
double duty providing regulated bias voltage.

Ive used both the original swamped grid network from the 230 and a tuned 
circuit. Either works fine, the 8072 is easily capable of 22 dB stage gain 
if only flea power is available.

I also would like to hear from others using this neglected bargain.

Carl
KM1H

----- Original Message ----- 
From: "Mark K5AM" <k5am at zianet.com>
To: <boatanchors at mailman.qth.net>; "Sandy" <ebjr37 at charter.net>
Sent: Friday, December 11, 2009 3:10 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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