8877 Tube Tester evolves into Linear Amplifer

[w8khk]

One of my ham friends in town was looking for a way to test 8877 tubes that he acquires as pulls from medical equipment, and approached me for a solution.  Tested tubes sell better than condition unknown devices.  I set out to build an emission and transconductance tester, but decided that in addition to emission, full power testing would be beneficial.  

I collected most of the parts in short order, but had no luck finding a socket with grid ring hardware, so I decided to modify an existing socket for the application.  I will include photos of the socket, before and after mod, and a few of the amplifier as the RF deck takes shape.  

I thought others might be interested in the method I used.  The amplifier will sit
in a 3-1/2' rack, with power supply in the bottom.  A pole pig and a 100uf 4kv oil cap are in place, and I am now building the rest of the supply, control, and metering circuits.

The socket looks like it was originally used with a 4-65A or 829 / 832 style tube.  I drilled the rivets holding the ceramic socket to the aluminum base/shell, removing the seven spring clips in the process.  I straightened the right-angle bend in the bottom of each spring clip, leaving just a slight bend, to provide tension.  I then added a bit more curve to the other end of the spring clip.  I then drilled holes for 6-32 machine screws between each 3/8" hole around the circumference of the base shell.  I attached the socket on the inverted base with 6-32 machine screws and nuts, then attached the seven spring clips to the circumference of the base shell with 6-32 machine screws.  

The finished socket mounts on the base of the amplifier plenum, which was fabricated from a Bud Minibox, original dimensions were 13" by 7" by 4".  I cut the top and bottom of the box down to 3" high, so the blower squirrel cage and tube/socket assembly could fit inside, with the anode cooling radiator protruding through the top of the enclosure.  Pressurizing the plate compartment allows cooling air to exit both through the anode radiator and the socket assembly, effectively cooling the pin seals.  (I do not know if Eimac recommends this method, but it is referenced in some of the handbooks.)  The nuts on the top of the ceramic socket prevent the tube from fully seating in the socket, allowing more air flow around the pins and seals.  (The pins still extend fully into the socket connectors.)  The seal around the anode radiator is provided by a sheet of G-10 glass epoxy board.

The additional pictures show the remaining RF Deck assembly steps.  Metering includes filament voltage, grid current, plate current, and plate voltage.  (I really do not like meter switches!)  The front panel includes a band switch on the left, and plate tune cap above the loading cap control.  I sacrificed front panel symmetry in favor of short interconnects in the tank circuit.  The loading cap is mounted on a 1/8" aluminum plate attached to the bottom of the tuning cap, and interconnects are either solid 3/16" aluminum plate or 3/8" copper tubing (from the bandswitch to the loading cap.)

I will provide updates as the project nears completion.  It is a low-budget project, mostly built from junk-box parts, as evidenced below:

Parts purchased at fests:

$80   B&W 850A Coil Assy        (Dayton OH 2010)
$25   Plate Tune Cap              (Dalton GA 2009)
$10   Two Panel Meters           (Marietta GA 2010)
$5     Ferrite Rod for Fil Choke  (Dayton OH 2006)
$1     7-Pin Socket                 (Dayton, OH 2009)
$1     Two Terminal Strips        (Marietta GA 2010)
$20    Plate Choke                  (Marietta GA 2010)
$6     Glitch Resistors              (Marietta GA 2010)
$3     Terminal lugs                 (Harbor Freight)

$151 Total Cash Outlay

All other parts, hardware and metalwork from scrapped HP computers and peripherals, and junkbox.

More pics to follow...

[w8khk]

More pictures of socket and amplifier

[w8khk]

Three more pictures of amplifier construction

[KM1H]

Thats a lot of work to test a tube . It is also an interesting way to convert those WW2 829B/3E29/832A sockets, shows creative thinking

About 20 years ago I picked up a Dentron DTR-2000L with a bad tube for $200 with the idea of converting to 6M and selling. Instead I added protection circuitry and grid current metering and have used it as mostly a test bed for those medical pulls. It can also act as backup to the 2 regular amps.

Several other DTR's have been converted to 6M but thats a different story.

Carl

[K5UJ]

That's some very good looking cabinet and sheet metal work there.   If anything I build looks half that good I'll be delighted.  I've wondered about those delrin ferrite core plate chokes before.  didn't know if they were any good or not.  By that I mean in part, I have not read or heard anything about how well they handle heating compared to porcelain for example. 

[WD5JKO]

Rick,

   I like your work, and innovation.

I too used a 4D32 socket with a 8877. This one is inside a Dentron MLA-2500. After I took that picture I had to space the socket back about 1/8" for more airflow. I kept the cathode 'rf-hot' and the filaments rf-cold' like Dentron did with the 8875's. I know that this was not recommended, but so far so good.

The RF choke has 4 ohms DC resistance, and acts like negative feedback since it adds 4 volts bias at 1 ampere. There was a parasitic until I added that little 6 hole bead in series with the other cathode choke.

http://pages.prodigy.net/jcandela/MLA_2500_8877/socket_view.JPG

Don't throw away those old 4D32/829 sockets!

Jim
WD5JKO

[k3zrf]

Great way to 'test' the tubes....wondering will you show test results as "tested in a QSO"?

Might bring an extra buck or two.

[w8khk]

Thats a lot of work to test a tube . It is also an interesting way to convert those WW2 829B/3E29/832A sockets, shows creative thinking

I agree, it is a lot of work.  But after looking at what is needed to fabricate a tester, with cooling, etc. I decided to put the extra effort into building a usable amplifier.  It is fun to get some use out of the ole junkbox!

[w8khk]

That's some very good looking cabinet and sheet metal work there.   If anything I build looks half that good I'll be delighted.  I've wondered about those delrin ferrite core plate chokes before.  didn't know if they were any good or not.  By that I mean in part, I have not read or heard anything about how well they handle heating compared to porcelain for example. 

Thanks for the kind words, Rob.  The metal surfaces are a bit rough from knocking around in storage all these years.  I am in a rush to get this thing completed, but after the fact I may remove the panels and black wrinkle finish them, along with the rack cabinet.  Then some contrasting knobs will set it off nicely. 

Not sure about how well that choke will work.  The guy who sold it said that it is fine for legal limit 75 through 10, but it has some resonances in the chicken band.  For me that is not a problem.  Will let everyone know how it works out under power.  It checks fine on the impedance bridge, but that may not tell the whole story!

[w8khk]

Rick,

   I like your work, and innovation.

I too used a 4D32 socket with a 8877. This one is inside a Dentron MLA-2500. After I took that picture I had to space the socket back about 1/8" for more airflow. I kept the cathode 'rf-hot' and the filaments rf-cold' like Dentron did with the 8875's. I know that this was not recommended, but so far so good.

The RF choke has 4 ohms DC resistance, and acts like negative feedback since it adds 4 volts bias at 1 ampere. There was a parasitic until I added that little 6 hole bead in series with the other cathode choke.

http://pages.prodigy.net/jcandela/MLA_2500_8877/socket_view.JPG

Don't throw away those old 4D32/829 sockets!

Jim
WD5JKO

Thanks, Jim.  I will be watching for parasitics when I fire it up, first at low voltage of course.  The filament choke I wound on the rod shows almost 5k reactance from 1.1 through about 25 mhz, then rolls off.  So I added the tubular ferrites to handle the higher freqs.  We will see how it works.  I have read in several HBs that the filament may be at RF ground for HF rigs, because the filament to cathode capacitance is small.  At VHF and above they recommend tying the filament to the cathode and using a filament choke.  I took the safe route, and added the choke.

Regarding airflow, I think I am good with the blower from an HP disk drive.  It has been said that if the tube stays in the socket, you do not have too much airflow.

I would be interested in what hardware you used for the grid contact. 

[w8khk]

Great way to 'test' the tubes....wondering will you show test results as "tested in a QSO"?

Might bring an extra buck or two.

Good idea Dave!  I think I will add a comment to the test sheet that says "This tube strapped a slopbucket." 

[WA1GFZ]

nice layout