[HomeBrew] Single Tube Transmitter Notes

Tue Apr 1 22:57:13 EDT 2014

Sounds like you have ALL the fun  (-:
David B.
W7DBH

** ** Please do NOT cross-post messages when posting to HOMEBREW **
**
**
** ----- Original Message -----
** From: "Carl" <km1h at jeremy.mv.com>
**
** I guess I'll have to recreate my 813 oscillator to show again it can be
** done
** and
**  sound fine. I had it running 100W on 80 CW using a couple of real FT-171
** BC-610 crystals and
**  VR tube regulated screen about 30 years ago. Output loading was
** stabilized
**  by going thru a bandpass filter (borrowed from the contest station) which
**  also provided the necessary harmonic rejection.
**
**  Since VR tubes have been steadily reaching end of life, even NOS, this
** time
** Id go
**  with a modern IC regulator and bypass the zener era of regulation.
**
**  Since I enjoy building, especially late 20's and 1930's circuits, another
**  was a PP NOS GE VT4A's self excited oscillator at 100W on 80 CW that took
** 2
** 1/2
** years of casual operation to confirm DXCC about 3 years ago. That wound up
** with regulated DC
**  filaments and an electronically regulated 1000VDC B+. That went thru a
** balanced to unbalanced
** tuner followed by the same bandpass filter into coax and a high inverted
** vee.
** Just a trace of chirp in the strongest winds and less than some modern
** transceivers was the final result.
**
** Carl
** KM1H
**
**
**
**
**
**>>
**>> ** Please do NOT cross-post messages when posting to HOMEBREW **
**>>
**>> Some suggestions for anyone thinking of building a single tube
**>> transmitter.
**>>
**>> I have found that you CAN get a fairly clean signal out of a single
**>> tube
**>> transmitter but it
**>> requires some attention to your antenna match AND to regulating your
**>> power supply. Also
**>> take care to limit crystal current. The use of a single tube
**>> transmitter
**>> is a compromise
**>> involving power output, keying, antenna coupling and crystal current on
**>> operation.
**>>
**>> Let's take these in order:
**>>
**>> 1) You do not want to overcouple to the antenna and push these little
**>> single tube rigs to their
**>> maximum power capability by heavy loading. Reduce the loading just a
**>> little and they are less
**>> likely to pull and chirp. It is especially difficult to build a clean
**>> one
**>> tube rig using larger tubes
**>> above a few watts.
**>>
**>> 2) Power Supply - needs to be large enough for the job. If you use the
**>> typical receiver
**>> transformer that can only supply 5 - 10 watts and try to run a single
**>> tube transmitter off it, the
**>> supply is going to diip a little as you key down and you will hear a
**>> yoop
**>> sound as the
**>> frequency pulls. There are two helpful things to keep in mind as you
**>> plan
**>> the PSU for a single
**>> tube rig transmitter. Have a stiff plate supply. One that does not drop
**>> voltage much when you
**>> key down. ALSO - if you use a tetrode or pentode it is important to
**>> regulate the screen
**>> supply, especially as you get above 4 MHz. For 160 and 80 you might get
**>> by without
**>> regulating it, but I still recoomend using regulation for any single
**>> tube
**>> rig.
**>>
**>> If you really want to use the larger tubes, you are going to need a
**>> regulator.
**>> It is much easier to regulate the screen than the plate voltage. You
**>> can
**>> easily come up with
**>> screen voltages of 75V, 90V, 105V, 150V, 165V, 180V, 225V, 240V, 255V
**>> or
**>> 300V using
**>> series combinations of the following regulators:
**>>
**>> OA3, OB3, OC3 and OD3 are octal regulator tubes providing 75V, 90V,
**>> 105V,
**>> and 150V
**>> respectively and need to sink 5 - 40 mA each. ideally set them to pull
**>> in
**>> the middle of that
**>> range. You want the resting current and the key down current thrugh
**>> your
**>> regulator, to both
**>> fall within that range
**>>
**>> OA2 and OC2 are 7 pin miniature regulator tubes providing 150V and 75V
**>> respectively and
**>> need to sink 5 - 30 mA each. Again, select your resistive dividing
**>> network so that they to pull
**>> quiescent current in the middle of that range.
**>>
**>> For example, if you want to provide a 225V screen voltage to your 6L6
**>> or
**>> 6V6 rig, put two of
**>> these tube sin series and do the mathemtaical calculations for a
**>> resistive dividing network as
**>> described in ARRL Handbooks or at various web sites such as:
**>>
**>> http://www.junkbox.com/electronics/sheets/GE_Glow_Tubes_ETI-176.pdf
**>>
**>> 3) Antenna coupling - Again, I prefer a pi network though some guys do
**>> OK
**>> with link coupling
**>> if it is designed and adjusted correctly. I find it is easier to get a
**>> match with a Pi Network
**>> output to the antenna rather than the link coupled designs of the
**>> 1930s.
**>> I can usually get
**>> more power out this way and be able to match a wider range of antenna
**>> impedances seen by
**>> the transmitter.
**>>
**>> 4) Crystal current. Much has been written about using various crystals
**>> but by far the most
**>> easily available of the these today are the FT243 crystals from AF4K
**>> that
**>> have been
**>> remanufactured using newer hc49 crystals inside. TO be sure these can
**>> work 100% fine in
**>> all transmitters using low level oscillators such as a 6CL6 - 6146 rig,
**>> or the usual 6AG7 - 807
**>> nostalgia transmitter that many hams have built either in their novice
**>> days or as a fun
**>> homebrew rig today. Many of the old Heathkits used this type of two
**>> simple stage design and
**>> rigs like these with 2 or 3 stages are by far the easiest to use with
**>> any
**>> crystals including the
**>> newer crystals.
**>>
**>> Bear in mind that many crystals have smaller quartz elements inside.
**>> This
**>> has been true for
**>> many years, long before we started doing it at AF4K Crystals. I have
**>> meny
**>> examples of older
**>> FT171B and FT243 crystals that had small adapters and much smaller
**>> crystals inside and
**>> can send photos of these to any doubters. David Ishmael, WA6VVL seems
**>> to
**>> be unaware of
**>> that in his article in Electric Radio this month. Do NOT assume that a
**>> large FT171B crystal
**>> has the large element inside and fry it in your 813 single tube crystal
**>> cracker (why anyone
**>> would want to build a single tube 813 rig is beyond me!!) A good number
**>> of the older FT171B
**>> crystals I have opened up have ana dapter and a very small crystal from
**>> an FT243 inside
**>> them! At least he does not mention this in critiquing the use of our
**>> smaller crystals.
**>>
**>> Some folks have had difficulty with their AMECO AC-1 or their homebrew
**>> single stage rigs
**>> either chirping or drifting as the crystals heat up with the modern
**>> smaller crystals inside.
**>> Thi is very definitely avoidable. I have heard clean AC-1 signals using
**>> our AF4K crystals.
**>> The crystals will very rarely ever break because they CAN take the
**>> current. They just get too
**>> hot if you hold the key down continuously with over 35 mA. Any crystal
**>> will. Crystal current is
**>> affected by several things. The applied voltage. The ESR (Equivalent
**>> Series Resistance) of
**>> the crystal and it's ratio to the other circuit parameters. Again,
**>> these
**>> will be affected by the
**>> circuit design chosen. The worst of all is the TRI-TET design also
**>> known
**>> as the "Crystal
**>> Cracker" by many hams because they had disappointment when their
**>> crystals
**>> broke in this
**>> circuit Yes, it happened frequently with the old original FT243
**>> crystals
**>> much to the sorrow of
**>> our early novices. We strongly recommend that you only use the safer
**>> designs such as the
**>> Pierce and "Colpitts" capacitive divider type oscillators.
**>>
**>> As to our crystals - a good number of the ones we have for sale are
**>> original larger FT243
**>> rocks inside, but many are not. Especially the ones outside the typical
**>> ranges used by the
**>> miltary in WW2. Some have been told that they fail, but that is not
**>> correct. Only if someone
**>> wants to use an 813 power oscillator and run 200 mA through the crystal
**>> or something silly
**>> like that. Even the old original FT243 crystals were recommended to NOT
**>> run greater than
**>> 60 mA of current through the crystal. If you look at some of the old
**>> ham
**>> construction articles,
**>> they often recommended the use of a 60 mA pilot lamp in series with the
**>> crystal to estimate
**>> RF current and limit it to a safe level.
**>>
**>> Interestingly - the study by G3UUR found that our crystals will
**>> withstand
**>> as much as 200 mA
**>> through them before failing and even then it was not the quartz itself
**>> thhat failed but the
**>> connecting welds inside the case.
**>>
**>> There are thousands of these crystals in use daily around the world in
**>> Heathkit, Drake,
**>> Hallicrafters, Gonset and Globe rigs; even in BC610 military 350 watt
**>> transmitters. My
**>> crystals were recommended for use in the older tube rigs by Dave
**>> Ingram,
**>> K4TWJ in his CQ
**>> Magazine articles before his untimely death a couple of years ago, and
**>> were also the subject
**>> of some serious scientific examination by Dr. David Gordon-Smith, G3UUR
**>> Materials Scince
**>> Professor in an 8-page article in Electric Radio Magazine.
**>>
**>> I stand behind these updated FT243 crystals. I can assure you that if
**>> any
**>> crystal fails you, I
**>> will replace it or refund the money if you are disappointed in any way.
**>>
**>> These crystals can easily withstand 40 mA. They only drift or chirp in
**>> certain rigs that are not
**>> adjusted well. For single stage tetrode or pentode rigs, regardless of
**>> what type crystal you
**>> use, I strongly recommend that you use a regulated screen supply. See
**>> above.
**>>
**>> Final notes on building One tube transmitters:
**>>
**>> There is a nice example of a CLEAN sounding one tube 6L6GC transmitter
**>> that is run at
**>> 350V DC on the plate and a regulated 225V screen supply designed and
**>> built by N4QR
**>> available here:
**>> http://af4k.com/6L6_XMTR_by_N4QR.jpg
**>>
**>> I contacted N4QR on this rig. He uses our remanufactured FT243 crystals
**>> for a great
**>> sounding signal on 80m. His 6L6GC rig sounded clean as a whistle.
**>>
**>> Some excellent additional notes by AA8V, Greg Latta may be found here:
**>> http://faculty.frostburg.edu/phys/latta/ee/6cl6xmtr/6cl6why6cl6.html
**>>
**>> See you in MAY 2014 for the special WES event featuring single tube
**>> transmitters!
**>>
**>> Mine will be on the air with a clean signal!
**>>
**>> 73 de AF4K, Bry Carling
**>> SKCC #1879 TX5
**>>
**>> http://skccgroup.com
**>> http://af4k.com
**>>
**>> Brian Carling, Radio AF4K
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-- 
David B.