[HomeBrew] Single Tube Transmitter Notes

Wed Apr 2 11:40:02 EDT 2014

The fun starts after the bugs are worked out! Before that it is a learning 
experience which ranges from pure evil to pure joy.

Carl
KM1H

>
>
>> ** Please do NOT cross-post messages when posting to HOMEBREW **
>>
>> 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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>> **>>
>> **>
>> **
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>>
>> -- 
>> David B.
>>
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