[HomeBrew] Single Tube Transmitter Notes

Tue Apr 1 21:17:30 EDT 2014

----- 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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