[HomeBrew] Single Tube Transmitter Notes

[bcarling at cfl.rr.com]

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