things first: SAFETY-
you play with 5 V and 12 V solid state circuits almost exclusively, you
may lack an appreciation for the danger associated with working with
250 volts, or even 120 volts, either of which can be lethal. This
is not a DIY article, it's a discussion of design choices and
observations in building a tube-type transmitter, not intended for
duplication. Hams who are not experienced in building tube
equipment should become well familiar with safety practices before
attempting such projects.
on with the show-
only built one tube transmitter
before, but it was from a Handbook circuit. I wanted to do a QRP
transmitter of my own design, more or less, and this is the result.
My goal was to have 5 watts or so out, CW only, crystal
Another goal was to be
able to use a miniature
crystal in a tube oscillator. Will that work? Some
say tube oscillators have too much crystal current for these
research said that a tritet oscillator has low crystal current, so I
figure 8, page 249 of Orr’s The Radio Handbook (1959),
substituting a 6CL6 for
the 6AG7. It sort of complicates things - there's a tuned tank
output so one more thing to adjust. That's part of the fun
though, right? And there's a parallel tuned circuit in series
with the crystal tuned 20% or so above the crystal frequency. How
do you work multi-band operation with that thing? BTW - find the
Bill Orr book and lots of other vintage tube technology books here -
Let's show the
schematic before further discussion
The amplifier stage is a more or less standard circuit with a pi-net
output and capacitor coupled input, shunt feed to the plate and cathode
keyed. The 5763 seems to be a favorite tube among glowbug QRPers,
after the 6L6 maybe. The grid leak and plate and screen operating
voltages were taken from the data sheet. OK then, a pi-network is
designed by specifying Q and input (plate) and output (50 ohm)
resistances. What should the plate resistance be? I assumed
it should be calculated like you do for a class C transistor amplifier,
basing it on the desired power out and supply voltage. My notes
on the subject:
There are plenty of calculators for doing a
I need to know what resistance to convert the 50 ohm load to. I think for class-A it’s a matter of a
load line, but I’m not sure about class C.
Can I use the same formula we use for class C transistor finals,
is Z = Vcc^2 / 2*Po, or supply voltage squared divided by twice power
Using the above with 250 volts
and 6 watts, I get 5,208
The 1969 ARRL Handbook has design
guides and charts for a
pi-network on page 158. You start with the ratio of plate voltage to
current in mA. That should be 250/34.3
or 7.3 in my case. Using those charts and a chosen Q of 15, with C1 on
side and C2 on the load side,
XC1 = 240 ohms, XL = 265 ohms, XC2 = 36 ohms. Using these values and a formula for
computing Rin from C1 and C2 (QST 1/84 page 48) in my spreadsheet,
indicate that R at the tube side is 3360 ohms.
Reg Edwards has a pi-L calculator
that also gives plate load
resistance for the power and voltage level.
For my case of 6 watts and 250 VDC, he gives about 5,200 ohms as
and 5,000 as “actual”, whatever that means.
It seems that “ideal” maybe have been calculated
using the formula I
After a posting to EMRFD, Randy AD7ZU
suggests a formula of
0.6 * Vsupply / I, where I guess I is the DC supply current. This gives me 4411 ohms so it lands in the
middle. Randy also suggests that a pi-net won’t keep me legal for
I should use a pi-L. The pi-L is
described in Elmer Wingfield’s QST 8/83 article so maybe
I’ll give it a
Glen Leinweber VE3DNL goes along with the rough
formula V^2 / 2Po
and thinks a pi-net would with Q of 15 would just about keep things
Tayloe also thinks the same formula would be used.
This was more of a pain
than expected. The data sheet shows 7 mA
at 150 volts. Is it OK just to use a series
dropping resistor? I eventually decided to
use four 36 volt
zeners, 0.5 watt rated. With 15k in
can get 6.66 mA through the zeners if the tube doesn’t draw
anything. I mounted the zeners on a
terminal block with
bare lugs crimped to leads cut short in the interest of power
heat sinking. Trying out the assembly on
the bench with 150 volts, I got 148 VDC immediately which drifted up to
VDC. I’ll leave it on for about an
to make sure the power level (about 0.25 watt per zener) doesn’t
blow up the
Tank in series with
This is what makes it a tri-tet,
I think. It’s supposed to be
resonant above the
crystal frequency by 20% or so, I think.
The coil I calculated to be 2.0 uH for 40 meters (maybe a little
for 30 meters) and I was going to use another real air core inductor
they’re such a pain to mount I cheated and went with a toroid
instead. I used 20 turns #22 on a T50-2
RFC to shunt feed
I needed another RF choke and was low on pi-wound
chokes. Came up with a couple 1 mH small
ones and a big 2.5 mH. I hate to waste one
that’s big enough for a 100 watt or bigger rig on this rig, but 1
mH only gives
about 22,000 ohms on 80 meters, which is only 4 or 5 times the plate
resistance, a little marginal. I decided
to use both 1 mH chokes in series.
They’re nice in that they have a 6‑32 stud for mounting on
Plate current or relative power
I initially wired the
B+ through a miniature lamp rated 12 volts
at about 60 mA, so I assume I’m only dropping 5 volts or so at 40
mA or less. But I noticed that using the plate
current indication lamp on the tube TX wasn’t effective at all in
maximum output power. There was not a
good sharp dip. So I decided to try
measuring the output current at the antenna jack and tuning for maximum
antenna current. I initially wanted to have that drive an LED and tune
for maximum brightness, but I found that LEDs go from dim too bright to
abruptly so I used a meter. The circuit is shown on the schematic and
there's some more discussion on my blog entry-
I noticed that the TX was keying my RF-sensed antennas switch when not
transmitting. With key up I measure 20 mW out. So there’s only 23
Not sure how this
Maybe capacitively coupled from
grid to plate of the amplifier tube?
don’t see it as a big problem.
B+ switch so I can put the TX in standby with the filaments still on.
I have to be able to do that anyway when I go to receive since my
oscillator runs continuously.
About voltage and
I was sort of disgusted getting just a
little over a watt
out on 40 meters with my 200 volt power supply that drops to 190 on key
down. So I hooked up to the bench supply
and with key down, cranked up voltage to design value of 250.
Now I’m able to get four, maybe five watts out. My design calculations were probably OK as
far as target power level, but one needs to remember to allow for
with unregulated B+ supplies.
Wow, it's a major pain in the neck, ain't it?
Mounting just about any component is an adventure in mechanical
engineering. For lots of hams with skills in this area, I know
that can be lots of fun and the final product can be really nice to
look at. But for me, if it weren't for Manhattan, Ugly, Wire Wrap
and Proto Board methods, I'd probably never get anything built.
How does it work?
Looks pretty good. I figured if the HC49/U crystal was being over
driven it would let me know either by failing completely or by chirping
excessively. But I don't seem to have much if any chirp, so I'm
calling it success. I've only had it on 80 and 40, so despite the
schematic, this is a two band rig.
I've had two QSOs so far, matching the TX perfectly with a
Hallicrafters S40A receiver.
And just because I love the looks of those pi-wound chokes, here's a
picture of the rear of the rig. Don't they look like futuristic
hi-rise apartment buildings, as in The Jetsons? Also shows the RF
metering board with the antenna output lead passing through the toroid.
There you have it -
Nick, WA5BDUA couple of updates:
I was very pleased to have my little transmitter described in CQ
magazine's QRP column, written by Cam, N6GA. It's in the April,
Cam also came up with a neat power supply
circuit (should be in the June issue) that uses something I didn't know
existed -- a little LR8 3-terminal TO92 regulator (the size of a 78L05)
that can handle 450 volts on the input side. He follows it with a
TIP50 pass transistor to get his 250V.
2) I received an
interesting email concerning my transmitter from David Newkirk, W9VES.
David has a lot more experience and knowledge in the area of tube
transmitter design than I do and offered his suggestions and comments
which I reproduce below. I haven't had time to try them yet, but
plan to. Meanwhile anyone want to follow along the lines of my
design might want to consider the ideas below.
From the email by David Newkirk, W9VES:
I've been enjoying your 6CL6-5763 transmitter writeup ( http://pages.suddenlink.net/wa5bdu/2_tube.htm
and have a few suggestions for how you may be able to get more power
output and reduce your backwave. These suggestions have to do with
determining (and controlling) the drive to the 5763, and neutralizing
Overdriving a power amplifier can
actually reduce its output; pentodes and beam power tubes (like the
5763) are particularly sensitive to this. An easy way to determine how
much drive your 5763 is getting is to install a 100-ohm resistor
between the ground end of the tube's 22-k grid resistor and ground, and
then bypass that junction to ground. Now, by probing that junction with
a DMM, you can easily determine the 5763's grid drive: 0.1 V across 100
ohms equates to 1 mA. I see from a quick look at the 5763's specs that
its grid current should be around 1.6 mA during normal CW operation. If
you're driving your 5763 much harder than this--say, at 2.5 mA or
higher--you may be reducing its output.
discover overdrive to the 5763, the best way to adjust it is to reduce
the screen voltage to the 6CL6. Maintaining a regulated screen supply
to the oscillator is a good idea, so I'd put an adjustable voltage
divider between Zeners and the oscillator screen, with the screen at
the divider tap. I recommend not adjusting the 5763's drive by detuning
A 23-dB-down backwave is not out of
line with your 5763 because it's not neutralized--that is, because
power can flow through it bidirectionally as a result of feedthrough
through its grid-to-plate capacitance. One result is that the
oscillator signal is getting through to the output network--at a reduce
level--and out to the antenna through the final's output matching
network. I recommend installing capacitive bridge neutralization to
take care of this. When you've neutralized the 5763, you'll find that
the tube is easier to drive *and* that your backwave is much reduced.
(The quickest way to try out neutralization would be to replace C3 with
a few hundred pF--say, 220 pF--and connect a 1-to-10-pF variable, of
suitable voltage rating, between the bottom of the oscillator plate
tank circuit and the plate of the 5763. Adjust the neutralization cap
for minimal variation of the 5763 grid current as the 5763 plate is
tuned through resonance--a good, old-fashioned electromechanical meter
shows this better than a digital meter. You can get close by adjusting
the neut cap to null the backwave.)
reason for neutralizing your 5763, however, is that in its current
configuration it's almost certainly operating as a locked oscillator.
That is, it's actually oscillating on its own at a frequency finally
determined by the crystal-controlled signal from the 6CL6. To prove to
yourself that this is likely the case, pull out the 6CL6 and key the
5763 (just dits; now it's getting no drive from the 6CL6 and will draw
too much plate current if it doesn't take off on its own). Very likely
you'll still have output--possibly more output than with the 6CL6 in
play! This happens because the grid-to-plate capacitance of the 5763 is
more than enough for it to act as a tuned-plate, tuned-grid oscillator
when its grid and plate are tuned to, or close to, the same frequency.
that's all pretty interesting. Especially, I think, that you can
measure grid current in that way. My understand is that RF grid
current only occurs on one half cycle, or close to it, so it has a DC
level. It's blocked by the coupling capacitor and so the DC
return path is through R3, where David would add the 100 ohm resistor
with which to make the measurement.
Back to WA5BDU page