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Old November 27th 16, 03:41 PM posted to rec.radio.amateur.moderated,rec.radio.amateur.homebrew
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Default [N2HTT] Wiggle room.


73, de N2HTT

///////////////////////////////////////////
Wiggle room.

Posted: 26 Nov 2016 09:56 AM PST
https://n2htt.net/2016/11/26/wiggle-room/


Ive been spending a lot of time messing with VFOs recently, both vintage
tube units and modern digital synthesis projects, all driven by the quest
for frequency agility with homebrew QRP rigs. My favorite homebrew
transmitter is the 2-tube W1TS rig, which has stubbornly resisted
cooperating with an outboard VFO. Even Ralph WB8DQT, the author of the
project, states in his documentation that VFO operation was not a design
goal for this lovely little transmitter. I wholly agree; I have tried with
a number of VFOs and a few different interfacing techniques, and I havent
been able to get sufficient drive from an external VFO. The thing is just
much happier with a crystal in its socket.

A while back, I had a thought: why couldnt you add the parts necessary to
create a VXO circuit around the crystal, and plug that VXO circuit into the
crystal socket? Youd get some variability with a crystal driving the 6C4
oscillator. It should work, right? I tried putting together such a circuit,
which never did work, since I didnt have any clue what I was doing. It went
on the pile of unsuccessful experiments, and I forgot about it.

Then just recently, I discovered acorn tubes. These tiny little tubes were
popular in the 1940s in VHF applications. Their small size resulted in less
inter-electrode capacitance, extending their useful frequency range into
the VHF. You can find them around on eBay, and from some of the sellers of
NOS tubes.

I got interested when I came across a couple of articles by Dave Ingram
K4TWJ (sk) in CQ Magazine describing QRP transmitter projects using these
little tubes. I tracked down a bunch of his acorn circuits (old CQ magazine
content is available behind a pay-wall here), and started accumulating
parts to build a 2-watter using two 955 acorn triodes in a push-pull
configuration. I have almost all the parts for this project and am looking
forward to starting it soon. So what does this have to do with VXOs?

One of the acorn tube articles described an optional VXO circuit that could
be substituted for the crystal in the oscillator. This got me thinking
about my failed outboard VXO attempt, and I did a search for VXOs for use
with tube rigs. I found a number of circuits: although mostly intended for
use with solid state QRP rigs, there was one result that hit the
jackpot. Â*I found a page showingÂ*a number of external VXOs intended to
drive tube rigs done by NT9K,Â*Â*Bill. (Scroll down this page to find the
VXOs. Bill has a very interesting website, well worth looking around while
you are there.)

This wasÂ*exactly what I had tried building on my own a while back, but
apparently his circuit worked!

I reached out to Bill and he replied to my email promptly, with the
information I needed to try to reproduce the VXO. As is turns out, the
original inspiration for the circuit was Dave Ingram again, who described
it in another of his CQ columns. Bill provided the critical missing
information how much inductance was needed and I built one of my own.
External VXO with HC/6 crystal onboard

I actually had most of the parts on hand, except for the 365 pF air
variable capacitor. These can beÂ*bought new from Antique Radio Supply
fairly reasonably, and they are nicely made units. ARS ships really quickly
too, so I had everything I needed in fairly short order.

Construction is point-to-point and the circuit is simple: the variable cap
and inductance are placed in series with the crystal, with a short cable to
allow plugging the circuit into the crystal socket on the rig. Using Bills
projects as inspiration, I mounted my circuit on a small wooden plaque
obtained from the local craft store. All the hardware is stainless steel
and brass. Two posts allow easy interchange of the inductor in the circuit,
which is actually quite handy as I tried a couple of different ones before
settling on 40 turns wound on a T-68-2 powdered iron core, about 10 uH.

I had the ceramic FT-243 socket on hand and used that because most of my
crystals are either vintage FT-243s, or modern HC/49 style crystals mounted
inside vintage FT-241 or FT-243 holders emptied of their out-of-band
crystals.

Many VXO circuits are designed to use more than one crystal, all on the
same frequency. It is easier to get more of a frequency swing with multiple
crystals. But I am only looking for 1 or 2 kHz around the crystal
frequency, just enough to work those guys you hear 500 Hz off your rock,
running tight passband filtering, who will never hear your call. For that
reason, and the fact that I dont have duplicates of any of my FT-243 rocks,
I opted for a single crystal socket. It would be easy enough to add another
socket in the future if this gizmo proves useful.

I enjoyed the Thanksgiving weekend at the alt-QTH this year, as we
entertained the entire family for Thanksgiving dinner. During the quiet
moments, I worked on this little VXO project, and by Friday evening it was
ready to try out. The W1TS transmitter lives at the primary QTH, so testing
would have to be done with my Knight T-60 transmitter which is currently
set up in the shack. I tried a number of different crystals and a couple of
inductors, and got a range of results.
VXO connected to the Knight T-60 Transmitter

The two inductors I tried were hand wound: one was 5 turns of 22 ga wire on
an FT-43-50 ferrite core, and the other was 40 turns of the same wire on a
T-68-2 iron powder core. The ferrite version measured 13 uH on my digital
LC meter I didnt measure the other one but the calculated value should be
about 10 uH. In testing these coils performed about the same, I went with
the red type 2 core, I think it looks nicer.

Results varied a lot with the type and activity of the crystal. Some of my
vintage FT-243 rocks only moved a rather disappointing 700 Hz, but the
smaller HC/6 types gave much better results with the same setup, swinging
about 3.5 kHz. The HC/6 style rocks are the ones in the metal can, with the
same pin spacing as the familiar FT-243s. I am looking forward to try this
setup with the modern HC/49 crystals mounted in FT-243 holders that I use
with the W1TS transmitter. I have a feeling that these will swing pretty
well, as they are very active compared to the old vintage rocks.
HC/6. FT-243, and FT-241 crystals all share same pinout

Connection to the rig is with a jumper cable that can be plugged into the
rigs crystal socket. I had a plastic plug on hand that fits the FT-243
socket (left over from an RF pre-amplifier project), and I used a short
length of 300 ohm twin-leadÂ*for the jumper. This has much less capacitive
loading than coax, but a short length of coax would have worked fine as
well.
300 ohm twin-lead terminated with weird plastic plug

Another plug idea that I have used often is to open up and empty a vintage
FT-241 crystal holder (these were mostly used for VHF radios, and are
plentiful at frequencies outside the ham bands), drill a hole to
accommodate the coax, and use the holder as a plug for the crystal socket.
You can do this with an Ft-243 crystal as well, but I think the larger size
of the FT-241s make them easier to use for this purpose.
Coax terminated with plug made from an FT-241 crystal holder

So Thanksgiving this year was great: warm house, big family dinner, snowy
landscape (yes, we actually had a footÂ*of snow the Tuesday before), and a
small homebrew project. Much to be thankful for.

73

de N2HTT


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