Hello Dimitrios,
Yes lets share ideas and progess, hope I managed to get any crystals
(very odd way of ordering and I don't know if I actually got any yet).
There is already one mighty fine built example on the net, but no
schematic provided:
http://www.cchidgey.pwp.blueyonder.co.uk/500_KHz.html
On a side note there seems to have been a goldrush after those X-tals
because now there is suddenly only one channel available within the
band giving 473.6kHz but this is very close to NDB SA lower sideband
at 473.5kHz.
BR
Paul-Henrik, OH1LSQ
Quoting Dimitrios Tsifakis <[email protected]>:
I am glad you found the eBay seller and your project is about to be
revived. A VFO, as David G4DMP mentioned before, is also another valid
option. It shouldn't be too hard to make a VFO stable enough for 472
kc, even if it is a valve one! As a test, I built a VFO for 7 Mc using
a 6C4 oscillator and a 6AG7 as a BA - it was perfectly good for CW
work.
I hope we can exchange more technical information about both projects
here. Maybe this will be a source of inspration for others that have
considered the possibility of using valves for a 472 kc transmitter.
73, Dimitris VK1SV
2012/12/18 <[email protected]>:
What a coincidence Dimitros!
After answering to your posting I did a search on Ebay for FT-241 crystals
and found the same seller - an order has already been made :)
Many thanks for bringing up this subject - I had lost hope already.
Paul-Henrik, OH1LSQ
Quoting Dimitrios Tsifakis <[email protected]>:
Hi Paul-Henrik,
Yes, the same thought has been on my mind for a long time now. The
problem
is that fundamental X-tals for this frequency band are nearly
unobtainable.
I have even tried to custom order some new ones for me and a couple of
friends but never succeeded - the quantities would need to be much larger
for this type of crystals. (They use a different kind of cut as compared
to
crystals for say 3500kHz which can still be ordered custom fairly easily)
Hopefully people with a stash of suitable vintage X-tals will wake up
when
the new band opens for real and put some up for sale if they don't want
to
use them themselves.
I bought a few crystals from this ebay seller:
http://www.ebay.com.au/itm/FT-241-CRYSTALS-Many-to-choose-from-You-pick-/260897401407?pt=LH_DefaultDomain_0&hash=item3cbeb26a3f&_uhb=1#ht_4373wt_1188
There seem to be a few left. They are sold as-is, but I found that the
majority of the ones I bought worked OK. There may be other sellers on
eBay.
I'd say go for it - I have built a couple of "MO-PA" transmitters for
80/160m, they are really pleasant and rewarding projects with a distinct
sound of their own. This is especially true if you also key the
oscillator.
I have built recently a similar transmitter for 40 m, using a 6CL6 and
an 807. When I keyed the oscillator together with the PA, I couldn't
get it to behave properly, so I added a switch to leave the oscillator
on and key only the cathode of the PA. I only key the oscillator when
I want to draw some attention with the home-brew sound!
In general, I have had difficulty sourcing a HV transformer suitable
for an 807. In the past, I found that one can build a voltage doubler
or quadrupler, with modern electrolytics, it is a viable alternative.
The 6.3 V for the filaments I found I can get from a 6 V transformer.
Hint: it's easy to add extra turns on a toroid type transformer :-)
73, Dimitris VK1SV
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