Re: LF: FT-241A xtals?

[M0FMT <m0fmt@yahoo.co.uk>]

Hi all

 

I have built a small QRP TX using tubes/valves for 500kc/s, it fits on the palm of the hand, sans PSU. Because finding Xtals for the band seems impossible I use two rocks and mix them, then VXO them onto frequency. 6.500-6.000 are pretty standard frequencies the difference equals 500kc/s (I have my 472 xtals ready ... :-). The tube line up is 12AX7 oscs - 6AU6 mix driver - 6CL6 PA. This line up gives 4Watts RF at the antenna socket. Its a fun rig for local use, strictly retro no solid state down to cathode keying using a relay. Recently used the second pair of contacts on the relay to key the osc cathode to eliminate the leaking through of carrier during RX. The effect on keying quality is minimal which came as pleasant surprise. Two way contacts have mainly been local but beacon reports using Opera have come from near EU counties namely PA0 land. 

 

This rig line up has been use in two other Tx's one 160/80m CW AM (choke mode with another 6CL6) (FM) FSK worked most of the UK on AM (3.615) and a 10m CW - FSK only version that uses a third O/T xtal but at full power I get chirp (work still to do). Both these rigs dispensed with the 6AU6 stage.

For a full retro style the use of none usable old style frequency crystals with internals removed and a modern xtal fitted inside the cases is a possibility but don't expect this to give rugged reliability if used in a "Power" oscillator design. If the older Xtal is of the plate variety and can be dissembled, raising the frequency can be achieved by edge "grinding" the slab i.e a light polishing. Skill is needed as the edge must be kept square for it to work and there is always the risk of fracturing or just stopping the xtal from vibrating by this technique. 

Please report success or lack of it. Tubes are here to stay!

 

73 es GL Pete M0FMT IO91UX

From: Dimitrios Tsifakis <dtsifakis@gmail.com>
To: rsgb_lf_group@blacksheep.org
Sent: Tuesday, 18 December 2012, 8:41
Subject: Re: LF: FT-241A xtals?

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  <phl@netikka.fi>:
> 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 <dtsifakis@gmail.com>:
>
>> 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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