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Author Topic: Tube VFO  (Read 14407 times)
AC5UP
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« Reply #30 on: August 11, 2012, 05:03:18 PM »

The 6AG7 is a tube only a Communist could love:      http://www.tubecollector.org/main/cv1882.jpg
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G3RZP
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« Reply #31 on: August 11, 2012, 10:03:14 PM »

Imitation is the sincerest form of flattery.......
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G3RZP
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« Reply #32 on: August 12, 2012, 03:51:28 AM »

BW, it is interesting that the tube in photo, despite being made in the USSR for a UK distributor, has a British military stock number on it....

At the time of the Falklands war, I heard on good authority that the ministry of Defence were somewhat dismayed to learn that the only source of spare klystrons for one of the Royal Navies' navigational radars was in Poland.....
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AC5UP
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« Reply #33 on: August 12, 2012, 05:31:50 AM »

This is true, but despite being electrically equivalent the British tube featured an enhanced internal structure which allowed it to better tolerate being shaken.

Not stirred.
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G3RZP
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« Reply #34 on: August 12, 2012, 06:46:03 AM »

Only that Russian 6AG7. Not the very few made in the UK - I think only Brimar offered them in the UK, and they may well have been US made, as many Brimar metal valves were.

A nice tube, the 6AG7. I must find a use for the number I have....
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W9GB
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« Reply #35 on: August 12, 2012, 10:06:01 AM »

Peter,

Here is one usage for the 6AG7, as a one tube amplifier optimized for use with DDS VFOs.
by Greg Latta, AA8V
http://faculty.frostburg.edu/phys/latta/ee/6ag7amp/6ag7amp.html

w9gb
« Last Edit: August 12, 2012, 10:07:50 AM by W9GB » Logged
K0IZ
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« Reply #36 on: August 12, 2012, 12:19:08 PM »

I have a BC-221 from long ago, periodically look at the wonderful dial and capacitor construction.  Picking up on this thread, I'm now thinking about converting it to a 80 or 40 meter vfo, using all the mechanical but changes to electrical values.  Seems I vagely recall an article years ago about doing that same thing.  Anyone remember?
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KB1WSY
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« Reply #37 on: August 12, 2012, 12:43:23 PM »

I have a BC-221 from long ago, periodically look at the wonderful dial and capacitor construction.  Picking up on this thread, I'm now thinking about converting it to a 80 or 40 meter vfo, using all the mechanical but changes to electrical values.  Seems I vagely recall an article years ago about doing that same thing.  Anyone remember?

There is an article in QST, March 1947. "The BC-221 Frequency Meter as a VFO" by Howard W. Johnson, W7NU. If you are an ARRL member it is in their online archive.

Note however that rather than *converting* your BC-221 into a VFO, this project involves leaving your BC-221 largely unmolested and adding an external power supply and transmitter exciter.

I believe that N2EY has used the LM dial (which I think is the one used in the BC-221) and presumably the tuned circuit too in projects and no doubt he will chime in!

73 de Martin, KB1WSY
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W8JI
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« Reply #38 on: August 12, 2012, 06:02:09 PM »

Peter,

Here is one usage for the 6AG7, as a one tube amplifier optimized for use with DDS VFOs.
by Greg Latta, AA8V
http://faculty.frostburg.edu/phys/latta/ee/6ag7amp/6ag7amp.html

w9gb


While it is a common design,found in many old rigs, I don't like that screen circuit at all. When the key is open, or the anode impedance very high, screen voltage will go to full B+ supply (about 450 volts).

This causes problems with excessive cathode voltages across the key jack.

It is a whole lot better to use a divider on the screen, or tap it off a lower voltage supply. Measure the cathode voltage some time with the key open, then put the screen down at rated voltage with key open, and check it again. The cathode voltage goes through the roof when the screen is allowed to pull up to grossly excessive voltages with open key.

One more resistor can make it a lot better, and greatly reduce clicks.

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G3RZP
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« Reply #39 on: August 13, 2012, 04:14:53 AM »

Plus you can destroy the heater cathode insulation, which can be a problem with cathode keying anyway. I much prefer grid block keying, but even then, you still should use a potential divider arrangement for the screen.
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N2EY
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Posts: 3909




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« Reply #40 on: August 17, 2012, 12:17:42 PM »

There is an article in QST, March 1947. "The BC-221 Frequency Meter as a VFO" by Howard W. Johnson, W7NU. If you are an ARRL member it is in their online archive.

Note however that rather than *converting* your BC-221 into a VFO, this project involves leaving your BC-221 largely unmolested and adding an external power supply and transmitter exciter.

I believe that N2EY has used the LM dial (which I think is the one used in the BC-221) and presumably the tuned circuit too in projects and no doubt he will chime in!

No doubt at all!

I have used the main tuning capacitor with its dial drive in a receiver and a transceiver. In both cases they did NOT come from working or repairable units! One was bought from Fair Radio in the 1970s, and the other at a hamfest.

I would not tear up a good LM or BC-221; there are plenty of junkers and parts units out there.

There are at least two styles of BC-221/LM capacitor. The first style mounts by means of three threaded holes on the front (three screws through the front panel). The second style mounts by means of threaded holes in the "bottom".

I have not tried the coils in tuned circuits.

For VFO use, the addition of a couple of buffer stages permits the unit to become a VFO and still be a frequency meter.

There are computer applications which permit one to re-create the calibration book, too.

73 de Jim, N2EY
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K0IZ
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Posts: 739




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« Reply #41 on: August 20, 2012, 06:02:41 AM »

Jim, Martin:  thanks for comments re BC221.  I looked over my unit, and that variable cap assembly is a work of art.  I'm going to check out that QST article.  Regards.  John.
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