Re: JFET matcher using an OpAmp
How important is gate capacitance?
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https://www.ti.com/product/JFE2140?bm-verify
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From
https://www.ti.com/lit/ds/symlink/jfe2140.pdf
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Low input capacitance: 13 pF at VDS = 5 V
? High gate-to-drain and gate-to-source breakdown
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The 2SK715 has 10pF gate capacitance.
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The jfe2140 has a slew of diodes for protection.
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I clearly don't know enough to have any idea if the jfe2140 is suitable for a loop antenna electronics.
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The jfe2140 costs $4.39 (on July 21, 2024) from Digikey.
It'd be interesting (and no I'm not asking) to learn what?
Everett uses in his clones.
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FWIW, I hate winding confusing transformers, as a dyslexic, it is way too much like tying obscure knots.
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Re: JFET matcher using an OpAmp
The TI JFA2140 is a matched pair of JFETs on a single die. No need to test and select.
Rick N6RK
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On 7/21/2024 10:11 AM, wn4isx wrote: On the off chance there is interest in matching JFETs for a clone of the Wellbrook loop antenna, I've posted a PDF of a simple and effective unit.
Layout isn't critical, I constructed mine on vero-breadboard with 0.1" centers and used point to point wiring and mounted it ~1/8" above an unetched PCB for a ground plane.
Note 1: I did not design the unit, I built one late last night and tested a bunch of 2N3819.
A lifetime ago, 1975, I was given several hundred prime TI 2N3819s. Sadly the 2N3918 isn't the most useful JFET but it was nice to test 50 and find 3 that matched extremely closely.
YMWV
Note 2: The 2n3819s aren't for sale.
The file name is
JFET-matcher-rullywow.com-Build-Doc-v1.01.pdf
It can be found in the files section.
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Re: Wellbrook ALA1530 Clone for sale
I should have said that the amp is a Wellbrook ALA1530LN.
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Everett
In a message dated 7/21/2024 3:11:43 PM Central Daylight Time, everettsharp@... writes:
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If anyone is interested I have a Wellbrook ALA1530 clone for sale. It is built with SMD JFETs. You can contact me off list if interested. everettsharp (at ) aol com.
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Everett N4CY
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Steve,
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Thanks! Yes, that is an "R", not a "k" on the trimmer on closer inspection!
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Mike M
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Martin,
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Comparing your schematic to Steve's I see a few differences:
- The secondary of T2 is directly connected to ground in Steve's and through capacitors in yours.
- 1.2k to ground at the output.
- As noted, 41pF instead of 82pF across loop.
- As noted R4 is 500k instead of 1k.
- R1 at input.
- Extra ferrite beads.
I assume R1 is to bleed of static charge. What is R6 for?
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Are all of the differences from tracing a different unit or are some things your own changes?
--
===================================================================
Mike M
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Wellbrook ALA1530 Clone for sale
If anyone is interested I have a Wellbrook ALA1530 clone for sale. It is built with SMD JFETs. You can contact me off list if interested. everettsharp (at ) aol com.
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Everett N4CY
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Re: Urban HF Antenna Dilemma: LZ1AQ vs. M0AYF
I might play a bit with the inductors and try to improve the pass band at around 3MHz, but I guess the overall behavior of the filter is quite acceptable. This is with a resistive source. THe loop is inductive and according to the simulation. the "flatness" does not change much between a 3uH loop and 50 ohm resistive source.
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
I asked Andrew Ikin one time if he could share what core he used in his preamps for the transformers. He told me he uses the Fair-Rite 2873000302, same as I've shown on my schematic. (Amidon "BN73-302"). That's what Everett and I always use too. Note this is a conductive core, and with the small gauge magnet wire needed for the transformers, it's easy to abrade the coating when winding, so the wire touches the core. You do not want any wire to touch the core--the circuit's IMD will severely degrade, particularly for the T2 output transformer. Use a multimeter set to ohms with one probe to the core and the other probe to any of the (stripped) wire ends--there should be infinite ohms, otherwise you need to rewind it and try again! We always use a thin Teflon tubing spacer when we wind the transformers so the wire doesn't abrade when winding. We buy it on eBay, from China. The proper size is "9L 3.0 x 3.4 mm". Here's one seller of it:
(I have no experience with the cores that Martin mentioned.)
Steve
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On 7/21/2024 9:26 AM, Mike M via groups.io wrote: Steve,
Schematic added:
/g/loopantennas/files/Wellbrook%20T1/schematic.pdf
Also, I notice that the 2SK715 had four ranks for IDSS and the 2SK932 has two ranks for IDSS.? Any idea of which rank was used by Wellbrook?
Thanks again.
--
===================================================================
Mike M
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Photo LZ1AQ-MPS2222A-3-8MHz+transformer-sweep.jpg uploaded
#photo-notice
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Hi Martin,
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On your schematic, at the junction of C1 (82pF) and C2 (82pF) they need to be grounded, or it will oscillate.
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Everett N4CY
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In a message dated 7/21/2024 11:58:32 AM Central Daylight Time, martin_ehrenfried@... writes:
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Mine added too, derived separately, useful for comparison purposes.
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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Martin, just a remark, in your wire numbering scheme of the input transformer two wires of different windings carry the same number "4".
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Regards
Fred
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Re: JFET matcher using an OpAmp
I found another JFET matching circuit at:
It is slightly more complicated but would be easier to use.
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Martin, on your schematic you've got the resistor feeding the 2 x
1N4148s as 500K, whereas Steve has it at 1K.
Should it have been 500R?
Mike - M0MLM
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Mine added too, derived separately, useful for comparison
purposes.
?
?
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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:
_
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Thank you. I should note that my group opened and traced out 3
different ALA1530LNs and all had a single 82 pF cap across the two
loop inputs. Apparently you had a variation if yours had two 82
pF? caps in series across the two loop inputs as your schematic
shows (and the midpoint of them not going to common/ground
either).
Steve
On 7/21/2024 9:58 AM, Martin -
Southwest UK via groups.io wrote:
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Mine added too, derived separately, useful for comparison
purposes.
?
?
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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
The phasing dots of the transformer are a key piece of information in the circuit diagram and crucial for proper functioning of the amplifier.
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regards
Fred
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Hi Mike,
Yes, it appears your numbering matches with the proper phasing of
the one-turn feedbacks. But RV1 is 100 ohms ("100R"), not 100 k
ohms. That pot is set so both Source sides are the same voltage.
All the 2SK715s that Andrew Ikin used were of the highest Idss
version, "W", from the Sanyo datasheet, in the 3 different
ALA1530LNs that my group opened up and traced out.
Steve
On 7/21/2024 9:26 AM, Mike M via
groups.io wrote:
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Steve,
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Schematic added:
?
?
Also, I notice that the 2SK715 had four ranks for IDSS and
the 2SK932 has two ranks for IDSS.? Any idea of which rank was
used by Wellbrook?
?
Thanks again.
--
===================================================================
Mike M
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JFET matcher using an OpAmp
On the off chance there is interest in matching JFETs for a clone of the Wellbrook loop antenna, I've posted a PDF of a simple and effective unit.
Layout isn't critical, I constructed mine on vero-breadboard with 0.1" centers and used point to point wiring and mounted it ~1/8" above an unetched PCB for a ground plane.
?
Note 1: I did not design the unit, I built one late last night and tested a bunch of 2N3819.
?
A lifetime ago, 1975, I was given several hundred prime TI 2N3819s. Sadly the 2N3918 isn't the most useful JFET but it was nice to test 50 and find 3 that matched extremely closely.
?
YMWV
?
Note 2: The 2n3819s aren't for sale.
?
The file name is
JFET-matcher-rullywow.com-Build-Doc-v1.01.pdf
?
It can be found in the files section.
|
Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Mine added too, derived separately, useful for comparison purposes.
?
?
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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Steve,
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Schematic added:
?
?
Also, I notice that the 2SK715 had four ranks for IDSS and the 2SK932 has two ranks for IDSS.? Any idea of which rank was used by Wellbrook?
?
Thanks again.
--
===================================================================
Mike M
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Re: Wellbrook traced-out loop amp schematics uploaded to Photos
Hi Mike,
One would need a full schematic with the T1 transformer labeled
per your drawing to be able to say if you have it correct or not.
My schematic in the files doesn't have transformer numbering on
it, just phasing dots.
Steve
On 7/21/2024 7:07 AM, Mike M via
groups.io wrote:
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Steve,
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Thanks for doing this. It would be a shame to lose these
designs now that Andy is gone.
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I added a folder and two files illustrating T1:
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Could you take a look and see if I have it correct?
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Thanks!
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Mike M
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Everett N4CY
Caaarlo