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cascode detector advantages?

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  • kyoritsu
    I ve recently been interested in double triode cascode regen detectors. They look neat. Went through comments here and elsewhere. One was they are a sort of
    Message 1 of 19 , May 1, 2008
      I've recently been interested in double triode cascode regen detectors. They look neat. Went
      through comments here and elsewhere. One was they are a sort of an extended pentode
      detector, spread out over two triodes, with screen grid-like regeneration control.

      If that's all, there doesn't seem to be much merit in the design. Perhaps there's an edge in
      using triodes in that they are not as noisy as pentodes. But on lower bands, say below
      10Mhz, tube noise doesn't seem to be a consideration.

      Cascode regeneration control smoothness: can't get enough of that, of course, one of life's
      small pleasures. Yet the 6BZ6 or the 6BA6 pentodes (especially the latter) are beautifully
      smooth with reduced voltage in my Span Master. Don't know how much better it could be.

      Actually, if you've got something like a 12AT7, a grounded grid RF amp plus triode regen
      detector would seem to be better design than using the two triodes in cascode.

      So, are there any advantages, theoretical or practical, to the cascode detector?

      Rob

      ps: I'll probably try one anyway. Another summer project.
    • deadgoose38
      Haven t built one for SW, but did for 2 meters. The cascode gives you a grounded grid isolation, which regenerative receivers need. I ve seen some circuits
      Message 2 of 19 , May 1, 2008
        Haven't built one for SW, but did for 2 meters. The cascode gives you
        a grounded grid isolation, which regenerative receivers need. I've
        seen some circuits floating around which should work nicely.
      • kyoritsu
        Paul, I ve looked at a couple of double triode cascode regen schematics. I can t see where the design provides isolation for the tank circuit from the antenna.
        Message 3 of 19 , May 3, 2008
          Paul, I've looked at a couple of double triode cascode regen schematics. I can't see where the
          design provides isolation for the tank circuit from the antenna. This design does look very
          much like a pentode (more accurately, a tetrode) regen detector: the LC tank is connected to
          the grid of the first triode, while the grid of the second triode controls the regeneration, ie,
          just like the screen grid of a pentode. I don't see what advantage the cascode detector
          provides. Also, doesn't the cascode design require a higher B+ supply, since you have two
          tubes in series? Still want to try, of course.

          Rob



          --- In regenrx@yahoogroups.com, "deadgoose38" <deadgoose@...> wrote:
          >
          > Haven't built one for SW, but did for 2 meters. The cascode gives you
          > a grounded grid isolation, which regenerative receivers need. I've
          > seen some circuits floating around which should work nicely.
          >
        • Paul W. Ross
          The grid of the first stage is grounded, which then provides a shield to the antenna. A similar strategy is used in the MFJ-8100 and Ten-tec 1253, but with a
          Message 4 of 19 , May 3, 2008
            The grid of the first stage is grounded, which then provides a "shield"
            to the antenna. A similar strategy is used in the MFJ-8100 and Ten-tec
            1253, but with a single FET working "grounded grid" - grounded gate.
          • kyoritsu
            I was thinking of something like this: www.dk3bi.de/Regen.pdf If I had one double triode to use for the detector stage, I think a combination grounded grid RF
            Message 5 of 19 , May 3, 2008
              I was thinking of something like this:

              www.dk3bi.de/Regen.pdf

              If I had one double triode to use for the detector stage, I think a combination grounded grid
              RF amp plus triode regen detector would be better than this cascode design. Wondered if
              there were any inherent advantages to the cascode regen detector over a single pentode.

              Rob



              -- In regenrx@yahoogroups.com, "Paul W. Ross" <deadgoose@...> wrote:
              >
              > The grid of the first stage is grounded, which then provides a "shield"
              > to the antenna. A similar strategy is used in the MFJ-8100 and Ten-tec
              > 1253, but with a single FET working "grounded grid" - grounded gate.
              >
            • Ian Wilson
              ... seems to me to be functionally the same as a pentode Hartley regen detector. It may be that control of the regeneration point is more gradual with the
              Message 6 of 19 , May 3, 2008
                This:

                > www.dk3bi.de/Regen.pdf

                seems to me to be functionally the same as a pentode Hartley
                regen detector. It may be that control of the regeneration
                point is more gradual with the cascode circuit. No way to
                tell without building it :).

                It wouldn't be difficult to put two tube sockets side by side,
                using the same circuit for both the published circuit and with
                a small pentode. Sort of like plug-in coils, but plug-in
                valves instead :)

                73, ian K3IMW
              • Ian Wilson
                A grounded-grid triode buffer stage followed by a regen triode detector would perhaps offer less total gain (you are really using the RF amplifier to provide
                Message 7 of 19 , May 3, 2008
                  A grounded-grid triode buffer stage followed by a regen triode
                  detector would perhaps offer less total gain (you are really
                  using the 'RF amplifier' to provide isolation between antenna
                  and resonant circuit), but I would expect it to be more
                  controllable.

                  73, ian K3IMW
                • Ian Wilson
                  Last reply and then I promise to go to sleep... Note that in a DC-coupled cascode, the cathode of the upper tube is at a potential of around B+/2. If
                  Message 8 of 19 , May 3, 2008
                    Last reply and then I promise to go to sleep...

                    Note that in a DC-coupled cascode, the cathode of the upper
                    tube is at a potential of around B+/2. If heater-cathode
                    voltage rating is an issue, you can 'ground' one side of the
                    heater supply to a resistive divider to B+/4 (bypassed with
                    a capacitor of appropriate size).

                    73, ian K3IMW
                  • kyoritsu
                    Hi Ian, This brings up something else I m not clear about, concerning grounded grid amps. The gain is said to be the same as the usual grounded cathode
                    Message 9 of 19 , May 4, 2008
                      Hi Ian,

                      This brings up something else I'm not clear about, concerning grounded grid amps. The
                      gain is said to be the same as the usual grounded cathode configuration (actually, slightly
                      more since the input signal is added to the output). The disadvantage is the low input
                      impedance.

                      However, in practice the grounded grid RF amp seems to give more or less only unity gain.
                      At least, in my experiments (high mu 6AV6, low mu 6C4). While the isolation is nice, some
                      extra gain for 14MHz and up would be useful.

                      Does the cascode regen design give more RF gain than a pentode? I thought regeneration
                      was the great leveler: with a high gain tube you need less feedback, with a low mu tube
                      you need more, but the total regenerative RF gain is about the same, for any regen
                      detector. (Again, this is an electronics primitive's understanding). The pentode does seem
                      to provide more AF gain over a triode (ie, in the grid leak detector function).

                      Thanks too for mentioning the filament-cathode voltage difference (and a solution). I
                      hadn't considered that at all.

                      Rob

                      -- In regenrx@yahoogroups.com, "Ian Wilson" <ianmwilson73@...> wrote:
                      >
                      > A grounded-grid triode buffer stage followed by a regen triode
                      > detector would perhaps offer less total gain (you are really
                      > using the 'RF amplifier' to provide isolation between antenna
                      > and resonant circuit), but I would expect it to be more
                      > controllable.
                      >
                      > 73, ian K3IMW
                      >
                    • Ian Wilson
                      Hi Rob, A grounded grid stage has - potentially - about the same power gain as a grounded cathode stage. However, this applies if you are matching to the input
                      Message 10 of 19 , May 5, 2008
                        Hi Rob,

                        A grounded grid stage has - potentially - about the same
                        power gain as a grounded cathode stage. However, this applies
                        if you are matching to the input and output impedances. This
                        is not usually the case for an RF amplifier/buffer stage.

                        If you are operating from a high impedance antenna (such as
                        a short piece of wire) then the grounded cathode stage will
                        give you more gain. If you have a low impedance antenna then
                        I would expect the grounded grid stage to give you more gain.

                        I would expect similar gain from a cascode and a pentode
                        *if they have the same effective gm*. I would think they would
                        be similar in practice using the tube types we typically use
                        for HF RX construction.

                        [Note: I've found it quite difficult in practice to compare
                        some of these things, so it's tough to get away from the
                        "it worked for me" approach <g>]. Do let us know your findings!

                        --ian
                      • John Berry.
                        Hi All.Recall seeing years ago a circuit for a Cathode Follower Regen detector.This used a 6SN7 2 triode tube.Grid #1 was conventional input coil through
                        Message 11 of 19 , May 6, 2008
                          Hi All.Recall seeing years ago a circuit for a "Cathode Follower"
                          Regen' detector.This used a 6SN7 2 triode tube.Grid #1 was
                          conventional input coil through series cap' with grid leak to
                          earth,Anode #1 to HT+.Both cathodes together through RF choke to
                          earth ? (bit hazy on this it was a LONG time ago !!).Grid #2 to
                          earth,Anode #2 through RFC to resistive load to HT+,with
                          Regeneration/tickler coil back to input coil through cap' to
                          earth !!.The basic idea was it gave a very high impeadance to grid
                          #1 ??.This was in some American Magazine.Anyone seen or even tryed
                          this out ??.John G1WOS.
                        • Ian Wilson
                          John, This is a long-tailed pair (although I m pretty sure that s a solid-state term). I would think it should work pretty well as a regen detector. If there
                          Message 12 of 19 , May 6, 2008
                            John,

                            This is a 'long-tailed pair' (although I'm pretty sure that's
                            a solid-state term). I would think it should work pretty well
                            as a regen detector.

                            If there were a cathode resistor, you could vary the gain
                            distribution between the two triodes by varying the (positive)
                            voltage on the grid of the second triode. That doesn't seem
                            particularly useful in your circuit, though.

                            73, ian K3IMW
                          • kyoritsu
                            Hi Ian, This summer I think I ll try both the cascode regen detector and a more traditional grounded grid plus regen detector, and see. Probably use a 12AT7.
                            Message 13 of 19 , May 7, 2008
                              Hi Ian,

                              This summer I think I'll try both the cascode regen detector and a more traditional
                              grounded grid plus regen detector, and see. Probably use a 12AT7. Can't do much right
                              now. (Something called work intrudes).

                              I do have some informal and subjective 'results' concerning different pentodes in my Span
                              Master. The regen detector is a 6BZ6. I've mentioned before that I get somewhat smoother
                              regeneration control with a 6BA6. (Hue suggested it was because the latter is a remote
                              cutoff, the former a semi-remote cutoff). 'Performance' otherwise seems the same. The
                              6BA6 requires a lower screen voltage for regeneration.

                              I then tried a 6AK5. The regeneration point is 'sharper' but still very manageable. This tube
                              requires the least screen voltage to go into regeneration. Now, this is very tentative and
                              very subjective, but it appears that this tube gives slightly better performance than either
                              the 6BZ6 or the 6BA6. That is, signals seem slightly 'brighter' against the noise
                              background on higher frequencies (15MHz and up). What I have been doing is listening to
                              a weak station at the optimum setting with the 6BZ6, then replacing it with the 6AK5 and
                              re-adjusting (both the tuning and regeneration positions change). The 6AK5 leaves me
                              with the impression it is the better performing tube. I've done this for several months now,
                              and my impression has not changed. Perhaps the circuit is better optimized for the 6AK5
                              than for the 6BZ6. The mains voltage in Tokyo is 100v/50cycles and perhaps this lower
                              voltage has something to do with it.

                              More recently I tried, with great anticipation, a 6GM6. Interesting tube, plate voltage is
                              lower than the others, rather high gm, lower plate resistance. Remote cutoff. Performance
                              was essentially the same as the 6BA6. Very smooth regeneration point. However, it
                              required the highest screen voltage to go into regeneration. This tube very noticeably
                              lowered the tuning, about 200-300kH, much more than the other tubes. Also consumes
                              more power and did run rather hot to the touch.

                              One interesting thing: the 6AK5 had the most consistent, the most level regeneration
                              setting across entire bands than the others.

                              I wonder how much individual tube performance is involved here. I have 3 6BZ6s, all
                              bought NOS. Two are unusable: one has a very high level of hum, the other is remarkably
                              microphonic (as in an unintended QRP AM rig, without all the hassle of switching between
                              TX and RX). But both my 6AK5s worked the same, and if there are any Span Master users
                              out there, why not give one a try and see how it works for you.

                              Rob
                            • Hue Miller
                              Rob, how about a 6EH7 ? High transconductance remote cutoff tube. This is a hot tube for RF use. Also, i saw recently a couple schematics for 1L4 single tube
                              Message 14 of 19 , May 7, 2008
                                Rob, how about a 6EH7 ? High transconductance
                                remote cutoff tube. This is a hot tube for RF use.

                                Also, i saw recently a couple schematics for 1L4
                                single tube regens. One used the "space charge"
                                circuit and other, not. Both used B+ in the 45 volt
                                range. I am curious to try both and see if there is
                                really any practical advantage to the spacecharge
                                hookup with this tube & voltage. ( The space charge
                                has screen and control grid hookups exchanged. )
                                -Hue
                              • kyoritsu
                                Hi Hue, The 6EH7 is 9 pin. I d have to remove the coils and bandswitch, remove the 7 pin socket, enlarge the socket hole... That s too much. But I ll keep that
                                Message 15 of 19 , May 8, 2008
                                  Hi Hue,

                                  The 6EH7 is 9 pin. I'd have to remove the coils and bandswitch, remove the 7 pin socket,
                                  enlarge the socket hole... That's too much. But I'll keep that tube in mind for another project.

                                  Rob
                                • Filippo Cattaneo
                                  TANK CIRCUIT ISOLATION, CASCODE AMPS, RELATED MATTERS The 2-triode regen design often called cascode ... www.dutch.nl/bdj/0V2/schema.gif
                                  Message 16 of 19 , Oct 6, 2009
                                    TANK CIRCUIT ISOLATION, CASCODE AMPS, RELATED MATTERS


                                    The 2-triode regen design often called "cascode"...
                                    www.dutch.nl/bdj/0V2/schema.gif
                                    http://www.dk3bi.de/Regen.pdf
                                    ...is known to work very well, but

                                    - it's not real cascode as there is only one active device in the RF loop, thus utilizes the stabler biasing advantages of cascodes but not the high-insulation they might offer

                                    - antenna isolation per se is not addressed by a single cascode past the tank circuit


                                    Also, when implemented in solid state there's always an issue with the amp moving between class A and C.

                                    Class C is inherently required if detection is to be achieved in that stage. Only a highly distorting regenerator will produce strong changes in its current correlated with the audio-frequency intelligence. The attendant and unavodable change in biasing and internal capacitance causes a (hopefully, but not necessarily, modest) shift in tank circuit resonance frequency.


                                    A change in frequency is also noticeable in source-coupled SS designs such as found in MFJ kits, and happens both when regeneration is controlled by directly varying amp gain by means of biasing or DC supply, and also when it's obtained by damping the RF transfer within the 2-transistor cell.


                                    Long Tailed Pair proper (DC coupled sources, single resistor to common) or Source Coupled regenerators are non-inverting and provide high impedance I/0 that can be applied directly to the tank circuit, often DC coupled to it.


                                    There are examples of SC/LTPs both with and without detection.

                                    MFJ/TenTec-like receivers have a SC regenerating detector:
                                    http://qrp.kearman.com/images/fet-regen-schematic.png

                                    LTP designs with a separate detector device would still work if the regenerator were totally non-distorting (and non-detecting) Class A with fixed bias:
                                    http://www.elo-web.de/elo/entwicklung-und-projekte/retro-radio/retro-radio-deluxe


                                    The Optically Coupled Regen also has separate regeneration and detection:
                                    http://supreg1.narod.ru/files/r38.doc

                                    So have the bipolar design family by Charles Kitchin and Paul Harden, the Wheatstone Bridge Receiver, the G.W. Short Homodyne and many others.


                                    Going back to cascodes: a true cascode stage

                                    - concurrently provides all-except-one among broad bandwidth, high gain, stability, and high isolation between the high impedance input and output nodes

                                    - contains a lower impedance node right between the two active devices, not as low-Z as in a source-coupled or long-tailed-pair, yet a better match for some antenna types

                                    - is a phase inverting amplifier.



                                    DOUBLE CASCODE NON-DETECTING REGENERATOR

                                    Picture this:

                                    - tank circuit between TWO cascode amplifier stages, both terminated at high impedance, both phase inverting

                                    - both cascodes stably biased for low gain, low-distortion operation across a broad dynamic range, including across the regeneration threshold

                                    - a resistive RF-throttle type regeneration control on the high-Z feedback path, well isolated from the tank circuit on both sides

                                    - antenna signal injected either past the RF-throttle on the feedback path or at the lower-Z node between the two active devices on the input-side cascode

                                    - RF (NOT AF) taken out either ahead of the RF-throttle on the feedback path or at the lower-Z node between the two active devices on the output-side cascode

                                    - detection by separate infinite-Z detector.


                                    POSSIBLE CHARACTERISTICS

                                    I expect this design will exhibit strong tank circuit isolation from antenna, feedback path, detector, little or no bias change in the amplifier semiconductors, and therefore

                                    - little or no frequency effects by RF input and regeneration throttles (input throttle should still affect regeneration a bit)

                                    - smooth regeneration control, with little or no hysteresis

                                    - no hand effect

                                    - perhaps better performance in presence of unwanted strong signals

                                    - somewhat worse internal noise due to the added semiconductors in the loop, maybe

                                    - probably easy to modify for homodyne mode, even with mode-switching between homodyne and straignt regen

                                    - probably easier to fit with a form of AGC.



                                    It will take me a month or longer before I'll have the chance to try this.

                                    Meanwhile, questions:

                                    Did anyone see this actually done?

                                    Comments?

                                    What about phase delay?






                                    P.S.: Non-English sources

                                    The LTP + Infinite Z detector combination seems popular in the German koiné, search for Audion, Empfänger.

                                    For a wealth of circuits, often objects of aggressive fair use of copyrighted material, one could do worse than searching for регенератор and регенеративни приемник.
                                  • Filippo Cattaneo
                                    This board is Cyrillic-averse?! To search for things regenerative in Russian and find a lot of Western stuff as well) you may start from these links:
                                    Message 17 of 19 , Oct 6, 2009
                                      This board is Cyrillic-averse?!

                                      To search for things regenerative in Russian
                                      and find a lot of Western stuff as well)
                                      you may start from these links:

                                      http://tinyurl.com/y9d5kam TEXT

                                      http://tinyurl.com/yats246 IMAGES


                                      --- <filippo_cattaneo@...> wrote:
                                      > For a wealth of circuits, often objects of aggressive fair use of copyrighted material, one could do worse than searching for регенератор
                                      ---

                                      WHOA!!!
                                    • Vladimir Lebedev
                                      Hi all! ... From: Filippo Cattaneo This board is Cyrillic-averse?! To search for things regenerative in Russian and find a lot
                                      Message 18 of 19 , Oct 9, 2009
                                        Hi all!
                                        ----- Original Message -----
                                        From: "Filippo Cattaneo"
                                        <filippo_cattaneo@...>

                                        This board is Cyrillic-averse?!
                                        To search for things regenerative in Russian
                                        and find a lot of Western stuff as well)
                                        you may start from these links:
                                        http://tinyurl.com/y9d5kam TEXT
                                        http://tinyurl.com/yats246 IMAGES

                                        Yes, this is in Russian.

                                        You can visite this radio amateur forum (in Russian)
                                        "Retro-radio regenrx with tubes, with thair own hands"
                                        http://forum.cqham.ru/viewtopic.php?t=10499 ( 134 forum pages, not only with
                                        tubes)

                                        "Retro-radio regenrx with tubes, with thair own hands #2" (145 forum pages,
                                        not only with tubes)
                                        http://forum.cqham.ru/viewtopic.php?t=17921

                                        "Retro-radio regenrx with tubes, with thair own hands #3"
                                        http://forum.cqham.ru/viewtopic.php?t=20367 ( 16 forum pages today)

                                        You can try translate this sites with "Google translate"
                                        http://translate.google.com/translate?u=http%3A%2F%2Fforum.cqham.ru%2Fviewtopic.php%3Ft%3D10499&sl=ru&tl=en&hl=en&ie=UTF-8
                                        (for next page click a page number on top of the page 1...134, not on bottom
                                        of the page)

                                        http://translate.google.com/translate?u=http%3A%2F%2Fforum.cqham.ru%2Fviewtopic.php%3Ft%3D17921&sl=ru&tl=en&hl=en&ie=UTF-8
                                        (for next page click a page number on top of the page 1...145, not on bottom
                                        of the page)

                                        http://translate.google.com/translate?u=http%3A%2F%2Fforum.cqham.ru%2Fviewtopic.php%3Ft%3D20367&sl=ru&tl=en&hl=en&ie=UTF-8
                                        (for next page click a page number on top of the page 1...16, not on bottom
                                        of the page)

                                        73! Vladimir, DL7PGA
                                        P.S Link to Forum "Old radio (valve soul)"

                                        http://translate.google.com/translate?u=http%3A%2F%2Fforum.cqham.ru%2Fviewforum.php%3Ff%3D30%26sid%3Df8e69ceb79fe1c5866c3c83ae8f70bed&sl=ru&tl=en&hl=en&ie=UTF-8
                                      • Filippo Cattaneo
                                        thank you Vladimir!
                                        Message 19 of 19 , Oct 15, 2009
                                          thank you Vladimir!
                                        Your message has been successfully submitted and would be delivered to recipients shortly.