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Oscillator/PA Cathode Keying, "Spotting" Your Frequency, and Chirp

Discussion in 'Homebrew and Kit Projects' started by G3EDM, Sep 5, 2021.

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  1. G3EDM

    G3EDM Ham Member QRZ Page

    This thread is split out from the thread "My First-Ever QSO -- 50 Years After Passing Ham Test" in "Ham Radio Discussions". It covers two issues but they are related because they both concern the cathode keying circuit in tube transmitters.

    My rig is the station described in "How to Become a Radio Amateur" (ARRL 1968).

    The receiver is a three-transistor regenerative set with an FET detector. Quite sensitive, but "broad as a barn door" and prone to have its frequency "pulled" by strong signals or changes in antenna loading conditions.

    The transmitter is a two-tube MOPA (master oscillator followed by power amplifier). The oscillator is a crystal-controlled 6C4 Pierce circuit. The PA is a 5763 pentode. The entire station is currently configured to operate monoband on 40m.

    Here is the transmitter circuit. I built it exactly as shown, apart from some improvements to the stability of the power supply (adding a voltage divider).


    One annoying problem is that it is currently difficult to "spot" the transmitter frequency on the receiver. It gets completely overloaded. I have got around this by transmitting into a dummy load (using a coax switch for quick changeover) and disconnecting the antenna from the receiver (which is done through the outboard transmit/receive switch).

    This makes "spotting" possible although a certain amount of "pulling" still takes place and the sidetone is not silenced, so there are two competing sets of tones in the headphones, a steady sidetone, and a variable tone as I tune into and out of the transmitting frequency.

    Jim, N2EY, sensibly suggested a better option would be to insert a SPST switch into the cathode keying circuit so that the key could be disconnected from the PA and only the oscillator would be keyed during "spotting".

    I will implement this using a small relay inside the transmitter, controlled from a new switch on the front panel of the T/R control unit. My transmitter is a "museum quality" implementation of the ARRL design and I want any change I make to be reversible, so I will not consider drilling a hole in the transmitter chassis to install a switch. The relay will be added under the chassis using a removable bracket.

    Problem solved, I think. But this gave me another idea.

    Other ops are reporting slight chirp on my signal. I have heard recordings of my own signal and I have to agree with them. Fortunately it is not at all serious and the signal is readable without the chirp being irritating.

    However, how about if I rewired things so that the oscillator is run continuously (when the T/R switch is in the Transmit position) and we key only the PA? I know that this is one option that is sometimes used, especially with VFO control where the oscillator runs continuously. Would that not reduce or eliminate the chirp?

    When the T/R switch is in the Receive position, it would of course be essential for the oscillator to be switched off (except during "spotting") otherwise it would completely blanket the signal in the receiver.

    Would this work? Is it OK to run a crystal oscillator in this way, or would continuous operation run the risk of overheating my crystals? They are all in FT-243 cases, but some of them are actually modern crystals mounted inside FT-243 cases.

    I think this modification could be made without any external change to my "museum quality" transmitter and in a totally reversible way.

    73 de Martin, G3EDM
  2. G3EDM

    G3EDM Ham Member QRZ Page

    Here's my existing Transmit-Receive switch. It also includes two attenuation controls for the receiver, and a slide switch for switching the attenuation in and out.

    There is room for another slide switch to the right of the existing one, for the new "spot" control that would trigger the relay inside the transmitter. Or I might use a more standard toggle switch because the switch will get heavy use.


    There is also plenty of room inside the chassis for any extra circuitry.



    I added this lead weight to the inside of the bottom plate to stop the box skidding on the desk when the T/R switch is flicked.


    73 de Martin, G3EDM
  3. G3EDM

    G3EDM Ham Member QRZ Page

    Another option, instead of adding a dedicated "spotting" switch on the T/R box, would be to replace the two-position rotary T/R wafer switch with a three-position switch: Receive-Transmit-Spot (Spot being the left position, Receive the middle position, Transmit the right position).

    73 de Martin, G3EDM
  4. KD2BD

    KD2BD Ham Member QRZ Page

    Hi Martin.

    A couple of thoughts...

    For better frequency stability, I would strongly suggest changing the oscillator from a Pierce to a Colpitts configuration. A further improvement may be achieved by changing the oscillator tube from a triode to a pentode (6AU6, 6AH6, 6BA6, 6BH6, etc.).

    In "spot" mode, you want to key the oscillator, only. With the final cathode disconnected from the keying circuit in "spot" mode, the sidetone will not oscillate. A neutralization circuit around the final can be employed to reduce the level of RF feed-through.

    Good luck with the project!

    73 de John, KD2BD
    G3EDM likes this.
  5. SWL37632

    SWL37632 QRZ Member

    I reviewed the schematic of your TX.

    You state that you have chirp with the existing xtal stage.

    You are most likely loading/changing the effective load impedance presented to the xtal stage which is resulting in the chirp. Or, your PS is not providing sufficient regulated current to the xtal stage.

    I 'get' your hope for maintaining your 'museum quality' expectations with the existing design and circuit.

    However and quite frankly, if you are married to this circuit....and it appears that you are..... "you have what you have".... which includes the fundamental design flaws and insufficiencies of the circuit.

    My 1960's car did not have power steering or power brakes or air conditioning and had lousy gas mileage. My 21'st century car has all of those power options and much better gas mileage.

    You may want to consider upgrading the circuit design.

    Last edited: Sep 5, 2021
  6. G3EDM

    G3EDM Ham Member QRZ Page

    Just about to sit down to dinner but just quickly.

    Making fundamental changes to the circuit of this transmitter is just not going to happen. I am going to be building a new, higher powered transmitter relatively soon. So for the time being I need to live with what I have, and I am insistent on preserving the basic design of the transmitter.

    The chirp is a minor issue. However, it might just be possible to fix it with fairly minor changes to the keying circuit.

    That's really my motivation. I got into the hobby because I love building and experimenting!

    73 de Martin, G3EDM
    SWL37632 and AD5HR like this.
  7. N2EY

    N2EY XML Subscriber QRZ Page

    In a word - UGH.

    I think it's definitely worth a try.

    Consider this change:

    1) The cathode of the 6C4 is completely disconnected from the keying line, and instead goes to the moving contact of a small SPDT relay.

    2) A 0.01 uF or similar disk ceramic capacitor is connected from the 6C4 cathode to ground just to be sure there's no RF running around loose.

    3) The SPDT relay has its Normally Closed contact connected to the keying line, and its Normally Open contact connected to ground. So, when the relay is deenergized, the circuit is just like the original, and when the relay is energized, the 6C4 oscillates all the time.

    4) Control of the SPDT relay is brought out of the transmitter through a suitable connector and cable to the TR control box.

    5) At the TR control box, there are two ways to energize the small relay: Either close the SPOT switch, or operate the TRANSMIT/RECEIVE switch to TRANSMIT.

    Or, consider this method:

    1) The cathode of the 6C4 is completely disconnected from the keying line.

    2) A 0.01 uF or similar disk ceramic capacitor is connected from the 6C4 cathode to ground just to be sure there's no RF running around loose.

    3) The 6C4 cathode line is brought out of the transmitter through a suitable connector and cable to the TR control box. So that circuit must be externally grounded to get the 6C4 to oscillate.

    4) At the TR control box, there are two ways to ground the 6C4 cathode line: Either close the SPOT switch, or operate the TRANSMIT/RECEIVE switch to TRANSMIT.


    Either of these mods will work. The main difference is what happens if you close the key with the TRANSMIT/RECEIVE switch in RECEIVE and the SPOT switch open.

    73 de Jim, N2EY
    SWL37632 and W7UUU like this.
  8. G3EDM

    G3EDM Ham Member QRZ Page

    That's what I thought!

    Not sure when I will be able to implement this experiment. Soon, I hope!

    73 de Martin, G3EDM
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    You might also just try killing the screen supply on the 5763....this will greatly reduce the output but still allow some oscillator buffering.
    G3EDM likes this.
  10. W9AC

    W9AC Subscriber QRZ Page


    Still another option is to switch between your FT-243 crystal and a modern DDS VFO with a buffered output stage meant for driving a Pierce oscillator stage. There are several DDS VFO kits available. This way, you can choose between frequency stability and originality as your mood changes. The DDS VFO output cable simply plugs into the crystal socket. I made a cable using low-capacitance coax and an empty FT-243 holder.

    My DDS VFO has a front panel OPR-STBY-SPOT switch. Photos are shown on my QRZ page. In standby, the VFO is keyed via a rear panel phono connector and is capable of full QSK. In the OPR position, RF is on all the time. The SPOT position is spring-loaded and spots only for the duration of holding the switch down.

    The DDS VFO is being used with several compromise designs from old ARRL handbooks and QST construction articles. It works especially well with a transmitter I built in 1970 as I was getting ready for my Novice license exam. It's a 6DQ6A-OD3-5U4 design out of a late '50s ARRL Handbook. The 6DQ6A is being used as a MOPA and as such, it's cheap and does suffer from some chirp even with a regulated screen supply. Parts for the transmitter were cannibalized from an old 1950s-era Philco TV set. With the FT-243 crystal, the transmitter has some slight chirp with the degree of chip dependent on 6DQ6A loading. But, with the DDS VFO, there's no chirp whatsoever - as expected.

    Anyway, just another thought that allows you a choice of easily switching between the original crystal and a DDS VFO to get modern keying stability.

    Paul, W9AC
    G3EDM likes this.

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