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

    @W9BRD: Dave, that is an extraordinary amount of information in your post. I am chasing DX in the early hours but later, will pore over all of this again as well as your website. Thank you so much!

    73 de Martin, G3EDM
  2. W9BRD

    W9BRD Ham Member QRZ Page

    The reason for this directive goes like this:

    When the plate tuned circuit of a screen-grid power amplifier tube is tuned to resonance with no load, the plate current dips to a very low value. When the plate is drawing abnormally low current, "the screen tries to act as the plate" as we used to say, possibly overheating and even melting the screen-grid wire. Related to this is the directive that one should never apply screen voltage to a screen-grid tube without applying normal plate voltage at the same time.

    And BTW old-timers learned this the hard way in the days when medium- and low-power screen-grid power-amplifier tubes were new. OTs had gotten accustomed to being able to tune their triode power-amplifier tubes with no load to find resonance; there's no screen-grid to kill in a triode. In writeups for power amplifiers in 1930s we even see plate current values given for "loaded" and "unloaded" as a result of this practice.
    Last edited: Sep 10, 2021
    KX4OM and N2EY like this.
  3. SWL37632

    SWL37632 QRZ Member

    The Eimac power tube manual suggests an overcurrent relay of some sort for the screen current.....focused on higher power tubes than what the OP is using but, as you write, appropriate for any screen type circuit in this application.

    For the OP's particular circuit, I would assume that a current limit resistor or a fuse or both would be appropriate?

    de Bill
  4. G3EDM

    G3EDM Ham Member QRZ Page

    @W9BRD: The station cabling is designed so that there is always a load on the tank circuit. There is a coax switch for choosing either the antenna, or a dummy load. (OK, it is a three-position coax switch, but the third position is never used so I would have to be unusually distracted to select that position.)

    There is a second dummy load inside the T/R switch -- a 25W, 50-ohm non-inductive resistor. When the switch is in the RECEIVE position, the cable to the transmitter is disconnected from the antenna and switched to have that resistor across it. So if I hit the Morse key by accident with the switch in the RECEIVE position, there will be a load across the TX tank. That is just as well, because from time to time I do forget to flick to TRANSMIT before transmitting (and get blasted by broadband noise in the headphones, because my receiver cannot cope with a signal that strong).

    Thank you everyone for all your contributions so far in this thread. There is an awful lot to absorb, and a certain amount of "Let's try A, let's try B, ....." troubleshooting is ahead. The station is functional but as you can tell, all sorts of tweaks could be made to improve performance. That's what homebrewing is about.

    73 de Martin, G3EDM
  5. G3EDM

    G3EDM Ham Member QRZ Page

    Thank you so much, your web page is a masterful and detailed analysis of these circuits.

    I will be doing this as soon as possible. It will involve adding a relay, should not be too hard.

    Excellent. Looking up the tube data, it is a pin-for-pin replacement.

    Perhaps worth trying if I can find the space under the chassis. IIRC regulator tubes run cool since they don't have heaters, so no ventilation needed. I will try the various other fixes first though.

    Very interesting, and relatively easy to try out.

    Will do.

    Am now ordering the parts for some of the above modifications. All in all, quite a few changes to make.

    I am also going to try to figure out why the key voltages are too high. Simply bringing those voltages back down to their design values could help, surely.

    Thank you so much!!!!

    73 de Martin, G3EDM
  6. G3EDM

    G3EDM Ham Member QRZ Page

    @N2EY and @W9BRD. As you know, I have switched over to a much better (but still regenerative) receiver. This is working well except that "spotting" is a nightmare. (I won't bore you with details of the "spotting" system I am using, but suffice it to say that it is easy to set the receiver to completely the wrong frequency because of harmonic images.)

    Quick question: If I wanted to do some "calibration spotting" to match my crystals to positions on the receiver dial, would it be safe to simply unplug the 5763 and therefore only the 6C4 would key? Or would the absence of the PA tube damage something? (This will soon be fixed when I alter the TX to run the oscillator continuously and only key the PA, and also add a "spotting" switch. But I am not quite there yet, it will happen, but in the next week or two. It does require adding a relay and so forth.)


    73 de Martin, G3EDM
    Last edited: Sep 14, 2021
  7. G3EDM

    G3EDM Ham Member QRZ Page

    @N2EY and @W9BRD: My head hurts trying to figure this out, but I think the modifications to the keying and spotting system can be done with a single relay, as follows.

    Remove the oscillator from the keying line altogether and instead, connect the cathode to one contact of a relay, with the other contact to ground. Energising the relay will start the oscillator. In this scenario only the PA is ever keyed. That's it. No further modifications to the transmitter are needed.

    For the transmit-receive switch, change the existing two-position rotary to a three-position switch operating as follows:
    • TRANSMIT: Relay contacts closed. Antenna connected only to transmitter.
    • RECEIVE: Relay contacts open. Antenna connected only to receiver. Antenna cable of transmitter is connected to a dummy load inside the T/R switch (25W power resistor, already installed) to provide a load in case the Morse key gets pressed inadvertently during reception.
    • SPOT: Relay contacts closed, and no need to key the transmitter for spotting on the receiver. Antenna disconnected from both transmitter and receiver. Antenna cable of TX is switched to the small dummy load.
    I think it is that simple. Is there anything wrong with the logic?

    BTW for simplicity I am thinking of powering the relay with a battery inside the T/R switch. That way, I use the ground bus as one side of the relay line and a single wire between the TX relay and T/R switch for the other side. I'll also eventually be adding a second relay (perhaps a latching relay) to switch between 40m and 80m, and that can use another single wire.

    73 de Martin, G3EDM
  8. KX4OM

    KX4OM Ham Member QRZ Page

    I found another variant of the W1TS transmitter circuit from QST Jan 2003. The article is "The Two Tube Tuna Tin Transmitter" by WD8DAS. Two notable differences from the original design are a 47 ohm resistor added to the oscillator circuit in series with the crystal, and an R-C network across the key. The author reported:

    "The oscillator ran loud and clear and clear and did not stall under any condition of tuning. Listening on a nearby receiver, I heard no key clicks. After adjusting the trimmer capacitor in the oscillator grid circuit, the CW note sounded near perfect and one of the best I've heard from a simple homebrew rig."

    The author cautioned to use only fundamental crystals. He was able to transmit on 80, 40, 30 and 20m with output ranging from 5 to to 10 watts.

    You might try these to very simple mods for evaluation.

    The schematic is blurry, as I made a screen capture from my 6C4-5763 files.

    Ted, KX4OM

    Last edited: Sep 14, 2021
    G3EDM likes this.
  9. G3EDM

    G3EDM Ham Member QRZ Page

    @N2EY and @W9BRD: Sorry one more thing. I forgot to include that we need an additional pole in the T/R switch for receiver muting:
    • TRANSMIT: Receiver muted.
    • RECEIVE: Receiver un-muted.
    • SPOT: Receiver un-muted.
    So obvious that I let it out of my original posts, above.

    73 de Martin, G3EDM
  10. G3EDM

    G3EDM Ham Member QRZ Page

    Greetings all.

    Today I begin the task of trying to "Tame the Boosted Pierce". If you have not been following this saga, I am attempting to improve my build of the W1TS transmitter, which uses a 6C4 oscillator and a 5763 power amplifier (PA). This particular version was published in ARRL's magazine QST in 1968 and was also the transmitter project included in my first book about ham radio, given to me when I was 12 years old: "How to Become a Radio Amateur" (1968).

    While I have been able to get on the air and score QSOs, the signal has been sub-par from the beginning, and actually deteriorated over time. Depending on which crystal I plug in, it manifests as:
    • Yoop. This is when, on initial key down, there is an upward shift in frequency, such that each dit or dah has a brief upward scale at the start.
    • Chirp. A tone instability that can affect the entire duration of a Morse pulse.
    • Drift. A gradual change in the transmitted frequency during the course of a QSO, usually in an upward direction.
    I am going to be trying various fixes and will then re-test everything so that I can tell what is making a difference, and what is not. In this initial post of the repair saga, I will list these planned fixes here. The list may change as we go along.
    1. Check the power supply carefully for problems and fix. This has already been done, and it is just as well, because an important resistor was found to have drifted from a design value of 6K to more than 40K. This resistor has now been replaced, resulting in an audible improvement in my signal, but not a huge one. It has, thank goodness, allowed me to resume limited on-air activity as long as I stick to a handful of particularly stable crystals. I will not post further on this fix.
    2. Change the value of the PA grid resistor from the published value of 10K to the higher value of 22K, which is what it should have been all along and in fact ARRL itself made that correction in later publications of the same circuit.
    3. Substitute a 6AB4 for the original 6C4 oscillator tube.
    4. Experiment with changing the value of R1 (the grid resistor on the oscillator), making it lower than its design value of 47K to put less stress on the crystal.
    5. Remove the oscillator from the cathode keying circuit, run it continuously while in TRANSMIT mode, and key only the PA. (This is potentially the biggest fix of all, but because I am still waiting for some mounting hardware for a relay and because it requires major surgery on my transmit/receive switchbox, will probably not be attempted until some time next week.)
    6. Finally, if all else fails, rebuild the PSU to include an OA2 voltage regulator supply a steady 150V to the oscillator.
    If all these fixes fail to make enough difference, I will still have a usable transmitter but will need to source some new, particularly robust crystals (untouched original FT-243 rocks) for some common frequencies, because otherwise my usable crystal collection is very limited.

    For most of these suggested fixes, I am following advice from @W9BRD who should be commended for his thoughtful and helpful posts here.

    I am posting the circuit diagram below, along with a drawing of the voltage divider in the PSU (which replaces what you see in the ARRL schematic). Let the fun begin.... I have already done Step 1 (no need to post further about it) so the next post, in a few minutes, will be Steps 2 and 3 which will appear in the next few minutes.


    Schematic of the new voltage divider, which replaced the PSU circuit shown above:


    73 de Martin, G3EDM

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