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favorite novice transmitter for about $100 these days...

Discussion in '"Boat Anchor" & Classic Equipment' started by K9UR, Oct 23, 2021.

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  1. W9BRD

    W9BRD Ham Member QRZ Page

    Here's just an example (simulated) of the modest (approximately 12 dB) rejection of subharmonic energy by a C-L-C pi network. The pi was designed using calculator at https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html to look like 3333 ohms, resistive, at 7 MHz, assuming a 50-ohm resistive load at its output, based on a Q of 12. Input C is 82 pF; inductance is 6.84 uH (with a Q of 200); and output C is 493 pF:

    pi_net_response_3333_ohms_to_50_ohms2.jpg

    A few watts of QRP can really get out, so a Novice operating running close to the Novice legal limit at 15 meters with a 7-MHz crystal and a 40-meter dipole at 3f with no antenna tuner in between to reduce 7-MHz output could easily have been cited for out-of-subband operation with a T-60 or HT-40. Of-of-ham-band spurs, such as those emanating from conversion VFOs with insufficient filtering, would have been even more "pink ticket"-hazardous.

    A classical parallel tuned circuit between driver and final is the way to add more rejection of the subharmonic (in this case, of 3.5 MHz) ahead of the PA control grid. Such a circuit also supports the straightforward introduction of classical bridge neutralization, also a good idea with high-gain tubes the likes of the 6DQ5 and 6DQ6.

    Adding such circuitry to an HT-50 or T-60 is not trivial; for starters we need the final grid tuning C to be insulated from the chassis to allow its participation in the neutralization bridge, and preferably we need a variable C there with a greater capacitance range than the stock part in either transmitter.
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    How many pink tickets were issued because one was using an HT-40 or a T-60?

    Again, if say I am wanting to operate on 14.250 MHz with a 7.125 MHz base frequency set by a VFO, what is the level of the 7.125 MHz component in the output signal in terms of dB down from 14.250 MHz signal? You do realize that most doubler/tripler stages have to operate in deep Class C with the fundamental frequency given to them?

    So neither you nor Janis ever measured the subharmonic(s) or the harmonic content on the HT-40, or T-60, or any other transmitter of the same era?

    I did measure the spectrum of the HT40 for the latest HT-40 rebuild seen below in the PDF. I used a 7.160 MHz crystal and tuned it up on 14.320 MHz. Power output was set to 20 Watts at 14.320 MHz into a 50 ohm NI load.

    The subharmonic of 7.160 MHz was 37 dB down from the carrier; the first harmonic at 28.640 MHz was 42 dB down from the carrier. Interestingly, the second harmonic was only 55 dB down from the carrier.

    Other harmonics past the second were too low to measure and in the noise.

    I put my 100MHz scope on the output and the carrier is a pure sine at the 14.320 MHz frequency.

    IMHO, your criticisms seem overstated considering the economics and the simple design for a Novice transmitter of this era.

    Pheel
     

    Attached Files:

    Last edited: Nov 15, 2021
    K9UR likes this.
  3. W9BRD

    W9BRD Ham Member QRZ Page

    Correct, in my case. I don't have a spectrum analyzer. In my case the final-amplifier tuning hysteresis -- a product of lack of neutralization -- and lack of a sensical driver plate/final grid tuning peak led to my rebuilding that part of the circuit with a classical parallel-tuned circuit and neutralizing bridge.
     
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    RF feedback from the Final is fed to the junction of C14 and C15 to curb any parasitic oscillations and is a Neutralizing circuit comprised of a lug going to the 2 lug terminal strip next to the Final.

    I have never experienced any neutralization problems in an HT-40 with or without a stub and if there had been any parasitic oscillations, they would have shown up on the SA.


    Pheel
     
  5. W9BRD

    W9BRD Ham Member QRZ Page

    When the output-power peak occurs at a different tuning point depending on whether one is tuning "up" or "down", positive feedback is indicated. I experienced that interaction with my HT-40; that's why I neutralized its final by building in a classical bridge that allows neutralization adjustment from "not enough" to "too much." Now as I tune the final, its output power peak occurs in the same dial position regardless of tuning direction, and its grid current is largely static except for a slight increase at output resonance, as expected. Case closed.
     
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Of the 6 total HT-40's I have worked on (now own two as I described earlier) there is only one Plate Tune resonance point and one Plate Loading setting which resulted in maximum output power.

    I would imagine that if one is tuning "up and down" with the Plate Tune and Plate Loading controls, certain combinations of settings of both controls could tune up on a subharmonic or some harmonic above the carrier but that would not prove a parasitic occurred, and certainly would not result in maximum power output. After all, the 6CX8 stage does have a "multiplier" function, in addition to its buffering and driving functions.

    I still have the HT-40 on the bench and the SA showed no parasitics when tuning "around."

    Your "Hysteresis" and other comments are not convincing because you have not shown any instrumented proof of the anomalies you say must be there.

    Pheel



     
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  7. W9BRD

    W9BRD Ham Member QRZ Page

    No worries; I'm not trying to convince you. When I fully neutralized my final, the tuning hysteresis you haven't experienced went away. Good for both of us!

    Out.
     
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I am not trying to convince anyone of anything, just asking them to provide supporting data as proof of supposed anomalies. Schematics of their fixes would be appreciated so they can be commented upon.

    One other comment: Back when NTSC or analog TV was being transmitted, there was a real effort by the manufacturers of amateur transmitting equipment to reduce interference and to protect TV Channel 2, 54-60 MHz.

    The HT-40 had a harmonic spur at 57. 280 MHZ, but this spur was 68 dB down from and wrt the carrier frequency of 14.320 MHz. So their simple Pi circuits and their L16-C50, L17-C51 trap system works very well.

    I measured this spur on a DX-60 with the same crystal and on the 20m band, and the 57. 280 MHz spur was 72 dB down from the carrier frequency of 14.320 MHz, using their complex low-pass output filter.

    So Hallicrafters did a good job of providing a reliable Novice transmitter that also protected NTSC Channel 2.

    Pheel
     
    Last edited: Nov 16, 2021

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