simple modulator

Discussion in 'Amplitude Modulation' started by KC2ZFA, Jul 16, 2020.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Please proceed and let us know the findings when loaded by the grids and with plate power. If you need more grid voltage there is no reason the 6.Xk loading resistor cannot be removed since the secondary is now being loaded by the equivalent grid resistance.


    Pheel
     
  2. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    everything is set up but I’m afraid to throw the switch :-(
     

    Attached Files:

  3. WZ5Q

    WZ5Q Ham Member QRZ Page

    Throw that Switch!!
    Drop the Blade!!
    Lets see the Sparks of Evil Carnage!!! :D:eek:

    Love the loads of clip leads by the way! ;)
     
    KC2ZFA likes this.
  4. W7TFO

    W7TFO Premium Subscriber QRZ Page

    A good modulator shows no signs of trepidation...

    73DG
     
    KC2ZFA likes this.
  5. K5UJ

    K5UJ Ham Member QRZ Page

    If it's wired up correctly and none of the clip leads etc. are touching anything else it should be okay. Just make sure there's a load on the transformer secondary either the rig or a big w.w. resistor that's around the intended load Z. I can't tell exactly what you are doing from the photo but that looks like a Thordarson CHT Tru-Fidelity transformer. That was their high end audio line. Make sure it has a secondary load.
     
    KC2ZFA likes this.
  6. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    OK, you guys convinced me.

    the mod xfmr is the T11M77 out of the CHT series (bought from Brett N2DTS years ago) and is connected with a 1.4 turns ratio and parallel secondary. It's older than dirt. The load's overall resistance for a first test is 5.4k.

    The test connection is below, except I use a 0.5uF 7.5kV coupling cap, and a 10:1 divider since I have a 100X probe.

    The ps is 1350V (choke 10H, capacitance 55uF) and capable of 400mA ccs. When loaded with 5k it drops to 1200 while putting out 230 mA.

    So, in terms of what to expect for the first smoke test:

    a) driving amp off, in receive mode, the resting current (there's a 50k resistor from filament ct to ground) should be practically zero. Yes/No ?

    b) driving amp off, in transmit mode (the 50k resistor is shorted out now), the bias switch to zero (I can do increments of 0.5 volts or so up to like 7 volts), the resting current should be 30? 40? 50? mA ?

    I really appreciate all the help you've all given.

    Peter
     

    Attached Files:

  7. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    One thing I forgot, the filament voltage comes up to 6.77 so I put a small variac to bring it down to 6.3.
     
  8. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    I gathered some courage and answered my questions :)

    a) driving amp off, in receive mode, the resting current (there's a 50k resistor from filament ct to ground) should be practically zero. Yes/No ?

    Answer: Yes

    b) driving amp off, in transmit mode (the 50k resistor is shorted out now), the bias switch to zero (I can do increments of 0.5 volts or so up to like 7 volts), the resting current should be 30? 40? 50? mA ?

    Answer: 50 mA, with 811 tubes, not 811As. Yesterday I received 2 811As (thanks John) and will check those too.

    Also, I added diode bias with the rotary switch and saw that 50mA go down and down.

    So far so good, no arcs and sparks.

    On to putting some audio on this thing.
     
    AD5HR and (deleted member) like this.
  9. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    so here's some groping in the dark, now using 811A's.

    at 140Vpp at the 811A grids I measure 1.2kVpp at the 5.4kΩ load resistor. The ps is supplying 1.2kV at 140mA

    at (a whopping) 240Vpp at the 811A grids I measure 2kVpp at the 5.4kΩ load resistor. Now the ps is supplying 1.15kV at 240mA. These were very brief tests as that 240 Vpp across the grids is too high.

    The 11M77 multimatch is set at 1.4 turns ratio.

    From the datasheet, with 140Vpp at the 811A grids I was expecting something like 3.2kVpp (for 235 watts across the 5.4kΩ load) at a plate draw of about 260mA.

    I'm either still measuring wrong (I've described the setup above), or I'm not understanding/processing properly what I'm measuring.

    :confused:

    In edit: I just remembered Master Pheel's suggestion to "If you need more grid voltage there is no reason the 6.Xk loading resistor cannot be removed since the secondary is now being loaded by the equivalent grid resistance." That may be my problem, parallel resistances when the B+ is on...check me please ?
     

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