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Viking ranger attenuator 4-400a grounded grid amplifier help

Discussion in 'Amateur Radio Amplifiers' started by KC0VVB, Nov 23, 2019.

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  1. KC0VVB

    KC0VVB QRZ Member

    Viking ranger with 6 db attenuator built per instructions from the thunderbolt manual. R1 120 ohms r2 120 ohms 47 puff cap and 470 ohms to ground. Max power from the ranger is about 10 watts. The impedance from the attenuator should be 350 ohms which should be ok for a ranger.

    Driving a grounded grid dual 4-400a home built amp with 2900 volts on the plate shows 100w carrier and 300w peak on the external watt meter.
    This is after the amplifier was tuned for max carrier from the ranger 50 watts dead carrier gave 500w out. I am confused by these readings.
    Erik Kc0vvb
     
  2. KC0VVB

    KC0VVB QRZ Member

    I checked the headroom on the amplifier for a few seconds with 50 watts dead carrier from the ranger showing 500w output. I keyed he Mike and spoke a few words and saw 800w peak. And then I stopped doing that.

    So the amp has plenty of headroom the way it's tuned and is reflecting the modulation from the ranger, for example 500w carrier and 800w peak
     
  3. KC0VVB

    KC0VVB QRZ Member

    With 100w carrier power the ranger is modulating 3 times the carrier power not 4.
    I built another attenuator for 3db r1 62 ohms r2 55 ohms r3 1000 ohms and the same 47 puff cap. The impedance was about 300 ohms. This allowed a 20 watt carrier from the ranger.
    200w dead carrier.
    And 400w on modulation peaks. In the amplifier now the ranger is modulating at 2 times carrier power.

    I'm confused. 100w carrier when modulated by a perfect sine wave which I haven't done yet I was just using my voice should be 400w peak. Not 300.

    And 200w carrier should show 800w on the peak reading watt meter. Ok so I'm just using my voice and 800w is a lot to expect but if 100w carrier shows 300w peak. I would expect 200w carrier to show 600w peak.
    Erik Kc0vvb.
     
  4. KC0VVB

    KC0VVB QRZ Member

    Ok so I have an issue some where.
    I'm pretty sure the linear is linear. It may be the way the ranger is tuned. It may need to be re tuned after putting max out put through the amp for tuning purposes and hooking up the attenuator. The modulation just seems odd.

    I may have an issue with the MFJ combination watt meter dummy load.

    Any ideas would be helpful.

    Erik kc0vvb
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm unclear about how you're tuning the amplifier.

    You know that a linear amplifier used on AM must be "tuned up" at the full PEP power level, right? And definitely not at the "carrier power" level -- ever!

    If you apply enough drive to achieve 800W key-down (carrier) output power, then reduce drive to 200W carrier power and do NOT retune, then when you modulate you should see the 800W PEP.

    If you "tune up" at the AM carrier power level, not only will you never see 4x carrier power as PEP, but you'll also produce a very distorted signal.
     
    WA5VGO likes this.
  6. KC0VVB

    KC0VVB QRZ Member

    Roger that Steve. That's what i am doing. Max 50 watts dead carrier from the ranger without modulation drove the amplifier to about 600w i figured i could get more power out of the amp with the tuning the way it was so i keyed the mike and spoke a few words just to see what it was capable of it hit 800 watts.

    Without changing the tuning of the amplifier at all. I put the 3db attenuator between the ranger and the amp. With the amp by passed, and the output of the ranger going into a dummy load. Dead carrier with the mike keyed and volume control all the way down 20 watts output from the ranger. When i turned up the mike gain and spoke normally calling CQ the ranger shows 80 watts peak into the watt meter / dummy load on the output of the amplifier.

    When i turn on the filaments and the plate of the amplifier and key the internal co-ax relay i get 200w dead carrier output from the amplifier. When i switch the ranger to the phone position and make noise into the microphone the watt meter shows 400w peak. Coming out of the amplifier. With a plate voltage of 2900v plate current of 250 ma and a Cathode/Filament/ grounded grid curent of 80ma.

    I am confused by the watt meter reading of 400w peak how could the output power be so low. 2900v keyed x.250 ma = 725 watts.
    Even modulated with just my voice at 200w carrier and this power consumption i would expect to see over 600w .
    Erik KC0VVB
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know what you mean by Cathode/Filament/grounded grid current. Cathode current (DC) = plate current + grid current. It would always be the same or higher than plate current.

    With an 80mA reading, that would be "grid current" only.

    Assuming your PEP wattmeter is actually accurate and would agree with a scope envelope waveform (maybe it doesn't), the symptom may be describing poor power supply dynamics or some manner of distortion which is limiting positive peak modulation, and may or may not be effecting negative peak modulation.

    If you haven't already done this, use a scope to observe the output RF modulated envelope. You don't need a single tone generator, although that's handy -- usually both PEP and distortion are observable just using your voice.
     
  8. WQ4G

    WQ4G Ham Member QRZ Page

    Post deleted
     
  9. KC0VVB

    KC0VVB QRZ Member

    Ok Steve;
    I will call it grid curent. I have a Heathkit sb-610 station monitoring scope.
    I can check for odd modulation coming out of the ranger. I just replaced the high voltage rectifier that could require a double check and reset of the modulator screen voltage.

    Another thought
    Not all resistive t attenuators use a 47 puff cap across r1 and r2. Could the Value of this cap be affecting modulation peaks?
    Should I consider removing c1 or increasing the value?
    Thank you for your quick and thought full response.
    Erik KC0VVB
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nah, that can't impact modulation. Any attenuator that includes a capacitor indicates the cap is there just to counter the inductance created by the design to make it more of a flat 50 Ohm load at higher frequencies; it doesn't do anything on 80m, but might help on 10m.
     

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