Simulating tube amplifiers

Discussion in 'Amateur Radio Amplifiers' started by W8JI, Dec 29, 2011.

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  1. W8JI

    W8JI Ham Member QRZ Page

    Actual tube with EF Johnson socket as in Heathkit:

    3-500Z-Jhnsn-long-leads.jpg

    This is normal hand wired grid wire, no capacitor.
     
  2. AG6K

    AG6K Ham Member QRZ Page

     How does this voltage burn out the suppressor resistor on the inboard 3-500Z Carl?

     
  3. W8JI

    W8JI Ham Member QRZ Page

    Come on Rich, we all know the HF energy, and heat from the tube alone, can damage a carbon resistor.

    Let's move forward. Imaginary or unconfirmed parasitics aren't the only thing that can cause heat.
     
  4. G0HZU

    G0HZU QRZ Member

    Hi Tom
    I'm going to be a bit busy over the next few days so can't look at this much but your plot above suggests you used a high impedance probe because the response above 400MHz goes above 0dB.

    Can you confirm this?
    Thanks
    Jeremy G0HZU


    Note to all :
    If there's still enough loop gain for the oscillator to oscillate at the VHF design frequency of the suppressor then the suppressor will toast.

    It only takes some basic sums to calculate the load impedance seen at the anode with a basic suppressor. It will be a few hundred ohms and the suppressor will toast.

    The whole design concept of the suppressor is to reduce the loop gain well below unity at VHF by reducing the anode impedance. If it succeeds then there will be no oscillation at VHF because there isn't enough gain around the loop.

    If there is still enough loop gain because maybe the suppressor has aged or someone modded the amplifier tuning tank wiring then the suppressor will toast.
     
  5. G0HZU

    G0HZU QRZ Member

    The current basic AC analysis model is quite good at showing the effect of the suppressor. Note that this is still a very basic system model but it demonstates the effectiveness of the suppressor on loop gain.

    Before is the dashed line and the solid line is WITH the suppressor.

    You can see the anode impedance drop to a few hundred ohms on the smith chart plot is a virtual VNA is temporarily coupled up to the anode.

    The loop gain falls a lot as well. Note that the suppressor only works over a fairly narrow bandwidth so there are places where it will not give the same reduction in loop gain.




    . [​IMG]
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  6. AG6K

    AG6K Ham Member QRZ Page

     Mr. Rauch -- Why wasn't the sister R-supp damaged ? Why was the replacement 47Ω resistor damaged? Why was R-supp #3 damaged? Was C-Tune's arcing coinciding with R-supp greatly increasing in resistance just a triple coincidence?

     There were no external signs of heat damage so whatever toasted the insides happened quite quickly.

    • Rich, ag6k
     
  7. KM1H

    KM1H Ham Member QRZ Page

    So let me get this straight.......
    Continued on its own thread Sue.

    Carl
     
  8. AG6K

    AG6K Ham Member QRZ Page

    •• I'm wondering how arcing could occur across switch contacts that are progrossively shorted by the switch?
    • Rich, ag6k
     
  9. W8JI

    W8JI Ham Member QRZ Page

    Rich,


    Hundreds or thousands of people, or even more, have found open or off value resistors with no signs of heating on the outside.

    Sometimes, one resistor in a bag of 1000 is bad. That doesn't mean the bag of resistors had one resistor with a parasitic.

    Ask around, you will find many people who have seen bad parts without the part looking like it was toasted. Especially carbon components.

    73 Tom
     
  10. G0HZU

    G0HZU QRZ Member

    Here's a Tube based version of the basic AC analysis. Note again that the correct AC analysis sets AC ground at the the grid structure deep inside the tube.This may seem a bit alien to some people but electrons don't know what chassis ground means and the feedback path is as per the diagram below.Also note that I've included a typical LPF input found on a tube design and this LPF makes the loop gain at VHF worse because the feedback can shortcut via C1 in the input LPF.I hope the modelling and analysis is slowly making more sense to people. I've not shown any heater wire connections in order to keep it uncluttered.

    BASIC_FEEDBACK4.gif
     
    Last edited: Jan 6, 2012
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