Simulating tube amplifiers

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

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  1. AG6K

    AG6K Ham Member QRZ Page

     The 40M/7MHz bandswitch contact is protected from 110MHz energy because the 40m inductance is a virtual RFC at VHF. The SB-220's 28MHz contact.and C-Tune are the most vulnerable to intermittent arcing from the 110MHz parasite.

    • Rich, ag6k
     
  2. AG6K

    AG6K Ham Member QRZ Page

     Thanks for the explanation Jeremy. I am more or a crime scene evidence investigator than a mathematical prestidigitator - so for me 110MHz damped-wave ringing, autopsy findings, and a crispy-crittered R-supp is more convincing. cheerz
    • Rich, ag6k.
     
  3. G0HZU

    G0HZU QRZ Member

    Hi Rich
    I still have a lot to learn about tubes but I also think there's stuff I already find very confusing.

    For example I am a bit surprised why grounding the grid connections with copper tape as Tom has done here will make stability better at around 110-120MHz.

    http://www.w8ji.com/tl922_mods_parasitic.htm

    I would have thought that the original 220pF capacitors as per the original schematic would have been better. i.e. 3 x 220pF leaded caps will give much lower reactance at 110MHz than that copper tape Tom has used.

    I did try both options on a basic amplifier model and the original caps are clear winners in terms of keeping the K factor high above 100MHz (ideally above 1)

    The manufacturer must have chosen these caps for that reason? Copper tape is always going to lose if the goal is to keep inductive reactance as low as possible at around 110MHz. At a guess Tom's mods make the reactance from socket pin to chassis WORSE (at 110MHz) by a factor of three?
     
    Last edited: Jan 10, 2012
  4. G0HZU

    G0HZU QRZ Member

    Tom
    You need to help me here. I found this explanation above.

    Far from being an engineering gaff as you put it, I think they are a way of offsetting the grid inductance in order to improve grid grounding at RF across lower VHF through maybe 160MHz.

    I have no idea what your super cathode analysis is all about because it appears like a load of hamgineering nonsense and I think you have seriously missed the reason these 3 x 220pF caps are there...

    If you knew why they were there then you surely wouldn't have replaced them with copper tape. Especially on a web page linked with VHF parasitics?
     
    Last edited: Jan 10, 2012
  5. G0HZU

    G0HZU QRZ Member

    It's just a guess on my part but maybe, just MAYBE this was done on purpose in order to add inductance in the chassis path back to the valve. This would tend to move lower the range of VHF frequencies over which the tank can be close to resonance with the anode capacitance and cause VHF instability.

    This would be in sympathy with the design goal of the 220pF caps (the ones you threw in the bin) as these caps will improve grid grounding at the lower VHF frequencies where instability can occur (compared to your copper tape).

    So the two things you proclaim as design gaffs could be intentional in order to promote better VHF stability. But that's just a guess on my part :)
     
    Last edited: Jan 10, 2012
  6. G0HZU

    G0HZU QRZ Member

    It would appear so...

    The copper tape mod is the worst one I've seen so far. Are there any others?

    [​IMG]
     

    Attached Files:

    Last edited: Jan 10, 2012
  7. G0HZU

    G0HZU QRZ Member

    I'm a bit surprised that there's been no response about the copper tape mod...
    http://www.w8ji.com/tl922_mods_parasitic.htm


    The reason it is a poor 'mod' is because the original 220pF caps were there for a reason (IMO)
    If you plot the reactance vs frequency of a 220pF cap then you see that it is about -7 ohms at 100MHz

    If you go down to 80MHz it has -9 ohms, at 60MHz it has -12 ohms.
    This effectively cancels some or all of the inductive reactance of the grid structure and also its connections. This is GOOD for stability.

    The penalty of adding the cap is it adds a few nH with its leads but this is still cancelled to a degree by the higher capacitive reactance of the 220pF capacitor.

    The copper tape ADDS inductance everywhere and will make the potential for instability across 60-130MHz much higher.

    Simple maths proves this...


    IMO a typical RL anode suppressor will be very good at reducing instabilty at the highest frequencies the tube shows instability (without the suppressor). As you go down in frequency the suppressor becomes less effective.

    The 220pF caps are there to provide MUCH improved RF grounding of the grid across 60-110MHz.
    (compared to copper tape) This helps provide extra stability in this area.

    In the region 110-140MHz I would expect the caps to still outperform Tom's copper tape but with less margin.
    As I said in a previous post a parallel RL suppressor at the cathode may be worth trying as this will reduce VHF gain as well as adding phase shift.
     
  8. G0HZU

    G0HZU QRZ Member

    I can demo the above effect with a basic amplifier model if it helps.

    Even the simplest of models demonstrates that the copper tape mod really degrades amplifier stability across 60-130MHz compared to using 220pF caps (with maybe 10nH lead inductance)
     
  9. W1QJ

    W1QJ Ham Member QRZ Page

    G0HZU, You can follow problems with arced band switches way back on all the amp forums concerning SB-220's and TL-922 amps. Try and find one compalint about the same band switch arcing for a Drake L4B or LK-500. The two latter amps all use the same switch until the Lk-500 somewhere along the line switched to the same switch that the SB-220 and TL-922 used. Once that mod was made the switch failed miserably in the field. In fact, every single one I have seen is toasted in the LK-500 with the same switch. The ones that are the same as the L4B are all prisitne after almost 30 years. The AL-82 also uses a similar band switch and arcing is uncommon in that amp also. Drake used the cap/inductor grounds, LK-500 did not. Eimac engineers put out a recommended schematic for an 80-10m 3-500 amplifier years ago and they recommend grounded grids. All the L4B and LK-500 amps I have seen have no problems with overheated Rsup. I believe that both QRO amps and Ten Tec amps use directly grounded grids also.
     
  10. GM3SEK

    GM3SEK Ham Member QRZ Page

    The problem with that "simplest of models" is that it's far too simple.

    In parallel with the two 200pF capacitors, the SSB-220 has a 1mH RF choke to provide a DC return. There obviously a parallel resonance between 1mH and 2 x 200pF which effectively un-grounds the grid at 250kHz, but that frequency is too low to cause any trouble. Far more important are the spurious resonances of that pie-wound 1mH choke at higher frequencies.

    To model the "grounding" of the grid accurately at all frequencies, we would need a complex sub-model for the RF choke which describes its alternating parallel and series resonances, and the alternating inductive and capacitive reactances in between. When the two 200pF capacitors are connected in parallel with this sub-model, there will clearly be some higher frequencies where the entire network becomes parallel resonant and the grid is effectively un-grounded.

    Heathkit didn't have a clue about these possible spurious responses, so please let's stop pretending that the grid "grounding" arrangement in the SB-220 was uniquely insightful. They made it work, but it was a more risky design decision than anyone in the 1960s could ever know.


    73 from Ian GM3SEK
     
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