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Grounding the Grids vs. Cap/RFC Grounding

Discussion in 'Amateur Radio Amplifiers' started by W4LAC, Jun 13, 2019.

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  1. W4LAC

    W4LAC XML Subscriber QRZ Page

    Speaking in terms of RELIABILITY, not power and performance, does direct grounding of the grids of a pair of 3-500 tubes help stabilize an amplifier? Does direct grounding of the grids reduce parasitic oscillation tendencies? Thoughts?
     
  2. K2XT

    K2XT Premium Subscriber QRZ Page

    Yes to your first question - the tube was designed to be a grounded grid amplifier, ground the grids. Which is not to say that 1000s of amplifiers have been built and work ok with grids lifted slightly, and dc grounded via rf chokes.
    Second question - it sure won't hurt, but the tube requires a parasitic suppressor.
     
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  3. W4LAC

    W4LAC XML Subscriber QRZ Page

    I'm wondering why so many manufacturers are capacitively and inductively coupling the 3-500 grids to ground instead of direct grounding. Are the RFC's being used as a sacrificial lamb, in cases of grid to filament shorts in the tube, to protect the filament transformer? I'm thinking about grounding them directly and adding a fuse to the primary winding on the filament transformer to protect it. Thoughts?
     
  4. K2XT

    K2XT Premium Subscriber QRZ Page

    Check this out.
    https://www.w8ji.com/grounded_grid_amplifiers.htm

    I've heard that designers at Drake, Heath, and others thought it provided some negative feedback to lower the gain of the amp on low bands so a user didn't overdrive it. There might have been more to it than that, but if you are homebrewing you should just ground the grids.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Depends on the amp. The SB-220 for example wouldn't need that. Possibly some other designs might benefit from it.
     
  6. G0GSR

    G0GSR Ham Member QRZ Page

    One reason would be if they are applying -ve bias to the grids.
    Older broadcast TX's using triodes had a "grid deck" but the grid decoupling capacitors were very low inductance, consisting of large brass plates with mica or kapton insulation. These grid decoupling capacitors were quite vulnerable to Anode-Grid flash-over.

    Frank
     
  7. HAMHOCK75

    HAMHOCK75 QRZ Member

    This subject seems a bit controversial. So far what I have learned from having both kind of amplifiers ( one with actual grid grounds and one with 200 pF capacitors on the grids ) is that I would not describe an amplifier with capacitors on the grids as grounded grid at all. Between 80-10M, I measured between 33 to 41% of the input cathode drive voltage is across the 200 pF grid capacitors depending on band.
     
  8. KD8DEY

    KD8DEY Ham Member QRZ Page

    Grid current monitoring ?
     
  9. KM1H

    KM1H Ham Member QRZ Page

    It seems some on here have no idea what they are talking about or simply do not understand the subject and are confused.

    First of all there are still many thousand Heath and Drake amps still in use that do just fine as designed and dont need any voodoo cures for parasitics as AG6K used to go on and on about....without having a clue about the subject. To him any arc was due to parasitics and he refused to believe that internal tube gas and worn out relays (pitted contacts and weak springs causing a momentary open circuit) could be the real fault. Toss in a kit built by many without the proper soldering and wiring skills and anything could happen.

    W8** is another off in his own world of misinformation and lack of knowledge. At least Heath used REAL engineers.

    The BIG problem with those old amps is that the original parasitic resistors have failed open or increased a lot in value and that will cause parasitics no matter what is done with the grids. The modern replacement is a pair of Ohmite OY series 47 Ohm resistors and the original coil.

    Another failure point was the old rectangular caps used at the grids in early kits would develop Silver Mica Disease or that cases would crack and allow moisture ingress and arc internally.

    The 26 Ohm DC resistance of the Heath grid chokes added a bit of Negative Feed Back (NFB) which reduced tube gain allowing for less than perfect construction practices. Those chokes also acted as a current limiter/fuse that saved the tube in some cases and could be returned to service if regettered. Those amps also beg for a current limiter resistor in place of RFC 2 which will eliminate a lot of component damage from an internal tube arc; 15-20 Ohms 20-25W real vitreous enamelled resistors are the only ones to use. Eimac and Amperex tubes were far from perfect and would outgas over time.....Chineses ones are the same.

    Another thing worth mentioning is that the SB-220 RF compartment is close to resonance at the 3-500Z parasitic frequency with RFC2 in place! That replacement resistor cures two problems at the same time.

    Heath went to 115pf caps in the SB-221 and later, a standard 120pf 5% 500V silver mica is fine...BUT I never understood the reason for the change...Ive used a GDO and spectrum analyzer and never found any difference.

    Carl
     
    W1BR, EA1DDO and K2XT like this.
  10. AC0OB

    AC0OB Subscriber QRZ Page



    That's exactly what Henry did they as they used a number of resistors and capacitors to share the DC and RF grid currents, respectively. They did not use any RF chokes as did HeathKit.

    Below is an equivalent circuit of a 2K-4 grid circuit. Interestingly, the circuit has a 3 db down staring point of ~ 3.5 MHz.


    Pheel
     

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