High Gid Current

Discussion in 'Amateur Radio Amplifiers' started by NE7X, Jun 5, 2011.

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  1. W8JI

    W8JI Ham Member QRZ Page

    Tom,

    DO NOT start randomly playing with resistors. The circuit I gave you, if it is really followed, will work.

    I am almost convinced you are relying on Henry's wiring or labeling to be, at least in some part, reliable.

    The meter path resistance, if you want a 1 amp meter, has to be 200 ohms with a 1 mA meter and a 0.2 ohm shunt. There is no way around this. You meter has an internal resistance of 50 ohms, so you need a 150 ohm multiplier in series with the meter. If you have to use something else, then you have a wiring error and it is NOT like the schematic I posted.

    For the plate current multiplier to affect the grid current, there has to be a wiring or labeling error or a bad component.

    The plate meter multiplier cannot possibly affect grid current unless the grid meter shunt or grid meter itself is connected incorrectly. This is just how it is.

    The correct series TOTAL resistance for the grid meter would be 20 ohms per 100 mA of full scale reading. If you want that meter to read 400 mA, you would need a TOTAL meter resistance of 80 ohms. Since the meter is 50 ohms, you would have to add 30 ohms externally. If you find you have to alter this, you have something wired wrong or a bad shunt.

    I'm pretty much convinced you are relying on something from Henry, perhaps a pictorial or perhaps how they said something was connected, and it is misleading you....or you have a simple wiring mistake in the meters.

    The resistor in series with the plate meter HAS to be 150 ohms for that meter to read 1 amp with a .2 ohm shunt.

    The resistor in series with the grid meter has to be 30 ohms for that meter to read 400 mA FS with a 0.2 ohm shunt.

    If you find disagreement with this, you have a wiring error or a wrong or bad component. If you change the resistance to make things work right, then it will never really get fixed properly.

    It sounds to me like the grid meter is wired wrong, and you are just missing the reason why.

    73 Tom
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Tom, Please go back and look at your "corrected" schematic. It appears to me that the B- passes through the plate meter and is then in series with the grid meter. Shouldn't the grid meter pick up the B- before it passes the plate meter? Have a look and see if I am full of it. Lou
     
  3. KD0CAC

    KD0CAC Ham Member QRZ Page

    If you guys do go off line to finish , I hope you post end results here , I think a few are fallowing this misery , we would like the end of the story :)
    73
     
  4. NE7X

    NE7X Ham Member QRZ Page

    Here is how I currently have the meter circuit wired and working. Both meters do not peg with 100 watts drive and compared to my other amplifiers, the meter reading values are within the same ranges.

    PLATE METER M101
    Multiplier resistor R115 = 150 ohms
    Meter resistance = 51.8 ohms
    Shunt resistor R116 = .2 ohms

    GRID METER M102
    Multiplier resistor R117 = 75 ohms
    Meter resistance = 51.2 ohms
    Shunt resistor R118 = .2 ohms

    However W8JI indicates having R117 set to 75 ohms and not 30 ohms implies there is still some circuit issue.[b/]

    W1QJ indicates the B- for the grid meter M102 should not be picked up after it goes through R116 (plate meter shunt) at the positive terminal of plate meter M101. It should be picked direct off the B- rail before it goes through R116. Both the Henry and W8JI schematics show the grid meter pickup at the same point, after R116 and at the positive terminal of M101.

    Surfing the internet looking at other grounded grid RF amplifier schematics, what I found:

    BIAS zener anode <== grid meter positive terminal / grid meter negative terminal ==> ground
    BIAS zener anode <== plate meter positive terminal / plate meter negative terminal ==> B- HV

    Looking at these other examples, it shows the grid meter is picked up before the plate meter shunt.

    W8JI, please clarify, what side of R116 should the grid meter pickup be?

    Thomas NE7X…
    http://ne7x.com
     
  5. NE7X

    NE7X Ham Member QRZ Page

    Looking again at both the Henry and W8JI schematics, it now see the BIAS zener anode connecting to junction of R117/R118/M101+/R116. This is where the positive terminals of both meters connect, then the two meter paths split. So the grid meter pickup up is before R116.

    BIAS zener anode <== grid meter positive terminal / grid meter negative terminal ==> ground
    BIAS zener anode <== plate meter positive terminal / plate meter negative terminal ==> B- HV

    So both the Henry and W8JI schematics appears to follow the same as the other circuit designs I found on the internet. However there is one difference, R118 shunt going to ground. I do not see this on any of the other designs.

    What role does R118 play?

    73s, Thomas NE7X…
    http://ne7x.com
     
  6. NE7X

    NE7X Ham Member QRZ Page

    The D1 BIAS zerner voltage is 12 volts, not 10 volts as original design. So this will put 2 extra volts at the junction of R117/R118/M101+/R116. Could this be the reason why the 30 ohm R117 multiplier resistor for grid meter needs to be a higher R value (75 ohms) to compensate for the extra 2 volts?

    73s, Thomas NE7X…
    http://ne7x.com
     
  7. NE7X

    NE7X Ham Member QRZ Page

    Just for clarification, now when I change the multiple resistor value of one meter, it DOES NOT change the reading for the other meter. That interaction issue was only observed when I was “adding” additional resistance in series between B- rail and R116/R115. That is when R116 and R115 were both .2 ohms. Here is the current configuration values. At this time everything appears to be working as expected

    PLATE METER M101
    Multiplier resistor R115 = 150 ohms
    Meter resistance = 51.8 ohms
    Shunt resistor R116 = .2 ohms

    GRID METER M102
    Multiplier resistor R117 = 75 ohms
    Meter resistance = 51.2 ohms
    Shunt resistor R118 = .2 ohms

    So the only question is why R117 is 75 ohms and not 30 ohms. Is this because of the higher 12V bias zener voltage? Original zener value was 10V. When R117 is at 30 ohms the meter pegs with >30 watts drive. 75 ohms, 100 watts drive the meter reads 350ma.

    73s, Thomas NE7X…
    http://ne7x.com
     
  8. W1QJ

    W1QJ Ham Member QRZ Page

    If you really had the meters in series like the one schematic showed, the grid meter would be reading the same as the plate meter and to tone it down you added resistance. I think W8JI got confused on which schematic he uploaded. The one he said had a mistake may have been the right one and the one he said was corrected i think was the wrong one. The one he says was wrong shows the grid meter not in series with the plate meter as it should be. The extra voltage on the bias zener would make no difference in the meter shunting or series. The only thing that will do is change the ZSAC on the plate reading in idle.
     
  9. NE7X

    NE7X Ham Member QRZ Page

    Looks like I may have only a 15 meter issue. To be honest, 95% of my testing was done on only 15 meters so I made the assumption the Ig meter issue was also true for all the other bands and did not test each time on a different band (pain in the butt retuning). MY BAD !!!

    Tonight I replaced R117 with a 30 ohm resistor like everyone was indicating it should be, and then tested on “all bands.” For reference, the meters scales: Ip meter = 0-1A / Ig meter = 0-400ma. Here are my test results.

    80 meters
    key, no drive
    Ip = 100ma / Ig = 0ma
    Key, 100 watts drive
    Ip = 700ma / Ig = 200ma

    40 meters
    key, no drive
    Ip = 100ma / Ig = 0ma
    Key, 100 watts drive
    Ip = 710ma / Ig = 325ma

    20 meters
    key, no drive
    Ip = 100ma / Ig = 0ma
    Key, 100 watts drive
    Ip = 800ma / Ig = 300ma

    15 meters
    key, no drive
    Ip = 100ma / Ig = 0ma
    Key, 100 watts drive
    Ip = 600ma / Ig = Meter pegs full scale.

    10 meters
    key, no drive
    Ip = 100ma / Ig = 0ma
    Key, 100 watts drive
    Ip = 500ma / Ig = 175ma

    PLATE METER M101
    Multiplier resistor R115 = 150 ohms
    Meter resistance = 51.8 ohms
    Shunt resistor R116 = .2 ohms

    GRID METER M102
    Multiplier resistor R117 = 30 ohms
    Meter resistance = 51.2 ohms
    Shunt resistor R118 = .2 ohms

    Any suggestions why only on 15 meters the Ig meter pegs?

    73s, Thomas NE7X…
    http://ne7x.com
     
  10. K9FV

    K9FV Ham Member QRZ Page

    Lou, Allow me to start off by saying I do not have anywhere near the knowledge you have on amps or electronics in general. BUT from looking at Tom's (W8JI) "corrected" schematic I'm having a hard time seeing both meters in series. Isn't P5 the B- line? There is no connection of meters to the P3 line.

    Follow the wire from neg end of C112, down thru R116 shunt, then up to the dot showing connection to R118 shunt, then on to amp. That is the B- line with the Ip meter in series with that line as it should be. The Ip current is measured in series with B- line, the Ig current is measured from B- to chassis ground?

    Now, from the dot showing the R118 Ig meter shunt to ground, HV/Ig meter above that line is wired with neg meter terminal to ground, in effect making the meter read Ig current(voltage drop) across R118 to chassis ground. I don't see how that is series both meters together.

    Am I missing something?

    Ya'll all have been very helpful and I've learned a LOT from this thread.

    Tom (NE7X) - your last post sure makes it look like the amp is working correctly with all meters reading correctly. I'm waiting for Tom or Lou to post as to a possible solution as to why only 15 meters is giving a problem. A tube problem that acts up only in the 21 mhz freq range? That seems unlikely since 28 mhz is working good.

    73 de Ken H>
     
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