3-500Z Home Brew problem almost solved and I need ideas.

Discussion in 'Amateur Radio Amplifiers' started by KM6CND, Jul 21, 2018.

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  1. KM6CND

    KM6CND Ham Member QRZ Page

    This has been a real adventure, filled with frustration and considerable expense.

    I will not post all the gory details, just the Cliff Note's

    Once upon a time, I traded an FT-817 for an FT-757GXII, along with the "tuner", meter and amp. The builder was an SK when I considered the trade. The executor of his estate was also a ham and had spent many hours in the garage with the builder. He basically forgot how to operate the amp, it seemed to work, so I made the trade.

    The amp worked fine for a time. I looked at videos on tuning an amp and seemed to get it right. Eventually I killed the tube from over driving it. I recently read a spec sheet for the 3-500Z that says 60 watts max drive and I was sending 100 watts into it all day long. My first diploma from the School of Hard Knocks

    I bought a new tube, and the amp worked fine. I had a Big Stick Dipole at the time, so I limited myself to 200 watts.

    I began to explore FT8 and the others. I continued to maintain a 200 watt limit. I had finished about 20 sessions when I saw the Cathode needle fall to zero. The consensus from fellow Zedders was I killed the tube by running these digital sessions. Diploma #2 from the School of Hard Knocks.

    I bought another tube and installed it, set things up for a test, key up and ZERO activity on the meter, I bypass the amp and the meter reacts.

    After much thought, I figured the RF is blocked by a possible dead door knob capacitor sitting just above the coil. I bought a new one and yes indeed the RF passed through.

    During one of those FT8/9 sessions, I saw the resistor (inside the inductor) burst into flames. Quick power down (Pray like mad) and replace the resistor.

    Presently I have a new tube and a new capacitor. With 30 watts of drive, I see 400 watts out. The problem is the Cathode and Plate dials DO NOT REACT when I turn them. I'll go to CW with 5 watts 10 watts, 20 watts, turn the dials and no reactions. There is one more capacitor at the bottom of the coil as it connects to the coax.

    I have detailed photos of the amp on my QRZ page. Any ideas are welcome.

    Thanks in advance.

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    First, make sure the capacitors inside the amp are actually rotating when you turn the knobs.

    Assuming you've already done that: The parasitic choke resistor burning up is an indication of an instability in the amp creating parasitics. If the 400W output you see is "parasitic oscillation" and not amplification of the RF input, then it can happen that the tuning controls won't do much, or maybe anything.

    You shouldn't have CATHODE and PLATE "dials," you should have PLATE (or TUNE) and LOAD "dials."

    Buying new tubes at $190 or so a pop is not a good way to troubleshoot an amplifier.

    You've posted photos of the amp and it's "schematic" before: The photos revealed beautiful construction, but the schematic always revealed design errors. And IIRC, the amp had meters labeled CATHODE and PLATE, which makes absolutely no sense -- nobody would monitor cathode current and plate current. In a GG 3-500Z amplifier, there's zero reason to monitor cathode current because cathode current is the sum of grid and plate current, and when you tune through resonance, grid current will peak at the same point plate current dips -- so it's not a useful indication of anything at all.

    A parasitic oscillation can burn up stuff, but unless it also creates huge grid currents in the process, shouldn't hurt the tube. If the amp has a parasitic, the objective would be to find out why and cure that. A bad plate RF choke, bad plate bypass capacitor at the cold end of that choke, lousy shielding, bad grid grounding connections and other things can cause problems.
  3. KM6CND

    KM6CND Ham Member QRZ Page

    The day the resistor fried, I wondered if the choke below it was also damaged. You may be on to something.

    I saw a solid state 600 watt Ameritron on Craig's List. I almost bought it, just to give up on this tube based amp...
  4. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Everything you're describing here is clearly operator error. First off, why would you need an amplifier on FT8. However, you should not be driving that amplifier with anything more than 10-15W on any high duty cycle mode, be it digital or AM. If you think you can, look at all the burned up stuff you have and rethink your position.

    One of the great things about FT8 is the ability to operate just fine at low power levels. anything more than 100W is just a waste, assuming you don't have an antenna system which needs power to overcome losses. Regardless, stop driving your amplifier to death. Learn the differences of power in different modes, everything does not equate to SSB PEP.

    And if you try to do this with that SS Ameritron I guarantee you will have that acrid smell of burnt up MOSFETs in seconds. Literally.
    Last edited: Jul 22, 2018
  5. KM3F

    KM3F Ham Member QRZ Page

    This is what happens when you have NO idea how it works or what your doing.
    Sorry to put it that way but it's the truth.
    You don't need ideas now but some education first so you can understand the ideas.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    I agree . . . this is a common problem these days!

    Years ago, I learnt how to Tune and Load a Valve PA Pi-Tank on a 10w Valve rig (by initially being shown, and then doing it thousands of times.)

    It was quite a while before I touched a rig that ran 150 watts input . . .

    And even longer before I dared to touch a high-power Linear Amplifier!

    The trouble these days is that a lot of people's FIRST experience of Tuning & Loading a Valve PA Pi-Tank is on a high power Amplifier . . . let alone how to monitor everything and drive it properly.

    I have repaired quite a few Amps for locals, who sadly really don't understand the basics.

    Roger G3YRO
  7. AI3V

    AI3V Ham Member QRZ Page

    Start by typing "care and feeding of power grid tubes" into your search engine.

    K9ASE likes this.
  8. N5MEP

    N5MEP Ham Member QRZ Page

    If you can find a copy of Bill Orr's Radio Handbook I think you will find a lot of information
    that will help you learn about tube amps and their operation.

    It does sound like you have a parasitic oscillation problem. You may also have an actual problem with the metering circuit that doesn't allow you to tune the amp correctly. A mistuned condition could contribute to the parasitic problem.

    As far as running a key down mode at 200 watts, the tube is just loafing. If you search online for an Eimac 3-500z data sheet, you will see that. The only problem might be that you don't get the tube hot enough to getter properly.

    Don't give up on it. From the photos you have a very nice amplifier. By the time you get it lined out you will have learned a lot, and will undersand it's operation.

    Larry N5MEP
    KM6CND and KD2ACO like this.
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That's an interesting amp!
    I do see the two fans that look like they are supposed to cool the tube seals at the base. I don't see a fan pulling air out of the chassis. Usually, one would set up baffles on the insides so that air really exchanges inside the box with strategic openings to keep the tube surroundings and power supply cool(er... it's all relative) as well. That tube cools mostly by radiation so the surroundings need to be "not screaming hot" so they work as a sink.

    What do the tube pins and mating socket hardware look like? Hi-Temp discoloration?

    Can you see the plate through that window? At 200w carrier, I bet the tube is screaming yellow after a few minutes unless the airflow is very impressive, which I don't think it could be with the apparent cooling arrangements.

    Steve's suggestion of oscillation sounds plausible. Do you have an RF sampler and scope on the output? I would be running those with any amp, but especially an unknown home brew, just to keep an eye on it and see what's coming out of the RF jack. This would answer the parasitics question in a hurry.

    I have an AL80B in place now and the manual says it can run at 500 watts output RTTY for 1/2 an hour. I'm skeptical about that. 150 watts AM seems like it's pushing that amp hard enough. I drive it at about 60 watts in for SSB and that gets me about 700 PEP out. 13 watts carrier in gives me ~150 out.

    I don't do FT8, but if I did, I'd run barefoot, or if my radio was QRP, 4 watts into the amp would put the carrier at about 50 watts for digi modes. The amp should be able to do that all day.

    edit: I really like Larry's suggestion of getting Bill Orr's handbook. There's lots of good reading in that tome. Here's the Tee-shirt to go with your degree!

    Last edited: Jul 22, 2018
  10. N5MEP

    N5MEP Ham Member QRZ Page


    If you look closely at the photo, you can see that the tube is in a chimney. This directs the air past the base, over the tube envelope, and finally over the anode connector. Tuned properly the anode probably would be a dull red at 200 watts output.

    Link to 3-500z data: http://frank.pocnet.net/sheets/140/3/3-500Z.pdf

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