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3-400Z Homebrew questions

Discussion in 'Amateur Radio Amplifiers' started by AC0LA, Aug 20, 2012.

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  1. AC0LA

    AC0LA Ham Member QRZ Page

    New to QRO and going through many learning experiences with the objective of having gear that I know how to repair while gaining some extra power. The bands I would like to have more power on are 160, 6, and 2 meters.
    Found this unit at a ham fest at a bargain price. The possible issues are. No tuned input. Loading capacitor plate spacing seems tight????? The power supply does not appear to be the original? The power supply uses 2 transformers in parallel. Lack of any interface and power cables between the power supply and the amp. Inexperienced operator confused about alc circuit, radio keying interface, and loading, tuning the amp. No documentation. I also regret the lack of 160 meters.
    The good things I see are many quality components and a quality build. Many hams are of the opinion that exciting an amp like this requires a tuned input so I would like to add one although would like to test things out first. I do have a small variac to bring it up slow.
    Would it be stupid to try adding 160 meters to this amp? I do have a fairly well supplied junkbox with most of the components to build a mono-band amp for 160.
    How can I evaluate the variable loading capacitor?
    What are the problems associated with using coax for the high voltage (B+) cable? Many of the cables I see with BNC connectors appear to be rated for 300 volts.
    Would it make sense to add another band switch for the tuned input. I would have most of the components on hand for that.
    The meters are installed in a shielded box. Is this required in amps?

    Apologies for the sloppy schematic. It is not complete but shows most of the components.
    Any advice or leads to references are appreciated.


    3-400_16m.jpg 3-400Schematic3m.jpg 3-400_17m.jpg 3-400_18m.jpg
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't see any pictures of the power supply. Where is the power supply? I also think the schematic is wrong. Based on your schematic, full B+ (3kV) would appear at the bandswitch and also at the LOAD capacitor. Based on looking at the LOAD capacitor, I'd think that would blow it up.:p Look again and make the schematic corrections. The only transformer I see in the chassis is one (1) which I'd think would be the filament transformer.

    Where's the power supply and what kind of connector does it use? There is a specialty high voltage connector that looks like a BNC but has an extended Teflon dielectric that adds much better insulation, called an "MHV" connector. Is that what the power supply and RF section uses? If so, you could use shielded coaxial cable but it should be PTFE (Teflon) dielectric cable. I've used RG-400/U for this lots of times, it handles 4kV just fine if you use MHV connectors (not BNC or UHF or N, etc). I wouldn't try adding 160m, at least not until I verify the original as-built system works perfectly and doesn't have flaws, bugs, bad components, etc. Making modifications to an unknown can drive you crazy, as you'd never know if the problems were there originally or were created by the modification. Take one step at a time and get it on the air.

    Not really, but "in the old days" most amps were better shielded than new designs are today because TVI was much more of a problem. TVI problems today are very rare since most are on cable or satellite, and even those who aren't are getting most of their reception up on UHF and it's all digital.

    View attachment 101653
  3. AC0LA

    AC0LA Ham Member QRZ Page

    3-400PS_HV27s.jpg 3-400PS_Sche32m.jpg 3-400Schematic3m1.jpg
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Yes, the female connector on the PS is an MHV receptacle. See how the Teflon insulation extends beyond the contacts? That's the clue. They're okay for a few thousand volts.

    I wouldn't use a UHF receptacle for HV, though. A Teflon insulated one might be okay, but the one shown isn't Teflon, it's phenolic or Rexolite. Also use of an antenna connector for HV is a bit dangerous, an accident waiting to happen.

    The PS schematic isn't right. You've shown the HV bridge rectifier in the wrong polarity. The way you show it, it would have two negative outputs and no positive terminal. I'm not wild about transformers in parallel like that, but I think hams have done this and mostly gotten away with it. Your schematic shows two resistors in series with the B+, and filter capacitors to ground between them, and no bleeder resistor(s) for the filter caps. I'd guess something is wrong there.

    Wonder what the other rectifier circuit is for? A 3-400Z doesn't need anything except B+.

    I also would guess you don't show the plate tank circuit quite right in your schematic: The LOAD cap, which you show in series between the tank inductor and the "output" line, should be between the tank inductor and ground, with the "output" line connected to that same point (where the tank inductor and LOAD cap meet and are connected).

    A tuned input would be good for IMD and to keep the load more stable on your transmitter; if you're using a vacuum tube transmitter with a pi-net ouput stage, not very important -- but if you're using a solid state transmitter, a tuned input helps keep the system working better, and also easier to drive. I wouldn't modify it yet -- get it working first to verify all is well, then think about mods.
  5. KM1H

    KM1H Ham Member QRZ Page

    The Tune cap is too small in capacity to reach 80M much less 160 and the spacing is too small for 2600V considering that the PEP RF envelope will be riding a bit higher.
    One thought is to make it a 6M monobander which would remove most of the haywire and a single band input network is simple. It could also be built as a 160 monobander with a toroid for the tank and a bigger Tune cap with a bit of fixed C to pad. The Load cap may be sufficient as is but its also easy to pad.
    The National R-175A plate choke is fine for 160.

    That blower is serious overkill, a muffin fan is more than enough to blow on the tube and a PC CPU type small one down below to cool the filament pins. Lose the chimney. That filament transformer looks big enough for 2 tubes once the blower is gone, check its ratings and be sure its not supplying more than 5.0V with the tube on. Several amps with 1 and 2 3-500Z's use a fan. The 4-250A/4-400A in GG can also be used in that same socket.

  6. AC0LA

    AC0LA Ham Member QRZ Page

    If the magic smoke stays in there during testing I will try it out before any modifications. Smoking a critical component could lead to major revisions.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd stay away from using colors other than black as they're hard to read.

    But anyway, the "second" rectifier shown in your PS schematic can't be used for the blower or filament transformer, as those are AC devices. DC would blow them up. I don't know what kind of relays are in there, but most hams would use 120Vac coil relays or 12Vdc (or thereabouts) coil relays, as they're the most common. But I don't know what he used.

    I'd check that carefully. If the builder used something other than low-voltage (like 12-14V) DC relays for input/output switching, that could be a hazard to whatever rig you connect the amp to, to key it. Even some 12Vdc relays might draw more current than modern transceivers can sink. And if the relay doesn't have a "kickback" diode wired across its coil, that can still damage some rigs' keying circuits. I'd look into that.
  8. KM3F

    KM3F Ham Member QRZ Page

    Respectfully, I hope you will allow me an honest opinion without to much insult.
    From what I see, you better put the project off awhile longer until you learn more about circuits and how to draw and understand them.
    The tube high voltage is to be very respected.
    The 3-400 is almost non obtainable these days if yours is not good.
    A 3-500 will set in the same socket but is taller. That may present a cabinet issue if done.
    Bottom line is your playing with you health if you make a mistake and don't use good construction practice and due reguard when working on the project..
    Good luck.
  9. AG6K

    AG6K QRZ Member QRZ Page

     Adding 160m to an 80m amp is possible if you have enough room to double C-Tune, double C-Load, double the HV-RFC, and double L-tank. Also, for the 160m tuned input, one needs to add enough pF to C-2 to cancel the fil choke's XL at 1.9MHz. Example: A 10uH filament choke virtually disappears at 1.9MHz by adding c. 800pF to C-2 of the 160m tuned input.
    - note- C-Load padder caps for 160m can either be 2kV or 2.5kV Cornell-Dublier micas or paralleled NPO ceramic doorknobs, or paralleled N750 ceramic doorknobs, but they should not be ceramic doorknobs that are intended only for coupling and bypassing applications - but not tuned circuits.
    Last edited: Aug 22, 2012
  10. N2EY

    N2EY Ham Member QRZ Page

    The 1964 ARRL Handbook shows a single-3-400Z amp that bears some resemblance to this one.

    73 de Jim, N2EY
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