Drake L4-B Resto

Discussion in 'Amateur Radio Amplifiers' started by WB1E, Sep 27, 2017.

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  1. WB1E

    WB1E Ham Member QRZ Page

    So far so good. The soft start works well on 240V & the VOX jack when shorted does key the relay. My home made plug for the the VOX jack works, but the jack itself is quite dirty and may need some additional attention, female spring shot?. Next, some filament current as I have now the chimneys and tubes installed, with the new bottom with foam tape to seal up the under chassis. (no tubes were rocked during installation).
  2. WA2LXB

    WA2LXB XML Subscriber QRZ Page

    -Main DPDT switch rebuilt-contacts were pitted, had carbon traces and overheating discoloration
    -W8ZR mod done by previous owner
    -Tubes & chimneys removed, all socket clips tightened and 3-500ZG pins cleaned with Scotchbrite
    -NOS carbon resistors installed in parasitic chokes
    -Weatherstripping replaced
    -Fan squirrel cage cleaned, motor bearings oiled
    -Added RCA jack in parallel to 2 pin VOX relay
    -Replaced L-4PS bleeder and glitch resistors
    -L-4PS power supply board added by previous owner
    -Cooling fans added to L-4PS
    -Added chassis ground wire between L-4PS and L-4B
    -Added DPDT main disconnect at station
    -CW and SSB plate voltages good
    -100W in, 820 watts out CW on 80M to dummy. Equal blush on tubes. No plasma. More testing to come

    Has anyone done both the W8ZR mod and the soft start mod?
    WB1E and WA7PRC like this.
  3. WB1E

    WB1E Ham Member QRZ Page

    Everything so far looks good. All the connections are fresh on the HV path. I need to check on the band switch one last time. I thing that I saw no issues there, but that was during the first 5 minutes of ownership. This is the very first time she has been buttoned up and possibly ready for some HV. Put a couple of hours on the tubes tonight running the filaments. Check on that bandswitch tomorrow an who knows. May have some linear operation

    No HV yet. Haven't hooked the input jumpers in the PS yet, mainly because the fact that this is an older XMF, windings should be warmed up and the moisture (if any) be driven off concerns me.
    I've heard of energizing the secondary to the point of slight heating to drive away moisture. Or the heat lamp trick. This was 6 months ago stored in a dry garage, and inside my house since. Perhaps a simple hi pot check, or something much easier- a Mhos test. Test the conductivity. From each winding to ground and to each other. This XMF performs only a single duty, HV. So can't warn it up using any other windings. OH and how do yall like those DIY anode heatsinks? Took me 30 minutes to make those.


    Attached Files:

  4. WB1E

    WB1E Ham Member QRZ Page

    It's alive!

    Just low plate current. HV=2.8kV and only ~300 mA dipped, going into a perfect matched dummy load. 300 watts on AM & 425w PEP on SSB. On 40m both tests. Did have the grid up to 220mA in attempt to follow Drake's tuning procedure. The numbers are more in "High Voltage" as noted. Driving with no ALC cable connected, with the pot to CW gives about 20% more output, at 100w ssb & 30 AM. The exciter did tune when prompted. I'm using a Bird system analyzer & the internal power meter inside the Amp which shows about 20% more than the Bird.

    Need to measure the HV DC? Getting good dipping using both load & tune peaked. Ideas? New tubes from RF parts.

    N4FZ likes this.
  5. WB1E

    WB1E Ham Member QRZ Page

    One thing I have noticed is that the LOAD is peaked around 0 or 0.2 against the hard stop. The 4 variable output caps are fully meshed and wanting more. There is a RFC is parallel. Good place to start looking.

    On 80m now with the same output: 460w PEP, Ig: 100mA / Ia: 300mA /HV: 2.6kV keyed. Input filters are reading 1:1 and the output is 1:1.

    More soon,
    N4FZ likes this.
  6. WA2LXB

    WA2LXB XML Subscriber QRZ Page

    Follow the L-4B manual closely and make sure that you're using a true PEP meter for your SSB measurements.

    Key down with switch in CW/Tune and ~1.0 SWR to dummy and 75 watts input from exciter should give you 1000 watts DC plate input on CW and AM. 100 watts exciter input should give you 2000 watts PEP plate input with a sharp whistle into the mic. Make sure you always have a 50 ohm load on the L-4B RF in and out. The XCVR is the RF in load during power testing.

    Consider trying to getter the tubes by tuning per manual for CW and going key down for 30-60 seconds if your dummy will take it. Watch to see if you get the plates (graphite anode) heating up to a dull red, and heating evenly. The gettering material isn't a coating on the glass in the 3-500zs, it's in the anode. You may want to leave your filaments on for a day and try the key down sequence until you see that dull red glow in the lower half of the plates several times. Then try tuning up again to see what happens.

    What's your exciter power into the L-4B?

    Although nobody I know of has a de-barnacleing setup, here's an interesting page: http://www.cpii.com/library.cfm/9#22 Click on this one: http://www.cpii.com/docs/related/22/AB21.pdf

    Here's the procedure I followed from AB2RA:

    Step 1, see if HV is OK with tubes out of amp. No arcing or smoke.

    Step 2, see if tubes light up with HV off. Check air flow.

    Step 3, with dummy load on input and output, try it with tubes in sockets, check for lit filaments, check for no HV arcs, Key ON and see if correct resting plate current and no arcs and sparks. Check for oscillation on all bands and various capacitor settings without the transceiver attached, dummy load on both in and out connectors.

    Step 4 if all above passes test, connect transceiver and low level drive to check amp operates correctly at low power and tunes properly on all bands.

    Step 5 run carrier key down just enough to start to get a blush on the plates. If OK, run it for a period of time long enough to getter the tube. Not sure what that time interval or plate current for a 3-500Z is.
    WB1E likes this.
  7. WB1E

    WB1E Ham Member QRZ Page

    Thanks Hugh!

    Wattmeter is a Bird System Analyst. The exciter is a FT2000D w/ autotuner, > then a ~MFJ wattmeter and autotuner combo, >then the Drake L4B.

    To describe power measurements, I will use "watts (Avg)" or "watts (PEP)" to denote the SSB or AM/CW operation. I believe the FT2000D puts out 100 watts PEP on SSB and 30 watts Avg for AM, CW not sure, but it is more than AM.

    I have run the filaments for a couple of hours so far with these 2 new 3-500 tubes purchased after discussion with Merit at RF Parts. The fan works well with a new impeller blowing a good amount of heat.

    Po for input from exciter- I've seen 500 w (avg) so far. Not sure if it was in AM or CW during an autotune function (CW). Have tested with similar results on 40m & 80m.

    All tests are with 1:1 on input and 1:1 on the output.

    Talked with it on SSB last night. 460 watts was all I could get (HV on high) at 2.8kV (2.6kV keyed)

    One weird thing is that the LOAD control wants to have more mesh, or capacitance as it will want to peak below the hard stop. Some times just a hair above, but I have seen that the plate & load do walk together a little bit. But with more tube "seasoning", hopefully this adjustment will behave normally.

    Care and seeding, Bob Orr, always great stuff!

    May have a look at the input filters. I noticed that on 40m, the dip was inside the 80m arc & on 80m, inside the 40 arc (plate adjustment).

    Hugh, I understand everything Step 1 thru 5.

    Roger wilco, tonight

  8. WA2LXB

    WA2LXB XML Subscriber QRZ Page


    Your 2.8 no load SSB plate voltage looks OK per page 22 of this link: http://www.wb4hfn.com/DRAKE/DrakeManuals/L4B/Manual_L4B.htm

    Check page 24 of the same link for knob pointer alignment on the Plate control. When the Plate capacitor plates (C46) are fully meshed the red vernier pointer should be resting close to 9 o'clock. Then try to tune the L-4B to 1000 watts CW into a dummy @21.225MHz and push the red needle to the 15 meter segment of the arc.

    I'm going to talk at my Elmer this week and ask her if I can use my antenna analyzer through a cold-iron, unplugged & discharged L-4B to tune the circuit and calibrate the Plate knob.

    It's all fun...congrats on getting the old girl back on on her feet!

  9. WB1E

    WB1E Ham Member QRZ Page

    Getting there....saw 8oo+ watts PEP on ssb, and also did the 1kW 15m plate pointer cal job. Took me awhile to figure exactly what Drake manual writers were trying to convey. It's like this, with C46 fully meshed, tune up for 1kW on 15m. My pointer was on 80m? So anyway what you're supposed to do now is hold the knob and then move the red pointer with your other hand's finger to the 15m arc on the front panel. OK, so now it points to 15m when you tune up on 15m. Need to see what happens on the other bands. Normally going into a manual tuner, this could ba an issue, I'll see tonight.

    My cantenna is starting to get very, very warm. AND it's easy to over drive this amp in CW. AM & SSB not so much. Did get a dull red glow in CW at 0.6 amps Ia. Loading and watching the grid is something new to me. Keeping Ig at 220mA and dipping the plate current, going back to the exciter and doing an autotune, and back to the L4B, dip and load 3 times because they want to "walk" a little bit, say 25% Po at least.

    The cat hair is burning off with a lesser amount of fizzle noises Hi. So I'm becoming more comfortable with this Ole Caddy. I have to go now and make some plates glow a little more on the air.


    73 all
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  10. W1QJ

    W1QJ Ham Member QRZ Page

    In the SSB mode (higher plate voltage) and 100 watts drive and properly loaded into a dummy load with good tubes one should see 1200 to 1300 watts PEP on a good peak reading watt meter. Even the carrier should be over a KW output.
    N4FZ likes this.

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