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Drake L4B tuning

Discussion in 'General Technical Questions and Answers' started by PA3ADU, Jul 29, 2020.

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  1. PA3ADU

    PA3ADU XML Subscriber QRZ Page

    picked up a very nice Drake L4B amplifier. Both the PA and PSU are like they left the factory yesterday. I replaced the PSU filter caps and it looks that everything is working perfectly.
    My question is about tuning the PA and the position of the Tune(CW) / SSB switch.
    Tuning according to the manual (CW and the switch in the Tune position), I got the following result:
    I/P - 66 W
    O/P - 666 W
    GRD - 240 mA
    PLT - 620 mA
    HV - 1600 V

    The gain looks right to me (10 dB) as expected from the 3-500Z's. But the grid current is at its maximum. More drive will overdrive the grid current.

    When switch over to the SSB position and tune with a CW signal, I got the following:
    I/P - 46 W
    O/P - 770 W
    GRD - 160 mA
    PLT - 600 mA
    HV - 2300 V

    So we have more gain (12 dB) and less grid and plate current which both looks healthier for the PA to me.

    Why did the Drake engineers wanted us to run the amplifier at the relative low HV (2.5 - 3 KV is more common) and why can't you use the "SSB" position for CW operation while it looks like the amp is less stressed. In my case it looks that tuning the amp in the "SSB" mode position is a better choice then for the "Tune (CW)" mode position.

    The idea is to run the amp at a comfortable low power at the end.

    Where do I go wrong?

    73, Nico PA3ADU
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Of course you can operate CW with the switch in the SSB position; in fact, I'd never use the CW position for anything on that amp (and I've had one, and never did).

    The switch is a "throwback" to the old days of amateur radio regulations in the U.S., which used to be "1000W maximum DC input power to the final stage" and the regulation was unrelated to output power. That law changed a long time ago (but after the L4B was designed) and it's now "1500W PEP output power" on all the HF bands except 30m and 60m, so the reason for the switch to exist went away.

    Your plate voltages sound lower than normal to me. That may be related to lower AC mains voltage (below 240V?) or perhaps the HV metering resistors drifted in value (to be higher) which will make the meter inaccurate.

    If you want to run at the lower voltage (CW) for some reason, to reduce Ig to a safe level, just increase LOADing by turning the LOAD control more clockwise. That will reduce grid current while having little effect on output power. However, a pair of 3-500Zs can handle 240mA grid current fine, so that won't damage the tubes.
  3. PA3ADU

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    Thank you Steve, I now understand that the switch is a legal rather than a technical addition to the PA.
    As I said, the tubes run more efficient and cooler at the higher (SSB) voltage so that will be mu choice.
    Btw, the HV measurement is from the internal meter under loading. Maybe the meter resistor is a little off. I'll check the HV using my HV probe again later but first measurement was that the meter was not far off with no load. Prim. voltage is 230V AC.

    There are some suggestions on the internet to remove the bleeder resistors and rewire the 120V circuit to reduce PSU heat (and power). Maybe the 100K could be doubled or the resistors parallel to the capacitors are maybe sufficient bleeders. Safety wise I can deal with some longer bleeding time and reduce this resistor but I am not sure what the effect is on the HV regulation but believe that the tubes are very forgiving in that aspect.
    Anyway, you comment and experience are welcome.

    73, Nico PA3ADU
  4. WB2WIK

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    Heat shouldn't be a problem: The power supply is in a separate chassis and enclosure and it breathes pretty well.

    I think the big bleeders total 100K, so at 2500V they'd be dissipating 62W or so, and they're two 50W resistors in series = 100W. I wouldn't be tempted to change them.

    If your HV readings were "key down" and not idling, they're probably right. Sounds like the amp is working fine.

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