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Author Topic: Tuning my L-4B  (Read 1755 times)

Posts: 101

« on: January 29, 2013, 06:09:17 AM »

Picked up the L-4B in an auction.  Tubes light up and running 240 VAC.  Plate voltage with no load using CW is 1900 volts and SSB is 2600 volts.  Plate current is approx .170 amps.  The amp seems to be working normally.  My confusion is about proper tuning procedures.  I also have the MN-2000 tuner but I also have a Daiwa meter between the amp and the tuner.  Given the tuner has a forward power meter as well as the Daiwa, I can pretty much see everything it is kicking out under a load.  Once again, everything looks normal. 

My question is about how to properly tune it.  The manual says apply power to .400 plate current and look for the dip.  Then do the same with load.  When I apply power, I see the out put going up on my meters but the plate amps are showing above the .565 amps.  If I tune the amp to maximize out put I seem to be in the .600 or .700 range.  Tweeking the plate back to .565 reduces my output on the meter.  So my question is simply..when I tune it do I adjust for maximum output while keeping grid current as close as possible to about 220 ma of grid current.  Or, is there a problem with my amp given plate amps when reduced to .565 I cannot achieve my maximum output?? 

I am sort of a newbie with this amp but I have used it on the air with many very good signal reports.  So I know it is working but am I hurting the machine in the process.  Can anyone provide me with some hands on experience as to proper tuning procedures? 


Posts: 173

« Reply #1 on: January 29, 2013, 06:25:28 AM »

I suggest
as reading material.   You might as well learn from an amplifier designer.. regards.. Fred

Posts: 2474

« Reply #2 on: January 29, 2013, 06:49:07 AM »

I have owned a Drake L4B since 1973.  First thing is to remove ALL "tuners" and connect the amp straight to a good dummy load - no exceptions while learning.  There should be nothing in line but your rig and the amp and the dummy load - NO TUNERS!

Select the band of interest on the L4B band switch.  You may as well start out on 40 meters.  Preset the L4B TUNE control in the center of the 40M arc.  Preset the L4B LOAD control at about the 10-11 o'clock position.  Turn your rig to the center of the 40M band - 7200 will do.

Apply around 40 watts or so from your rig.  The amp should "key" and you should see plate current around 400-500 mA.  You should also see significant power out (400-600 watts) on the internal wattmeter in the L4B.  Turn the TUNE control (very slightly) for MAXIMUM watts out on the L4B wattmeter.  Turn the LOAD control (very slightly) for MAXIMUM watts out on the L4B wattmeter.

Apply up to 100 watts to the L4B.  Turn the L4B TUNE control for MAXIMUM watts out.  This will be a VERY SLIGHT adjustment.  Ditto for the L4B LOAD control.

If everything is OK you should see around 1200 watts out on the built in L4B wattmeter with the HV switch in the SSB position.  Plate current will be around 800 mA and grid current will be around 200 mA.

Dick  AD4U

« Last Edit: January 29, 2013, 06:51:34 AM by AD4U » Logged
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