[Central-Electronics] 600L Problem Diagnosis

[Laurence M. Szendrei]

Hi Barry,

If you have a Vector Network Analyzer (these days lots of folks own the 
quite affordable nanoVNA), you can do a simple single-port measurement 
indicating how well the couplers are matching the plate circuit to 50 
ohms at the amplifier output. Since the VNA ports are 50 ohms, you would 
connect the S11 port to the 600L RF output connector, and temporarily 
connect a load resistor (small 1/4W or 1/2W carbon comp or metal film 
are fine) from the plate of the tube to ground. THE AMPLIFIER IS NOT 
POWERED UP FOR THIS TEST - IT IS STONE-COLD, AND UNPLUGGED FROM THE 
MAINS FOR SAFETY's SAKE. ALSO MAKE SURE PLATE SUPPLY FILTER CAPACITORS 
ARE COMPLETELY DISCHARGED. The value of the resistor should be the 
specified plate voltage divided by [the specified peak plate current X 
1.6]. The factor of 1.6 is approximately correct for class B/AB2 
operation, and should be close enough to reality for this test. Sweep 
the VNA over the appropriate frequency range for all the couplers you 
want to look at, selected via the band switch on the 600L. You can then 
look at the resulting graphs of return loss or VSWR (which are different 
ways of looking at the reflected power), and a Smith chart which will 
show the complex impedance over the sweep. The ideal result would be a 
VSWR of 1:1, and return loss at the noise floor, over the frequency 
range of interest, indicating a match from the PA plate circuit to 50 
ohms resistive (and I guarantee you won't have that in the real world). 
You can compare results for the couplers on the bands which are behaving 
themselves with the couplers for the bands which aren't.

Make sure you remove the load resistor and VNA from the 600L before 
powering it up, or annoying things will happen! ;-)

I haven't done this with my two 100V transmitters yet, although it's on 
my bucket list. But I have used this procedure with a homebrew 
high-power class C amplifier with a pi-network output to find the 
optimum settings for the input and output capacitors, and the inductor. 
For class C the fudge factor is 2 rather than 1.6.

73,
-Larry/NE1S

On 8/17/26 9:33 PM, Barry Graves via Central-Electronics wrote:
> New member here.  I'm a CE fan and have a couple of 20As, 10Bs, slicers, MM, and most recently a 200V and 600L.
> Recently a problem cropped up with the 600L on 80M band.  The amp has low output and more alarming, the grid current pegs on key up and the reflected power deflects the meter to the left (both conditions on the 600L meter).  A quick test on 40M was perfectly normal so this seems to be limited to 80M and not a meter problem.  I'm hoping this is not an output coupler problem.  Looking for advice on what to check.  Is there a test method and expected measurement guide for the broadband output couplers for example?Cheers,BarryKJ5GQM