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Author Topic: how I gained 20 watts output on my Valiant  (Read 369 times)

N4MQ

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how I gained 20 watts output on my Valiant
« on: March 06, 2023, 07:23:49 AM »


The short answer:  I changed the plate current shunt resistor and provided a GOOD solid ground connection to the chassis

The back story:

While beginning the restoration on my second Valiant (#2), I checked the shunt resistor used for reading the plate current and found it about 30% high.  I had stocked 0.2 ohm 1% resistors and swapped it into the chassis and went on to verifying all other components.

In the back of my head I thought I should recheck the new resistor, and it was VERY close to the 0.202 factory value, well good enough.  I also decided to check the shunt resistor to the chassis and NOT just across the resistor – wow what a surprise I got. The reading was about 300 milliohms or 50 % high on the new resistor.  The photos will show the resistor readings and the factory rivet connection and the photo of the better lug ground connection. 

The rivet connection to the chassis had a resistance about 100 milliohms to the chassis.  This is a terrific error due to age and condition causing a POOR connection. This was remedied by using a star washer and large stud mounted ring terminal.  Now the reading is correct, beware of the plate shunt resistor value!!

Readings were taken with my eBay 4 lead milliohm meter which can read to micro-ohms, basically a Kelvin ohm meter.  This is useful for testing relay and switch contacts resistance, resistors, and connections generally.

How I got more power:
I went back to my first restored Valiant (#1), popped the cover off and measured the shunt TO THE CHASSIS and read 0.241 milliohms or about 20% high.  This means that the 330 ma loading on AM was actually only 264 ma, limiting the output power because of low plate current.  Ebay milliohm meters are between 50$ and 100$ for modest models.  After re-tightening of the ground lug on this rig and loading it to 330 ma again, my swr meter now shows output power of about 120 watts.  I never believed the swr meter previously and just assumed that it was reading low.

Also remember to shunt your plate meter with a pair back to back diodes for  meter protection in case the shunt resistor from the plate transformer center tap to ground opens up.  I know, it will pop the meter instantly.

 Woody  N4MQ – on QRZ.
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