The AM Forum

Group, Suspect the plate meter shunt value is off even though it measures close.want to check with external meter, would as I have read inserting it in the plate lead be the place for this . Then if my thinking is correct, multiply by 3? Also can as the manual states, this have 200 watt output in CW ?       thanks, John,Wa2fns

No way!
That is at RF, you could insert the meter after the plate choke and bypass cap if everything can take the 800 volts or so.

A better way is to use a low voltage supply to run current through the circuit and measure that.
I have an old Heathkit adjustable power supply with meters on it and current limiting, and make meter shunts up all the time, it takes under a volt to read 300 ma.

With 3 tubes, you want to be able to measure the current through each one, one could be dead.
I pull 2 out and run one and see what current/output power each tube has.
All three should be close.

General rule of thumb, 50 watts output from one 6146, so three should do 150 watts OUTPUT, a little bit more maybe in CW if the voltage is high enough.

AM is about 700 or 750 volts at 200 ma I think, (2 tubes) 140 watts in, 100 watts out.
For three, its 300 ma, 150 watts out.
 

Your test meter should be connected in series at the cold end of the plate choke (the end with the bypass capacitor).  Make all connections with the Valiant unplugged and discharge the filter caps (bleeder resistors sometimes go open) before starting.

Lower than expected output with early Valiant transmitters on 160 and the low end of 80 is because the plate coupling/DC blocking capacitor is too small.  Originally a single 500 pf unit was used while later production used a pair of 1,000 or 1,200 pf in parallel.

Early units also read incorrectly high about 30 mils and the service note indicated either to change the circuit or load to higher plate current to compensate.  I think the service notes are on edebris but if not PM me and I will send the information to you.

Thanks guys for the info and education.Will check further, am close on CW at about 125 W out, 425 mills per the meter, with fresh tubes.     John

What is the plate voltage?
If 700 its very poor, 700 x .425 is 297.5 watts input.

If the rig was 70% efficient you should get 209 watts out.

Three 6146 tubes have 150 watts of plate dissipation.
278 - 150 is 128 watts out minimum to not exceed the dissipation.

At 600 volts, you should get 178 watts out (70%).

300 watts in and 125 out is very poor....

Haven't checked in some time but will again.Have changed the finals to 6293s, they were less money than 6146s and are as I have read, a direct plug in.No power gain though, just more rugged. ........      John

A healthy Valiant runs 275 watts input on CW and close to 200 out on the low bands, a little less as you go up in frequency.  Kit built Valiant transmitters invariably have meter shunts that are off in value, the builder was expected to end up with a very short piece of tinned resistance wire of exact length net the tinned area and many failed miserably.  Fortunately most are too long which results in the transmitter being run at a safely lower power.

Input and output are of course lower on AM where it is rated at 200 watts input but you should have at least 125 watts output on AM, a bit more on the lower bands with good tubes.

Make sure the tubes are neutralized to avoid problems, I set it roughly on 20 meters and then do the final touch-up on 10 meters.  If you are too far off it is possible for it to take off on 10 meters which is why I do the rough setting at a lower frequency.

Suitable precision shunt replacements are available from Mouser (.2 ohm for final plate current, .4 ohm for modulator plate current). 

Remember the Valiant has quite a bit of voltage at the key terminals on CW, it is current limited to a safe value but it will not feel pleasant.  The grounded side of the key line should be connected to the side of the key with the most exposed metal to reduce your odds of shock. 

I would be careful with the 6293's. I believe the interelectrode capacitance of these tubes might be different from the 6146/6146A tubes originally used in the Valiant. Neutralization might become a nasty problem.

With the values you have I suspect either the shunt is off, tube(s) are weak, or lack of drive, or possibly a voltage problem in screen or plate circuit. Is clamp circuit working properly? The resistors in the old Valiants and Vikings were noted for changing values considerably. Measure resistances IAW resistance chart in manual at the 6146s. Anything more than 10% off investigate.

Here is a test that may help:

With one tube plugged in set grid drive to 3 ma and load to exactly 100 ma plate current. Note power output (it should be close to 50 Watts) with a good Wattmeter. Repeat with the other tubes, note differences in power output. Hold key down for 10 seconds, if output drops that is sign of weak tube.

If all is good then with three tubes in final and  9 ma grid drive and 300 ma plate current you should get slightly less than 150 Watts output. Why? Increased load on power supply will drop plate voltage some.

A good 6146 will produce 50 Watts under above conditions in a Valiant provided nothing else is defective.

The builder made the shunts?
That is crazy and asking for trouble!

Have made my own shunts many times for home brew rigs...

At some point in production they started providing pre-cut shunts but I well remember going back through the Valiant I used in 1975 following the original assembly instructions to cut the shunts to size.  My father supervised a precision measurements lab so I had access to a bridge that made it fairly simple to cut the shunt to the correct low resistance value but that was a piece of gear that wouldn't have been available to most of the builders back in the 1950s.

My RCA TT-5 says they are the same values.  The only difference according to the book is the 6293 is built for rectangular wave pulse modulator (radar)

That's good. Maybe I was thinking of 6293A's of maybe I was just having a bad dream.

I agree as my restored Valiant with 3 fresh 6146A 7.5 mA grid, 300 mA plate puts out 148-150W on my calibrated Bird.