The AM Forum

    I have a Valiant that has grid current on the finals, plate current close to 250 ma with a dip, new 6146 finals and all other tubes test good, but only 10 watts or so output.  I have
checked all connections in the output tank, the 500uuf blocking cap and still nothing above 10 watts.  Come on guys, what am I missing here? 
   The last 3 days have been at the hair pulling
level and I don't have much left, any ideas for solving this problem with the Valiant?  Bias has been checked and is correct and all other voltages appear to be normal.

I’m not real familiar with these, but the 6146 has a screen grid. If something is pulling that near ground, you’ll get nothing out. It likely is fed thru a dropping resistor and has a bypass cap after that/at the socket. Check that cap and see if it’s shorted or sorta shorted...

Ed

How deep is the dip? If there is a normal dip, that means that the loading is ok. Please check your output meter at an other transmitter, this is weird

Thanks for the tip on looking at the screens.  The only thing I can think of is there was a tiny metal shard that was causing a short.
When I moved a wire to get to the screen connection, the darn thing started working correctly.  Checked for cold solder joints and found none.  Miracles do still happen I guess.

Hmmm....  suspicious indeed.  So you're saying the only thing that changed is that the full power came out.. with no other meter changes?

No matter what the screen was doing, if you were seeing 250 mA in the plate with a normal dip, there should have been 100+ watts into the load.  The screen just goes along for the ride and simply makes the tube easier to drive.

I'll bet there is still an intermitant in there that will rear its head again, probably in the tank circuit/ relay/ load after the tube.   The Twilight Zone question is: If only 10 watts were coming out at 250 mA, in what dimension was the remaining 90+ watts of heat ending up?

OR, could it be that the fixed grid bias voltage was intermittent, causing the tube to idle at 250 ma with just enuff drive to produce 10 watts out?  (Tubes showing color)

Or, is the screen clamper involved here?

T

One thing for sure, it’s not going back in the cabinet for a while just in case!

usually loading cap issues. Those fixed ones you switch in.
Try bypassing them and strap on a 1000 pf. variable just for a test.
Don't overlook shorted RF Choke on the output. (Cooked)
Got to be in that tank circuit if you have normal drive & voltages on the plate & screen.
KC

This thing has gotten to be like a person with multiple illnesses.  Every time I find one problem and repair it, another one pops up.

I once had a Valiant on the bench that had two intermittent components:

Once was the R13 clamp tube potentiometer which was dirty and the other was a faulty C37 which would open then heal. Cleaning R13 and adjusting the screen voltage for about 175 volts cleared up one problem, but C37 had to be replaced.


Phil - AC0OB

Remember those meter shunts used in those things? That's a common problem. Very inaccurate meters.
Trying to think of what we missed.
That loading switch, bad contacts and bad caps.
The clamper tube. Is that doing it's thing.
Is the bandswitch messed up? 80 going in, 40 coming out?
Some of those were kits. (Non riveted tube sockets) so maybe final tank screwed up?
It's possible it never ran from day one. If you were a local I would suggest several people get together and just try every stupid thing you can think of.
KC

Still thinking.
Look around the driver tube. Check all caps. Screen bypass cap went bad in one I had.
And make sure the band switch on the driver output is correct. 80 meter tap selected on that one when 80 meter tap is selected on the final.
And look at that bypass cap at the bottom of the input coil needs to be working.
Not much else there. But guessing input and output coils may not be synced.
KC

Don't put the Valiant back in the cabinet = everything will be O.K

Had a nice 30 minute round table with the valiant yesterday, 130 watts out, great audio reports.  I turned it off for a short time and came back to no output just as before.  This thing is driving me nuts. 

Roy, check every solder joint and tube socket pins in the RF section, kit or no, I have see bad joints in factory built units.  They contracted with outside individuals to build the things and mistakes were made.

In the interest of maybe helping someone else in a similar situation, I will reveal the cause of the dead Valiant.  It was a dead Bird, not the feathered kind but a 43 species.  After a lot
of searching everything that could cause me to lose output, just on a fluke I switched to another Bird 43 and viola, 130 watts.  I didn't realize a Bird would go bad.  I learned a lot in the
process of modifying the Valiant.  Thanks everyone for the suggestions for curing the problem.

Roy,

    That was an unexpected problem!  If you had an RF sample to a scope, then that pattern of X numbers of CM deflection would not have changed when the Byrd flew the coup.

    As to the Byrd, the symptoms you describe are common at my QTH with my Byrd watt meter.
The problem here is the calibration pot in the Slug element gets a dirty spot over time. I've had to pop off the slug cover, and inspect the pot. First make a mark to approximate the adjustment slot position, and then with a tweaker, cycle the pot back and forth end to end several times. Then reposition pot back as close as you can to the mark made earlier. Viola....The Byrd stays home!

Jim
Wd5JKO

I had a Bird 250W HF slug go intermittent also.  A hair dryer played against the top of the slug enabled the metallic label to be carefully peeled off with a razor blade.  A ten turn pot was seen, accessible via an existing round channel in the slug.  A squirt of contact cleaner and working the pot back and forth restored the function.  Afterwards, the label went back in place without a problem.

73,
Ed

I have a Valiant on the bench, that yesterday, I couldn't get ANY power out of.   I found that you have to hook the Watt meter up to the "Proper"  RF connector to get the needle to move.

Corrective action:

New labels with BIG letters.
(https://photos.smugmug.com/EF-JOHNSON/Valiant/i-mg5TLBw/0/ee88d958/L/P4170004-L.jpg)

M,

I've done 'stuff' like that before. I don't think that I want to undergo the anguish of the Valient.


Why didn't you put your call on the back?  I see that your name is there......

KLC

Haha, I did that once on my upgraded Apache.  ;D

Didja know that the SSB exciter input connector does not give you any RF out?  ::)


Phil- AC0OB

You know....
Every time I set up a transmitter I put a couple of turns of 18 gauge, maybe 1" in diameter near the final tank and route it to a BNC jack. That is a very handy thing to have for hooking the scope to and monitoring your signal with.

No comment on the meter......
I would never do that.
:)

I knew the Valiant could be used as a linear, same as a B&W 5100B, and maybe a couple others of the day, but just about all transmitters just had one PL-259 on the back and the first one I saw got the cable. ;)    

Kinda like your keys, why would you keep looking when ya already found'em?

Luckily nothing seems to have suffered unduly because of my error, glad for that.