Viking Valiant-- Low Power Output

[w2hld]

I referbed a Valiant and was using on 40 and 75 meters for a few months. Power output was about 130 watts.. somewhat lower than expected but still ok. That power began to drop and it was last at 120 watts. I developed a problem with the HV and after getting that corrected, noticed that my output could only reach 100 watts at best...often lower.
The RF bias is proper at - 70 volts and the plate voltage is ok at about 630 vdc. The 6146 screens are a bit low but acceptable. The Valiant loads fine, and I have all the drive I need and keep the grid at 8 ma. So everything going in to the final is proper... however, not the output. Any thoughts on the reason for the "missing output power"?

[Pete, WA2CWA]

How does it perform on 20, 15, 10 meters?
What's your HV?
Check your Clamp circuitry.
Check or replace output tubes.
One or more mica caps in the output could be breaking down.
Testing on a dummy load?

[WQ9E]

The original plate meter shunt in a kit built Valiant is also suspect so you might want to measure current draw (at the cold end of the plate choke) to confirm you are actually loading to as much current as you think.

As Pete said make sure the clamp circuit isn't pulling the screens down.  I generally set by loading to full power with rated drive and then adjust the clamp control to the point it just starts to pull down the power and back off from that a tad.

If this is an early Valiant it will have a single 500 pf plate coupling cap, this is too small for 160 and the low end of 80 meters.  Later units used a pair of 1,000 or 1,200 pf units in parallel.

[w2hld]

Thanks for the comments and suggestions. I suspect the clamper is not adjusted properly. I made this adjustment in following with the setting outlined in the manual. I measured the screen voltage and its about 40 volts off of the value expected. I'll skip the manual and go with the suggestions here. To answer some of the questions... I'm connected to a resistive dummy load through a Bird meter to measure the output power. I really don't think its the 6146's but I plan on replacing anyway since I don't know the age of the tubes and when I checked on a transconductance checker some were usable but marginal. I don't operate 160 so the coupling cap really is of no interest to me... but I understand the need to change it out for operation on this band... a very expensive capacitor! I'll make the adjustment to the clamper later today and report back. Thanks again for you help.
Mike, w2hld

[w2hld]

Roger gets the golden banana! It looks like I've been fooled into believing that I was at the rated current level for the 150 watt output. It appears that the meter reading on the Valiant has been incorrect and when it was indicating 350 ma the actual, measured,Ip was144 ma. The input power at a plate voltage of 638 vdc calculated as 91.8 watts. Using a 75 percent efficiency, the Bird on the output should read about 69 watts... and it did.
Now, I need to replace the shunt on the meter and my diagram is not very clear. It looks like the shunt is R58 and the value is 0.202 ohms. Is this the proper shunt resistor for the plate current measurement? I've looked for it and have not yet found it in the rig. I can get 0.2 ohms easily from a local distributor. This value should be ok for the application. Any other approach would be appreciated.
Thanks again to all for your assistance with this problem.

73
Mike

[WQ9E]

Mike,

You have identified the correct shunt and .2 is plenty close, the Valiant meter movement itself isn't lab grade so two thousandths of an ohm for the shunt doesn't matter.

These shunts were made from nichrome wire and will usually be installed as a little U shaped wire loop, sometimes the builder put a piece of insulating tubing over it.  As I recall this one is installed on a terminal strip but just trace the wiring from the center tap of T1 and you will find it.  The location will also be identified in the construction manual if you can't find the shunt easily, I expect that there is a copy on edebris.

[w2hld]

Thank you again, Roger... you have been of enormous help with this mystery.

73
Mike, w2hld

[WQ9E]

Mike,

You are very welcome!

While you are in there you might want to replace the shunt used for the modulator.  It is likely your Valiant was kit built (it seems like a huge number including both of my Valiants were kits) and the builder had to precisely cut a very short piece of wire and tin it to end up with the precise length and many are off.  Fortunately most people cut them long so the error leads to reduced instead of damaging excessive power.

If you have an early production Valiant carefully examine the wiring from the low voltage transformer to the 866A filamentary cathodes.  In the early production regular hookup wire was used and it breaks down with the voltage over time resulting in shorts and ultimately a dead LV transformer.  If the wires used for this aren't clearly insulated for higher voltage then either replace or sleeve them.

Enjoy the Valiant, I had one as a novice back in 1975 so it remains a sentimental favorite

[w2hld]

Interesting in that the shunt used for the modulator meter reading is exactly 2X that of the final IP shunt... .404 ohms. I bought a half dozen of the 0.20 ohm resistors, 3 watt, and figured I could put two in series for the modulator.
These are 1% units and at the high end would put me right on at .202 ohms.
You are correct about the 2.6 volt filament wires to the 866's. I had replaced the 866's with solid state units and left the filament connection back to the low voltage transformer in tact. Big mistake. The cloth covered wire cracked open and, yes, shorted on and off to ground. Took me a while to find it and it was at this time that I disconnected the 866 filament feed from the LV transformer. I then noticed the problem with the final not producing the proper output based on the meter readings.
By the way, I've forgotten more that I remember about this stuff... you saved me a great deal of time looking for the "resistor" for the final IP shunt.
I had forgotten that for the low value of resistance a "high resistance wire" was use "back in those days" Thanks to you, we found the black covered wire at the terminal strip that terminated the center tap of the HV winding.
I could have spent months looking for that shunt
Sure do appreciate your help, and perhaps I can return the favor.

Mike

[W8VG]

Those meter shunts are "pesky" little creatures....had two Valiants and had to replace em in both.  First time around, the mod tranney started talking back.  Took me a while to realize wasn't really fully loaded  .  Also had hard time wondering what that little curly cue wire jumper was doing where the shunt resistor was supposed to be! 

Have fun with the "Val".....you'll learn alot from getting one of those going......and they can be made to sound nice too. 

[Jim, W5JO]

I hope all the joys of owning a Valiant that Philip had are absent in your life.

[w2hld]

Thanks, Jim... if there were no problems there would be no learning process.
Its like the new rice box radios... turn it on and operate.. but most have no idea of how it works!! So while these problems are somewhat annoying, there is a great deal of satisfaction in making it play.

Mike

[WQ9E]

Mike,

I sent you an email file with some notes on the Valiant I put together after working on a few of them.  Some of them may prove helpful as you go through your transmitter.

[w2hld]

Roger: I received the document and read it through. Very good summary, and I have incorporated many of your suggestions in both my current Valiant and several Rangers. I found out the hard way about the VFO shaft.. had to rebuild the coupler on a few Rangers.. quite a pain! I've replaced the drive control with a source follow mosfet circuit that takes the pot out of the "bleeder" activity and now runs cool to the touch I've also use thermistors to limit the surge current... and this also works well to limit the inrush current on Astron power supplies... stops them from jumping off the desk upon turn-on.
Thanks again for sharing all your experience with this great transmitter.
Ah, one last thing... I built and used a Viking II in 1959 as my first AM transmitter when I received my General ticket. You're right... its the best!

73
Mike

[W3GMS]

Mike,

I sent you an email file with some notes on the Valiant I put together after working on a few of them.  Some of them may prove helpful as you go through your transmitter.

Rodger,

I would appreciate that email as well when you have a chance. 

josephfell@verizon.net

Thanks!
Joe, GMS

[Pete, WA2CWA]

Why not just post the information in the AM Handbook Section under Transmitters for all present and future Valiant owners to benefit.

[ka4koe]

Tremendous amount of deja vue going on here. This all sounds so familiar.