Sudden Rise in Plate Current

[ka4koe]

Was on 1885 KHz last night trying to communicate with Don, around 11:45 PM Eastern, who commented that I was "not that strong" FWIW. SWR was around 1.7/1 on the 40' vertical,  120 watts carrier @ 300 mils plate (approximate), 7.5 mils grid.

Coarse loading around 5 (may have been too high).

Chatting away and plate current pegs suddenly. Immediately stopped transmitting and waited a few seconds. Decreased loading a couple of notches ato get the plate current back to around 300 mils...still 120 watts out, 7.5 mils grid.

Weak tube or did I make it weak? No arcs or sparks or other noises; just the pegging of the meter.

Other Inflammation: 40' vertical, Base loaded with MFJ coil, 9 - 50' radials (need more), and MFJ 994BRT remote tuna.

Philip

[N2DTS]

MFJ tuna on 160 AM?
That is all you have to say....

[WQ9E]

If the plate meter suddenly pegged something changed either inside the transmitter or the antenna system.  It is a bit late now but a good practice is when first installed use a MFJ analyzer or equivalent and log the settings and for the transmitter do the same thing for all bands into a known dummy load (like the ubiquitous Cantenna) and then when something like this happens it is easy to go back and see which one changed.

In general the odds are something happened with your antenna EXCEPT in your case your particular Valiant is cursed so it might be the rig

[ka4koe]

I usually do just that, Rodger. I have a spreadsheet setup. It was late and I forgot! That's my excuse and I'm sticking with it. For various reasons I don't take my eyes off the plate current when I am transmitting.

[VE3LYX]

I had this happen twice recently. I use lamp indicators (instead of  meter across the shunt as I cant really see a meter unless I get real close) so I know instantly when something goes wrong. In my case I was on 80M both times running my HB 811A linear. The first time I forgot to reduce the drive when switching back to the amp. Easily cured. Last Sunday I had it happen again. I was short of time so had to wait till I returned to find out why. I found my tuner (I use a longwire) had rotated on the input or tune side on its own probably due to the cold as it is a ball bearing cap and I had used a foam behind the knob (hidden of course) to provide enuf friction to keep it from self rotating but it was so cool downstairs in the early morning it had allowed the cap to fall over so to speak. After correcting that I also discovered the coax was not tight on the input. Not like me to have that happen. I will have to pay more attention. I have learned that when power levels rise things are very unforgiving and happen in a flash, sometimes literally.
Don

[W1AEX]

Any chance the MFJ Remote Auto-Tuner decided to take off and re-tune while you were transmitting? I used a Palstar AT-Auto (differential tuner) at one time and on 160 meters the Q was so high that if I hit a sibilant peak it would often initiate a re-tune sequence which would really make things exciting here if I was running a few hundred watts of carrier in the AM mode. When I remembered to switch the tuner over to manual mode I could avoid that scenario but I ended up getting rid of it due to its tendency to arc on 160 meters.

Rob W1AEX

[N2DTS]

I generaly rate MFJ stuff as good for 1/2 of what they say, then anything rated for ssb you need to derate to 1/4 of the power on AM at LEAST.

So the 994 is good for 600 watts pep, I rate that as 300 watts pep if you want it to last, and 1/4 of that is 75 watts.

Any bad swr it has to deal with will lower that unless you tune it at low power.

I used to smoke traps and baluns rated for 1KW or more with a DX100 all the time when young and stupid.

[Jim, W5JO]

I generaly rate MFJ stuff as good for 1/2 of what they say, then anything rated for ssb you need to derate to 1/4 of the power on AM at LEAST.

So the 994 is good for 600 watts pep, I rate that as 300 watts pep if you want it to last, and 1/4 of that is 75 watts.

Any bad swr it has to deal with will lower that unless you tune it at low power.

I used to smoke traps and baluns rated for 1KW or more with a DX100 all the time when young and stupid.

He said MFJ Coil not auto tuner.  I have a 998RT and run 1500 watts + SSB and 1100 AM peak through it without any trouble at all.  Have been using it for over a year.  I like the tuner at the tower rather than on the desk.

[W1AEX]

He said MFJ Coil not auto tuner... 

Actually, he did say MFJ auto tuna (tuner):

"...and MFJ 994BRT remote tuna."

[K5IIA]

I was transmitting one night and my plate current did that. After a few other times of current going up one of my padding caps smoked.

So if you can't find what caused it. Maybe check your padding caps in the valiant for heat or any change of value.

73
Brandon

[WA4JK]

Those Padding caps are a week link in the Valiants and they are aged. Find some door knobs 5Kv and replace them.

[Jim, W5JO]

He said MFJ Coil not auto tuner... 

Actually, he did say MFJ auto tuna (tuner):

"...and MFJ 994BRT remote tuna."

He did but he didn't say it burped nor did he say it  was the coil.  Easy to blame a brand rather than trouble shoot.  With all the trouble this transmitter has had, I would look there first, but to each his own.

[WQ9E]

He did but he didn't say it burped nor did he say it  was the coil.  Easy to blame a brand rather than trouble shoot.  With all the trouble this transmitter has had, I would look there first, but to each his own.

Agree completely Jim.  There are so many possible failures that could cause these symptoms in the transmitter, feedline, antenna system that jumping on part of it because it is "brand X" is likely to lead the OP down the wrong path. As posted previously by another the coarse loading caps (and switch) are prime suspects with the Valiant.  But I wouldn't be surprised if it turns out to be a bad coax connector (or connection within).

[N2DTS]

Yes, it could be a lot of things, but running 130 watts of AM on 160 meters into a low power auto tuna is pushing things I think....

[kb2vxa]

Whatever burped it's a good idea to swap out those fixed loading caps just the same. Once upon a time in my shop I had cat fits with them, I had some difficulty finding replacements in the junk box but after a bit of fiddling, problem solved. IMO when they produced the Valiant 2 they should have left well enough alone, the Valiant 1 had a big honkin' air variable that never gave a bit of trouble. Maybe a retrofit if you can find something suitable?

[W2VW]

The Vally-ant one had sucky padder caps too Warren. You are probably thinking of a DX-100 verses DX-100B.

[kb2vxa]

OOPS! You're right, my bad. The really buggy one in the Vally Ant was that silly looking "helicopter cap" for the lack of a better term. The DX-100 is a better transmitter IMO and reminds me of VJZ Irb with his wind chimes in the morning. I saw them at the estate sale, he didn't take very good care of them.

[ka4koe]

Silly me....the aerial decided to lean over on its own at a 45 degree angle. Most of my caps have been replaced. I need to find a 330 PF/10KV doorknob and that will finish my work on the band turret.

BTW, I have a large series coil (approximately 100 uH) at the top of the 994BRT tuna(yes, I said tuner), to take care of the capacitive reactive nasties on 160m.

VSWR didn't change significantly during the little event, but apparently was enuff to incite mischief. I always keep an eye on Ip and VSWR whilst I am transmitting (actually 2 eyes). Why else would my eyes point in different directions?

The TX has had two months now without a failure. Since I've said this on this forum, things are now jinxed and it will break again shortly.

Hey, look at it this way....can anyone think of a better TX to test one's mettle? If I'd gotten a perfectly restored unit, then I would've learned nothing, zilch, nil, other than the proper loading procedure..

I'm sure some folks say...."oh no, Philip has a problem AGAIN!".

PAN

[ka1tdq]

Another quick note on auto-changing plate current can be a sub-par DC blocking capacitor on the plate choke.  I built another dual 3-500 amplifier back in the day and plate current would rise dramatically about 5 seconds after I started transmitting.  I would need to continually dip the plate as I talked to keep power output up.  After I would receive, my next transmission would have the plate capacitor at starting position and I would need to repeat the cycle.

Tim and Steve helped me troubleshoot this one on the air.  I was using a crappy old TV doorknob capacitor which heated up and changed value during TX.  I swapped it out after I turned the power off using a good Soviet style cap and things worked well.

Jon
KA1TDQ

[ka1tdq]

And you can get Russian doorknob capacitors off of eBay for about $10 plus shipping.  I use them in most of my projects, such as tuners, transmitters and amplifiers.  I bought a good vacuum cap too for much cheaper than the price sold here in the states.

Jon
KA1TDQ

[ka4koe]

Sure, but you have to get several!

[WQ9E]

Sure, but you have to get several!

Unless you are planning to stick your tongue into the plate supply in the near future you WILL be able to use a lot of them in the future for repair and building!  They have an excellent shelf life