The AM Forum

Hello, me again. The Valiant is up and running well, VFO is finally very stable. Still have to work on hooking up a Mic for the AM mode.

I wanted to really "SEE" what this thing was putting out so I bought a Oscope, got it today and did some fiddling. I made yesterday a RF Sampler circuit I saw on Youtube...see 20-40-80M scope shots attached. I see a little flat topping there, my TS-570 was a perfect sine wave...well looked perfect at least.

The finals are really not neutralized, I have tried the method from manual but really not doing that well I think. I would like to hear any suggestions on a easy method of removing the PA Screen and Plate voltages so I can use the Scope to properly neutralize.

Any thoughts on the waveforms? How to setup scope for other tests, neutralize tips would be greatly appreciated.
73
John-N2LK

Hi John,

Have you tried "cold" neutralizing it?   After tuning it up on freq, turn off all power and then inject a signal into the 6146s' cathode pin.  Connect the scope to the loading capacitor hot side before the antenna relay.  Hopefully you can then adjust the neutralizing cap so that the scope signal gets nulled to a sharp dip. (minimum signal)  You may have to manually key a relay to get a complete circuit, I dunno.

I have never done a Valiant this way, but I have done many big rigs with directly heated cathodes (filaments) with good success.


As far as "hot" neutralization, do you see a dip in plate current as maximum power output occurs as normal? Does changing the neut capacitor affect this relationship?

Why do you feel the neutralization is not working well?  A poorly neutralized rig may still work OK on the lower bands. On the higher bands it may become unstable RF-wise. But generally, it will not cause the audio modulation to flat top unless the final really takes off and loses its mind... :-)


Tom, K1JJ

Thanks Tom for the ideas...W8JI comments on neutralization helps me understand your point on the low bands, and frankly I am not too concerned but still, that is the last step in the LOOONG but fun process of bringing it back to life...I will try the cold method and put a close to that chapter.

Being totally new to this level of experimenting, wanted to show you guru's what the signal looks like in case there was anything quite glaring staring me in the face that I dont recognize....frankly I was surprised how well the waveforms looked!!

73
John-N2LK

John,

   On the scope pictures, you might slow the sweep to about 1ms / division. That way you will see any low-level audio amplitude modulation on the carrier. If you see a periodic pattern, try setting the scope trigger to line sync to see if the noise on the carrier is power line related. If it is, the line sync will trigger the pattern such that it is still.

Jim
Wd5JKO

What do you have the final plates loaded to?  Keep in mind that today's line voltage is higher than it was in the 50s so reducing the final current is wise unless you have a variac to drop the supply voltage.

Loading it to the CW settings is pretty rough. 300 mA might be preferable.

Flat topping could also result from a weak driver tube so there are a few things to check. I've discovered that it's important for the 5763 tube to be in good shape.

For neutralization, the Valiant procedure calls for comparing changes in grid current as plate tuning is adjusted. Basically, detuning the plate circuit should never cause grid current to increase and should have minimal effect overall.

Different, but all the voltages can remain on the tubes. See page 15(?) of your manual.

   Yup,

           What N1BCG said.. Looks like a Screen
   Issue to me. Drive too.  2 much something.
   Even Fil. E. 

        A rippling screen/Plate supply will do that
   too. 

   Jim made a gud point as well, slow the sweep
   time and send it some of your favorite Tones.
   Very revealing!

   Keep at it.. You'll get him..


/Dan

Thanks Dan...being a bit of a newbie in this arena can you explain just a bit your 2 comments:

"Even Fil. E."--filament something ???

"feed your favorite tone", I guess feed on Ch. 2 of scope some tones from my signal gen??

Pardon my lack of knowledge, this is truly the beginning of my learning experience.

I have another driver tube, may switch that in and see if the scope looks any different...
Thanks
John-N2LK

So I changed out the driver tube today, readjusted the grid voltage back to -70vdc, readjusted clamper and fired up the rig on 80 meters.

See pics attached...WOW, very pretty sin wave....like my TS-570....

Thanks all for the guidance in this saga.
73
John-N2LK