Johnson Valiant Expertise Needed

[Rodger]

Hi Bruce,

I would recheck the grounds around your 5763 in case the problem is 
originating there, substitute a different 5763 if you have one.   Double 
check your final grid bias and screen voltages to make sure that those 
are in the ballpark.  You might also try using your 6146 modulators to 
sequentially substitute for your final 6146 tubes in case one of the 
tubes has an odd defect..  As I told you in an earlier email, the only 
instability I found in either of my Valiants was an odd oscillation in 
one (probably TPTG) around 220khz and it was so low in level as to have 
little effect on the plate current.

Another thought, if you still have the original "turnstyle" selectable 
loading caps in yours they may be leaking/shorting as the RF imposed 
upon them increases.  If it is original it may well be the problem.

73, Rodger WQ9E

 It soundshudlerb1 at netzero.com wrote:
> Hi All, just trying to finish up a resto on my very expensive Valiant. Lots of new parts and she's looking good, but have run into some weird problems someone may be able to help with. 
> Loading into a 50 ohm dummy load, the rig seems very unstable and certain settings of the plate control and loading send it into oscillations on 160 mtr or 80 mtr bands. Not so noticeable on 20 or 15. I have done a neutralization adjustment by disconnecting the plate and screen voltages and using a RF probe in the antenna jack, I can null very nicely. 
> At a very low grid drive, the scope on the output shows clean RF, and the controls load smoothly, no erratic dips etc. When you bring the drive upwards to a normal level, all kinds of crap end up on the carrier and the plate dips erratically or not at all....so it has parasitic oscillations or something. I can get 200 W out of it under certain control settings, but never clean dips on the plate meter with maximum output. 
> I'm at a loss why it is so crappy on 160 and 80, while the higher bands seem to be free of oscillations. 
> No loose grounds or hardware, this rig is in nice shape and completely unmolested, so, I'm again at a loss why it is so unstable. 
> Thanks 73, Bruce WA3MKC
>
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