Valiant Neutralization

[ka4koe]

I can't seem to neutralize my Valiant. Replaced the bad B+ choke caps. Extremely high Ip corrected.

I have the neutralizing capacitor fully meshed and the grid current increases with an increase in plate loading and vice versa. The manual tells me this means the neutralizing capacity is too small, and that more capacitance is needed. Tried swapping 6146 finals with pair of 6146 modulators (straight 6146s, no suffixes) and no success. Clamper and grid bias voltage set per manual at 10 mA and -70V respectively.

Any suggestions? Something is not correct. To get a decent level of plate current on 80m with good output, for instance, the coarse loading is at zero. Similar issue on 40m. 160m and 20m seem to be okay with respect to plate current vs. output power ~ 140W.

Tried adding some capacitance across the variable L14 (22 PF) but feel it is likely too large. I shouldn't have to do this kludge to get the sucker to work, should I??? Getting reasonable plate but power outPOOT is only around 50w.

Coarse loading caps replaced months ago by snubbers as suggested by WQ9E. Also installed HV doorknob 1000 PF for 160m loading months ago. Only caps not replaced are old paralleled bandswitch caps for 80m.

Philip KA4KOE

[WU2D]

The neutralization circuit is a bridge. the method that they propose is more of a check, not really very useful for setting the neutralization for the best null or troubleshooting.

With the unit unplugged and the bottom cover off. Inject a 14.25 MHz weak CW signal into the output connector from a 50 Ohm generator with a short piece of coax. Attach a small (10 pf) coupling cap to the top of the driver tank and run clip leaded coax from it (braid grounded to chassis)to the input of your R-390A tuned to 14.25 MHz. Peak the driver and output tank. Reduce the generator until you are at 3/4 full scale on the signal meter. Now play with the neutralization cap and see if it is nulling at all. Now you can see what you are doing. A scope will work too rather than the receiver.   

[N2DTS]

Also keep in mind, there are 6146's, 6146a's, and 6146b's.

I think its not uncommon to have problems with the b's in old rigs.

[WU2D]

Phillip, How did you make out with the neutralization? We are curious. After cleaning the variables and trimmer and tube sockets, checking the bypass caps and the bias, you really are ready to do the nulling test. 

The Valiant has three 6146 finals in parallel with a .5 - 10 pF neutralizing cap connected directly to the plates after the parasitic suppressors, fed back to the bottom of the driver tank which is bypassed with a 1000 pF disc. This is a (Collins type) Bruene grid neutralization system.

If the set does not null, replace the 1000 pF cap with a 820 pF. This lower value will increase the feedback. At some lower value of this capacitor 6146B's will nicely neutralize. Do not go below 300 pF or so, or you will start to lose drive.   

[ka4koe]

Plain 6146's. Inserted some more C around the neutralizing cap and although it brought plate current down, power out-POOT also went down to about 1/3 maximum. Never could get it neutralized after that either. I've got other issues at work here in the coarse loading part of the tank, eg. bad caps (smoke released) in the coarse loading. The old micas hadn't POOFED when I tried to neutralize it the first time. So, I have to fix THAT first as soon as I get parts.

I keep reminding myself how much fun this is and how much I'm learning about the repair of Demon Hell Spawn Old Transmitters. And believe me, I AM learning.

Keep smilin', since it's only radio.

 

Feeleep

[N2DTS]

Maybe its something to do with 6146's, I can not get my home brew rig to neutralize, and I tried everything.
Not sure I ever got any 6146 rig to neutralize well, while my 4x150 and 4D32 rig do real well.

Many rigs did not bother to neutralize the 6146 final...

[KA2DZT]

Philip,

I agree, the bypass cap at the bottom of the grid coil is too large.  Reduce it to 500pf to get more feedback.  I think I used around 300pf in my rig but I don't use a direct connection off the PA plate.

Leave the neutralizing cap as is and reduce the bypass cap.

Fred