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 1 
 on: Today at 12:09:33 AM 
Started by KD6VXI - Last post by KD6VXI
Had a question posed to me.

Let's say you have a freakishly large amplifier.  Capable of a LOT of headroom.

And lets say you have two seperate stations.  Two antennas, feedline and exciters.  You have bpf on each radio to protect it from the other upon TX, etc.  Separate  input circuits.  The tube is the common element.

Would it be possible to amplify say a 15 meter and 10 meter signal, using separate pi networks on the output for each?

I've simulcast a second am signal before by amplitude modulating a 60 kHz tone through an analog series modulator, so I know two close in signals can't be generated and amplified.

After seeing the 3cx3000A7 amp post, I'm sure you can deduce where the question is coming from. 

On the top, it seems doable.  It would require a large cabinet for two pi networks, but any ideas?

--Shane
KD6VXI

 2 
 on: Yesterday at 11:26:41 PM 
Started by W2PHL - Last post by Steve - K4HX
Eric: One possibility,

http://www.qrpkits.com/10wamp.html



 3 
 on: Yesterday at 10:43:32 PM 
Started by N2LK - Last post by N1BCG
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.

 4 
 on: Yesterday at 10:39:54 PM 
Started by N2LK - Last post by WD5JKO

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


 5 
 on: Yesterday at 10:21:12 PM 
Started by N2LK - Last post by N2LK
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

 6 
 on: Yesterday at 10:04:39 PM 
Started by WBear2GCR - Last post by WBear2GCR
Yeah, thanks John!
I "knew that" - but...
...anyhow that display makes perfect sense. Cheesy
Worked on the #1 unit today - turned out the switch on the first Atten position was toasty.
It's a KAY, teflon "Mega Switch" (says so on the face of the panel bushing!). The same switch
used in those precision RF attenuators that have a bunch of toggle switches. Just like those units
someone put too much power on the input and smoked a resistor. But the switch showed open, after
a bit more checking and pulling the switch.

So, I drilled the rivets and decided to have a look inside.
neat switch!

It's got a Teflon barrel with gold rings on each end that is permitted to roll from one end of the switch to the other on a gold plated track- then it bangs into the sides where there are some gold plated contacts that go through the wall (teflon).
On this switch the gold contacts were blasted where the contact was made.

So, since it is Teflon, I rotated the contacts in the body to find a clear gold surface and put the switch
back together with 2-56 hardware. Replaced the two dead 1% metal film resistors. 4 hours later... all good!

That's the short version.

Tomorrow we see if it "hears" now - it was stone cold deaf before! HA!

                  _-_-

 7 
 on: Yesterday at 09:43:53 PM 
Started by N2LK - Last post by K1JJ
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


 8 
 on: Yesterday at 09:13:39 PM 
Started by N2LK - Last post by N2LK
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

 9 
 on: Yesterday at 07:40:52 PM 
Started by W4AMV - Last post by N8ETQ

     

         Yo'


      Nothing wrong with the trusty 5408
'cept I hear they are obsolete. 6A10 or 10A10's are
a good sub. if you can take the larger footprint
and lead size.

                I read somewhere years ago that due
to the consistency from unit to unit credited to
improved manufacturing technique's that the R/C
combo across each diode is unnecessary. I have not
bothered with those in years and have not had any
issues either.


                I have also had good experiences
using "Zero Crossing SSR's" for input power to the
supply, Eliminating the need for a soft start setup
when hitting a high uF Cap bank.

GL es 73

/Dan



 10 
 on: Yesterday at 07:35:50 PM 
Started by WBear2GCR - Last post by w3jn
R-392 is a 455 KC IF.  R-388 and 51J-4 are 500KC IFs.

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