The AM Forum

Neutralization.  I need a good lesson.  What brings this on?  10 meters was open today.  I made some fine contacts on my Kenwood TS-570 on AM up there.   But when I tried to tune my Heathkit DX-60 up there on 29.000, some strange things were happening. 

My first guess is maybe this 6146 needs some taming.  Actually it's an 8298 (ruggedized version of the 6146) that I recently put into the rig.  Works FB on 80/40/20 but when I tried to put it on 10 meter (via the HG-10B vfo), the grid current was very shakey!  The needle on the meter was merrily dancing around.  Is this indictative of the need for neutralization or maybe I have some other issue?  I sure would love to get the rig up on 10 meters.

73 John, KX5JT

Try the DX-60 with a crystal first to determine if it's the rig or the VFO. If the tube is a 8298A, which is equivalent to a 6146B, I would try a 6146 or 6146A. Some of the Heath transmitters/transceivers that were originally fitted with 6146's were unstable on the higher bands when 6146B's were substituted.

I agree, Swap the tube.  I had the same problem with my Ranger. It likes the old RCA or Amperex tubes. When I used the B's It would not load on the high bands.

C

It's a Motorola 8298 which is the 6146A equivalent... I have a few 6146's most original or A versions, one is a B and I don't even want to try that one...  Okay.. I'll try some of the others.  Hmmm... let's see... I have a few 40 meter xtals..  I suppose I can throw a 7.290 xtal in and should put me on 29.160...  I'll check those things out. 

Thx, John

dont get a nose bleed up on the ultra- highs, John.  :D

The xtal works so apparently the vfo is not stable.  Bummer.  I was hoping to be able to move around.  Now I need to figure out why the vfo wouldn't be stable.

check teh caps, then check your moving bearing  surfaces on the main tuning cap. bet you'll find they are full of schmaltz. clean em out with your favorite goop blaster and recheck. Also lookit any vr (OA2, OB2) tubes and make sure they got a nice steady glow to them.

get done with all that and check the tubes and you should be gold. if still NG attack he tube sockets and wiggle some rx's around while you listen to he note in another rxer.  nothing much to a vfo. after all NG check your connection cable from vfo to TX.
 
8)  nothing to it, holmes.

Back to the original question.........Does anyone have the explanation of why neutralization is needed and how to do it?....Good subject .......

Why rehash something thats been covered in depth in most every HB ever published?  If you dont have one its high time to spend some cash.

Once you understand the tech stuff then its time to ask rig specific questions as they are often different.

Carl
KM1H

Ok. Glad the tube was not the issue.  Time to dive into the VFO. Tubes, Caps, resistors,  Cleaning and maybe replace the VFO cable?  I had a VF1 that would act up like this and after a went through it and replaced the cable, It had a strong signal.  I wish I new more about Neutralization to help you with that.

Clark

Indeed.  I'm going to do some reading in the West Coast Handbook edition 19 tonight (it's online).  Although I don't believe it's an issue at this point as I did before.  Looks like the VFO needs some TLC instead.  C

Post a link to that please.

C

Oops, sorry it's the 15th Edition Clark.  I have a copy on my site here:

http://kx5jt.net/files/radio_handbook_1959.pdf (http://kx5jt.net/files/radio_handbook_1959.pdf)


I really need a hardcopy of this or another edition.. I much prefer an actual book to a huge pdf file but it'll have to do for now.

Over the years Ive managed to accumulate most of the West Coast and a majority of the ARRL HB's. Also all QST's from 1926, all HRM's and 73, and CQ from 1945 to the early 70's.

Its so much better with the real thing as I can read it in many comfortable places ::)

No matter the tube, how about Neutralizing the thing? Here is a method I like because it is less cut and try. You can easily see what you are doing.

Leave the DX-60 turned off and remove the cover, bottom plate and cage top. Temporarily remove the B+ from the final by disconnecting the HV connection below the plate RF choke or somewhere else in the feed before the choke bypass. Merely pulling the plate cap will not work because the circuit needs to be in place for the adjustment. Do not transmit or key during this test.

Next connect a signal generator set for 29 MHz preferably through a 50 Ohm pad to the SO239 connector. Thats right - directly to the antenna connector. Yes you can do this on a lower frequency like 40M too.

Attach coax to a receiver to the top of the final grid tank or better yet to the plate of the driver tube through a 50 pF cap. Find a nearby ground for the braid.

Tune the RX for 29 MHz and peak the plate tune and load and the grid tune for maximum S-meter. Keep reducing the sig gen output so that you have an S meter reading towards full scale S9 + 20 when everything (tuning, drive and final plate tune and load) is maxed out.

Light the fils - remember no B+ on the final!

Now using a wooden popsicle stick, push the neutralization wire first towards then away from the tube and look for the position that causes a deep null on the S meter. If you need more, add length to the wire and try again.

You are now neutralized. You have balanced the bridge. The capacitance from the wire to the plate now equals the right value (not the same value necessarily because the DX-60 uses a ratio of capacitances) to cancel the plate to grid capacitance which causes the TPTG feedback and all of the nasty oscillations!

Hey thanks Mike!  

That's great info!  I really hope I can find a decent signal generator at one of the hamfests in the next couple months. :)  I don't suppose I could use my kenwood rig to do that, 5 watts would be far too much.

Great info Walt .....Although a lot of us understand neutralization and the capacitance between grid and plate and own many handbooks, It is great to see it rehashed here.........especially particular to the heathkit rig.....Thanks...Steve KL7OF

One could use an oscilloscope here if you prefer.  I do this to neutralize my stuff.  

Who's Walt?   8)

When I built the push pull 812a rig, I put rf drive into the grid, and had a receiver on the output, looking at the S meter, and got it very well neutralized.

I don't think I was ever very lucky with beam power tubes in pie net...

Brett

I do it slightly different.

Unplug the power cord so you dont make a mistake.

Connect the signal generator to the high side of the input tank thru a .001 cap. Hook your receiver, scope, spectrum analyzer, etc to the antenna connector.

Tune the input and output for maximum signal tranfer

Adjust the neutralizing wire or capacitor for minimum signal.

This also works fine for amplifiers, simply hook the sig gen directly to the input connector.

Interelectrode C is there with and without filaments or DC. Ive measured no difference in the neutralizing wire or cap position in amps thru 6M with all power off or with it running full output and the SA taken off thru a directional coupler and attenuators.  In some cases there may be a slight difference but the neutralization doesnt have to be perfect to be effective. And its a lot safer with no HV involved, especially with amps.

Carl
KM1H

I like the "no HV involved" method that Carl uses...I've used a grid dip oscillator on  BC rigs.

Mike's alterego?  ::)

Thats the kind of info I needed. Thanks alot for taking the time to post your meathods.


C

could be the tube itself. Somewhere on this board there was a large page of information about various manufactures having problems with a lot of replacement types for the 6146. It could have been off this site? Maybe someone can recall the the information that I'm referring too. 

It became a very big deal with the mobile applications of the tube variant.

I think W2JBL may have some knowledge about this problem.

Two of us had that problem with the tube. We suggested it, He said that was not the problem.

C

Whether you do it forward or backward, the technique works because it is a bridge circuit. Just make sure that you have 50 Ohms across the output connector if you use an RX or Scope on the output, and do it forward.

I like the backward method because the generator is naturally Low Z and a scope or lightly coupled receiver is naturally high Z. So I hook the gen to the output and scope to the grid.

The "generator" can be a single 2n3904 transistor oscillator with a crystal for the purposes of this test. Link couple off the tank into a 100 ohm pot and attach the tap to a pad to a SO239 connector to set your level.  This will give you a variable output with a 50 Ohm impedance. Here is a simple circuit. Got any crystals?

Mike WU2D

Most vintage service grade signal generators are a completely unknown output impedance and with most attempting to offer some sort of match into 300 Ohm and higher receivers. The impedance varies widely (wildly with some into the 1000 Ohm + range) with frequency.  With those driving forward into the amp minimizes or eliminates the issue.

Carl
KM1H

 

Well I gave my VFO a good cleaning and used some contact cleaner on various areas.  It's much more stable up on 10 meters now.  My grid current behaves.  The DX-60 loads a whole differently up there and of to get clean audio and a nice envelop I have to sacrifice a lot of power.  So a few watts into the SB-200 which also suffers quite a bit on gain up on 10 meters yeilds about 35 to 45 watts of clean resting carrier with good swing to about 150 watts p.e.p.  This isn't too bad actually because when 10 is open it doesn't take much to put out a respectable signal.  At least I now have the advantage of the modified DX-60 audio and bandwidth.  The TS-570 sounds good but it is 2.4Khz TX (on ssb) so that's a pretty restricted 4.8 khz on AM.  It's very high quality communications audio but the DX-60 is closer to 6.5 or 7 Khz before rolling off.  Much more pleasant to copy.

I appreciate the tips and I will still seek an rf signal generator for the bench at the Orange, TX and Rayne, LA hamfests coming a few weeks.

73 from the marsh praries of South Louisiana
John KX5JT

Good job.. Sounds like you got it going. Sounds like my VF1. It just needed some cleaning and a new cord. 

C