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Author Topic: FT-902dm neutralising 6146b's using Kenwood method.  (Read 2857 times)
ZS5WC
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« on: April 10, 2013, 02:46:33 AM »

 Huh I was wondering if you could not use the Kenwood Hybrid method to neutralise the 6146b's in an FT-902dm.
There is no screen grid switch on the Yaesu however,--but what If I reduced the bias to ZERO mA , and then listened on another RX whilst in TX mode for minimum noise, whilst tuning the neutralising cap?..
(If you know the Kenwood method, you will know what I am talking about.)

Would this not do the same thing?.

73 de William
ZS4L
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VE1GA
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« Reply #1 on: April 10, 2013, 05:43:37 PM »

Just for the record I'm not familiar with the FT-902DM or the Kenwood prescribed methods. However here is the method I've preferred to use through the years. It's simple and effective.

1) Set the transmitter to its' highest operating band (10M).
2) Tune up at full power into into a dummy load.
3) While watching plate current dip also monitor power output. (VSWR Bridge in forward position works well).
4) A properly neutralized transmitter will yield maximum power output at plate current dip.
    If this isn't the case, make small adjustments to the neutralizing capacitor until this is realized.

Note: Always use caution as there are high voltages involved.

73 es GL
de:  Leigh, VE1GA

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K1DA
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« Reply #2 on: April 11, 2013, 09:26:38 AM »

Can you kill the heaters on the 6146's and do it that way?
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KE3WD
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« Reply #3 on: April 11, 2013, 12:05:34 PM »

Can you kill the heaters on the 6146's and do it that way?

?


It is the 6146's that require the neutralizing, so that's not going to work.


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KG6YV
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« Reply #4 on: April 11, 2013, 06:17:29 PM »

Just find the screen supply wire and disconnect from both tubes.  That will duplicate the Kenwood SG switch.  YOu can then neutralize without burning up your finals. 

Greg
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KD0REQ
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« Reply #5 on: April 15, 2013, 06:13:43 PM »

you don't want the finals powered for neutralization.  you want to see how it works with the driver only.  see here for the usual process

http://www.w8ji.com/neutralizing__amplifier.htm

bottom of the web page first explains the physics of the 6146 or similar tetrode stage, and then process.
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K1DA
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« Reply #6 on: April 16, 2013, 06:53:42 AM »

The basis of this method is looking at feedthrough from the driver and reducing it to a minimum value.  I used the method with power triodes such as 250 th's all the ime.  ou are adjusting for MAaXIMUM negative feedback.  To say the tubes must be powered shows you don't understand the concept, but that you have all the answers.
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G3RZP
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« Reply #7 on: April 16, 2013, 01:19:45 PM »

It's best to have the heaters alight, as the heat can affect the inter electrode capacitances. The measurement of minimum indicated plate current and max output can be misleading , as usually, it is cathode current that is measured and minimum plate current is maximum screen grid current.
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KE3WD
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« Reply #8 on: April 16, 2013, 02:22:58 PM »

The basis of this method is looking at feedthrough from the driver and reducing it to a minimum value.  I used the method with power triodes such as 250 th's all the ime.  ou are adjusting for MAaXIMUM negative feedback.  To say the tubes must be powered shows you don't understand the concept, but that you have all the answers.

So why must neutralization be performed whenever the output tubes are changed? 

As it states in the 902dm Service Manual. 

TC01 is the neut cap, providing that negative feedback from Plate to Input of the power tubes. 

The interelectrode capacitance can differ from tube to tube set, among a few other parameters, and I fail to see how that can be accounted for if the tubes are not present. 

Then there are the various Sweep Tube arrangements, unlike the venerable 6146, I've seem some of those darn things able to takeoff all on their own. 

Whatever, the Service Manual clearly explains first tuning up at 29MHz, finding the right side of the dip, and then adjusting Neut Cap for lowest Current figure *while the thing is transmitting into a dummy load* - and I have found that to work just fine in quite a few different hybrid rig situations over the years. 

But really, to state that I don't know what negative feedback is, well, either puerile or puzzling, pick one. 


73
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K1DA
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« Reply #9 on: April 17, 2013, 08:09:17 AM »

You really don't get it, do you.  Nobody said to take the tubes OUT.
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G3RZP
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« Reply #10 on: April 17, 2013, 08:50:56 AM »

Variation in capacitance from tube to tube, or differences between same type number sweep tubes from different manufacturers doesn't matter in TV sweep circuits. It can be a really bad problem in RF amplifiers, though.

Personally, I prefer to leave sweep tubes alone...
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K1DA
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« Reply #11 on: April 17, 2013, 09:03:13 AM »

W8JI has ALL of this covered, including "cold neutralising" of beam power type tubes.  I jest LOVE people who read half a post and then "take exception" to what they thought it said.
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KE3WD
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« Reply #12 on: April 17, 2013, 04:33:21 PM »

When it comes to the cal and adjustments, I'll go with the Factory Service Manual instructions unless and until a Service Bulletin is released stating to do otherwise. 

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