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DIODE IN B+ LINE FROM MOD XFORMER TO PA




 
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Author Topic: DIODE IN B+ LINE FROM MOD XFORMER TO PA  (Read 361 times)
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W2PFY
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« on: June 20, 2018, 08:09:27 PM »

I have seen many examples of a diode in series with the B+ line from the secondary of the mod transformer to the PA tube. It was used in some HF mobile rigs and there is some mention of using a diode in very early BC transmitters back in the 1930's. What is the purpose of this single diode? Does it allow only positive pulses from the audio wave to the PA and at the same time eliminate the possibility of the negative part of the cycle from approaching 100 percent negative? Is the negative part of the cycle eliminated?

I know of circuits using three diodes for negative cycle loading and then there's the three diode keep alive circuit that I believe Steve WA1QIX developed?

Will the single diode application cause distortion? Any thoughts on this will be appreciated. I plan to build the QIX circuit for my BC-610D but will this cheap & dirty single diode do harm or good?

I am not an engineer so you smart fellers out there must have an answer Grin Grin Grin

   
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KD6VXI
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« Reply #1 on: June 20, 2018, 09:42:36 PM »

That diode is there to prevent a collapsing field from destroying the mod xformer.  The inductive kick can be quite large, even on the CB radios....  I believe I measured about 60 volts coming back when I tried removing it.

On xmitters that are full bore TX constantly, it's not an issue.

--Shane
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« Reply #2 on: June 20, 2018, 09:55:07 PM »

I prefer dispensing with that diode and instead use a soft knee circuit to damping the ringing.

A soft knee circuit such as this also "loads" the transformer on negative voltage excursions such that the secondary does not see an open circuit.


Phil - AC0OB

* Soft Knee Circuit for AMPHONE.pdf (43.85 KB - downloaded 75 times.)
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