[AMRadio] GK500 OTA almost

Donald Chester k4kyv at charter.net
Fri Jan 6 14:41:24 EST 2017

> I believe you may need to NEUTRALIZE the final.     Any grid driven Class C
> final will need to be neutralized.     Sometimes you can avoid
> re-neutralizing going to new final , but maybe not.

Some grid-driven tetrode and pentode finals can eek by without neutralisation if carefully laid out.  Given the "value engineering" that went into the GK-500, it doesn't surprise me that this may have been needed but wasn't included in the stock design, just like the parasitic suppressors. 

> To neutralize you need to REMOVE BOTH PLATE & SCREEN VOLTAGE.....

Something else, seldom mentioned in the handbooks but which I found from experience, is that with a single-ended neutralised TRIODE final, the +HV lead from the plate supply must be PHYSICALLY DISCONNECTED from the PA.  Just switching off the power supply isn't enough.  If there is DC continuity from the tube's plate to ground through the power supply components, as soon as grid drive is applied, you may see a phantom plate current indication even though there is no +HV on the final. With MV rectifiers, this phantom current may be enough to cause a visible blue glow in the tubes. 

When grid drive is applied to the final, some of the electrons attracted to the grid from the cathode when the grid swings positive at  RF cycle peaks, will miss the grid and continue on to the plate even though it has no positive DC plate voltage.  With DC continuity, these electrons will seek ground, causing a pulsating DC plate current to flow in step with the RF peaks.  This pulsating DC appears as an RF voltage in the tank circuit, and can make accurate neutralisation impossible, since the tank will still show the presence  of RF feed-through even when the neutralising capacitor is set properly. Physically disconnecting the +HV lead will make this problem go away, since the electrons no longer have a path to ground.  I  have not noticed this in the case of a push-pull triode final; evidently the phantom plate currents in each of the two tubes balance each other out. I see the phantom plate current in the 211 driver stage of my HF-300 rig, but none in push-pull final, and it neutralises OK without the lead  disconnected, although I usually do so anyway just to make sure.  I have had  limited experience with screen-grid tubes, so I can't say whether or not this is ever a problem  with  tetrodes and pentodes.
Don k4kyv

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