The AM Forum

When neutralizing an RF deck, I know that the plate supply needs to be off, but should it also be disconnected? In my reading on this subject, I can't seem to find an answer on exactly what should be done with the connection to the power supply. FWIW, to this point, I have simply disconnected the plate supply lead at the back of the RF deck, but I'm not really sure that I should be doing this. Maybe I should be leaving the plate supply connected, but just turned off. What do you guys think?

Thanks, Ron

Hi Ron,

For cold neutralization (recommended) the HV supply will have no effect on the neutralization tuning whether the HV supply is connected or not.  Neutralization is an RF function and the path through the plate choke and bypass cap is blocked for RF.

Turning on the filaments usually helps to make the input RF signal stronger at the plate.   

When you get the RF signal dip when the neutralization cap is adjusted correctly, watch out for stray pickup which can cause erroneous readings.  The sharp dip means you are balancing out the feedthrough well and should be good to go.

As a final check, when the rig is running, look for a plate current dip near max power out as well as a grid current peak at the same time. This tells you everything is pretty close.

Later -


T
 

Here is a neat trick:

1) HV out COLD.  Period.  Make sure it is.  Drain them bleeders!

2) Attach a high impedance DC meter (DVM or VTVM) as follows:  Positive lead to B-, or ground.  Negative lead to HV lead at the BOTTOM end of the plate choke.  Anode caps stay on, and some sort of small load is connected to amp output.

3) Apply drive sufficient to swing the voltmeter.

4) Set neutralizing cap(s) for minimum voltage indicated on meter.


If you are on, plate dip and grid peak will coincide.

You hook up this way because the final tube with HV off acts like a  diode, rectifying the RF drive current.

Have fun!

I always use a griddipper set at diode mode near the PA coil. Adjust  PA tuning for max signal and Neut for minimum.