Steve: IMHO, neutralizing the finals with the plate and screen voltages removed is the quickest way to neutralize and the easiest on the finals. Several older rigs have a final voltage disconnect plug to accomplish this (e.g. Central Electronics) and I always build in a similar disconnect on my homebrew transmitters. Richard's cautions are well placed, you've got to remove BOTH the plate and screen supplies and both are at high voltage so caution is advised (also, make sure the HV caps have dissipated). You'll need to measure the RF voltage at the output of the final amp. The easiest place to get this at the antenna connection and the easiest way to read it is with a scope (though most instructions will have you construct a low reading RF volt meter with a uA meter, diode and capacitor). Just remember that you will be tuning the final for maximum RF, then neutralizing for minimum RF. 73 Lee WB6SSW In a message dated 10/23/2012 9:10:13 P.M. Pacific Daylight Time, [email protected] writes:
----- Original Message ----- From: "Steve" <[email protected]> To: <[email protected]> Sent: Tuesday, October 23, 2012 6:17 PM Subject: [Drakelist] Neutralization >I have a Drake T4 that needs neutralization . I have tried >the approach given in the manual and while it seems to work >I thought I might use this as an opportunity to learn how >to do what is called a Cold Tube neutralization where you >do not apply High Voltage to the plate. Seems that you can >Neutralize the tube with only filament voltage, bias and >screen voltages applied. Has anyone done this and if so >what is the preferred method of doing it. > > Steve NU0P I think this is a confusion of the traditional method of neutralizing where the tube is fed with FR but with no plate or screen voltage. You can remove the screen voltage and neutralize with plate connected or disconnect both but if you remove plate voltage without also removing screen voltage the tube will burn up. The idea is to set up the amplifier for normal drive and tuning and then disconnect the HV. Then measure the feed through of RF on the plate side and adjust the neutralizing capacitor for minimum feed through The neutralizing capacitor forms a bridge circuit with the interelectrode capacitance of the tube and is adjusted to balance to prevent positive feedback from output to input with consequent oscillation. Some care is necessary because you can't disable the HV supply in the small Drake transmitters so you must disconnect it at the feed-through caps for the tubes. That is dangerous both because its a shock hazard and because of the danger of accidental shorting of the supply. The handbook method is fussy but works. -- Richard Knoppow Los Angeles WB6KBL [email protected] _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
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