Re: [Drakelist] Neutralization
- Original Message - From: To: Sent: Wednesday, October 24, 2012 9:08 AM Subject: Re: [Drakelist] Neutralization 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 My Kenwood TS-520S has a switch which disables the screens for neutralization. That seems to shut the tubes off well enough. I've tried neutralizing a Drake T4XB both by disconnecting the screen only and by disconnecting both screen and plate. Both work. I used a scope to find the null. One could also use a receiver in the same way is setting carrier null but that can be picked up with a sampling loop while neutralizing really requires a direct connection. I think the reason special voltmeters are prescribed in older literature was simply that suitable instruments were not generally available, at least not outside of an electronic lab. These days sensitive RF voltmeters and scopes that to into the stratosphere are common so one need not worry about building anything. I will say that for high power amps a grid-dip meter that can also work as a wavemeter/detector is helpful. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Neutralization
Lots of good on what I was looking for. I have done the neutralization the way it is outlined in the manual and think I have it pretty good. This T4 was a rust bucket basket case when I got it and it had several burned up cathode resistors, two in the Driver cathode cicuit alone and one in the plate circuit of the driver. I replaced those and began the neutralization per the manual. As I moved the neutralization cap all of a sudden the radio started sparking and smoking. The same 3 resistors had burned up. So I am thinking that some kind of wild oscillation is taking place when I hit a certain spot on the neutralization cap. So I replace the resistors again. And the very same thing happens again. So I look at the charred mess and the black smoke deposits in the radio and try one more time. This time I replace the resistors and remove the neutralization cap and find that at a certain rotation of the cap the plates short together. So what was happening was that I was getting HV from the PA tubes into the plate circuit of the 12by7 driver. No wonder those resistors cooked. I Put it all back together and find that I can get no output at all now. It turns out that the filaments of several of the tubes along with the #47 pilot lamps had all burned out. So I replace those tubes and lamps but still no output. I replace one more tube in the Tx chain and she comes back to life. She really has been abused but is putting out respectable power now and is neutralized to a pretty good extent. Now I want to try the neutralization scheme which removes the HV and Screen voltages. I will be careful... I have learned the hardway before at least once or twice. Steve NU0P On 10/23/2012 20:17, Steve wrote: 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 ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Neutralization
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, 1oldle...@ix.netcom.com writes: - Original Message - From: "Steve" To: 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 dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] Drake Station for sale
Drake Station for sale: TR7A S/N 12042 PS710648 RV71140 SP75 1154 MS7 MN4 3470 Desk Mic. Original manuals. All in excellent condition. Enquiries: p...@sympatico.ca ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
