I would do some checks... Plenty of grid drive? Low drive will limit modulation peaks.
Power supply, is the voltage dropping a lot under modulation? It should not drop a lot, 50 volts or less would be my guess. Are the output tubes any good? On all my rigs, homebrew and the 32v3's, the carrier power on an average wattmeter goes up under modulation, a lot on the home brew rigs. Modulator phasing also comes into play, most people have an asymmetrical voice, with a lot more peaks in one direction than the other. I never saw a rig drop power output under modulation as much as your 5100 is doing, not without something being very wrong with it. Brett N2DTS -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom Elmore Sent: Monday, October 11, 2004 2:26 PM To: Discussion of AM Radio Subject: Re: [AMRadio] B&W 5100B The B&W 5100 transmitter I recently acquired does not have the original modulation transformer but a replacement made by Stancor. When I modulate the transmitter with a 400 Hz tone about 50 % into a dummy load I see a significant decrease in power at least 25 watts. I measured this with both my Bird and Coaxial Dynamics wattmeter. I don't see a return to normal power until I remove the modulation. I'm wondering if the primary and secondary on the modulation transformer are possibly backward. The original owner included some documentation for the mod transformer that shows it's possible to use either the primary or secondary winding connected to the plates of the modulator tubes in my case 6146's for different types of transmitters and impedances. -Tom ----- Original Message ----- From: "Donald Chester" <[EMAIL PROTECTED]> To: <amradio@mailman.qth.net> Sent: Monday, October 11, 2004 9:32 AM Subject: Re: [AMRadio] B&W 5100B > > >It's normal for power to deflect downward on AM during modulation. You have > >to have a peak reading wattmeter to see the peaks..... Sounds like you have > >good modulation. > > > > With an average-reading wattmeter, theoretically the power should shift > upwards 50% with 100% modulation with a sinewave tone. An rf ammeter in > line should kick upwards about 22%. A field strength meter should show no > change, since it reads average voltage output. > > With voice modulation, the upward shift in power will be much less, since > the duty cycle of a voice signal is much less than that of a pure sine wave. > The rf ammeter will show little, if any deflection with voice modulation. > Whistle into the mic or play some music, and the meters will show more > positive deflection. > > If the carrier level (field strength) meter, rf ammeter or wattmeter show > any downward deflection during modulation (voice, tone or music), something > is wrong. The transmitter is showing poor modulation linearity. > > There may be a slight downward shift in carrier strength due to less than > perfect voltage regulation of the power supply, especially with plate > modulation. This should be no more than 5 percent. The power and rf line > current should still kick upwards. > > Don K4KYV > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net
