> From: "John Coleman" <[email protected]> > The output voltage of a modulator is determined by its plate supply > voltage > and the modulation XFMR turns ratio.
> You need to think in terms of voltage transformation. If you are using a > common power supply on the final and modulators, or more exact, the > modulator plate supply and the final plate supply have the same voltage, > then the ratio that you use is what determines the maximum modulation. That is true irrespective of the nominal impedance of the transformer, the p-p load impedance of the modulator tubes or the modulating impedance of the final. Of course, the tubes have an optimum p-p load impedance at any given plate voltage, and a transformer has an optimum primary and secondary impedance at which it works best. But the actual transformation is based on turns ratio. The impedance ratio stamped on the nameplate of the transformer is a nominal value, and a good transformer should be able to work at up to twice the nominal value and as low as half the nominal value with little degradation in performance. The impedance transformation ratio is the square of the turns ratio. For example, a transformer with a 2:1 turns ratio has a 4:1 impedance ratio. > 100% modulation occurs when the audio voltage from the modulation XFMR is > 2 > X the plate supply. Actually it's when the peak output voltage, the combined audio and DC voltage from the winding adds up to 2 X the plate supply voltage. The peak a.c. output voltage from the transformer is equal to 1 X the DC plate supply voltage. When it is in the same polarity as the DC voltage, the two voltages add together to produce a sum that is 2 X the plate voltage at the positive modulation peak. At the opposite peak of the audio cycle the polarity is reversed and the two voltages cancel, leaving zero volts on the plate of the final. This is the negative modulation peak. > > At maximum drive the modulator tubes conduction (assuming they or big > enough) takes the plate voltage close to 0 Volts at the peak of the audio > for that conduction cycle. Nothing you can do will take the voltage lower > than zero. As one tube hits the Zero volt peak then the other tube will > hit > the 2 X plate voltage point. That is theoretical. In actual practice, there is nothing you can do to pull the instantaneous plate voltage below about 20% of the power supply voltage. In the case of screen grid modulators, the plate voltage can never be pulled to a lower voltage than the DC screen voltage. As one tube reaches maximum conduction, the instantaneous voltage on the plate of the other tube will reach about 1.8 X the DC plate supply voltage. > Something between 2:1 and 1:1 is what is needed. You need a little extra > to > make up for the fact that the modulators will use some power in plate > dissipation and you will want a little head room for voice lopsidedness > (everything is not a perfect sine wave). > > Experience information from Don, K4KYV, indicates that between 1.2:1 and > 1:4:1 is generally a good choice. 1.2:1 will give you more head room but > will require more modulator current perhaps larger tubes. 1:4:1 will > probably just be enough audio with very little head room, but will require > less modulator current and lighter demand on the modulator tubes. If you > chose 1.2:1 for plenty of head room then choose modulators with a little > more current capability or double up (push pull parallel). Using a higher step-down ratio of 1.6:1 will just barely allow you to reach close to 100% modulation with no headroom whatever, but the modulator tubes will run more efficiently. Somewhere between 1.2 and 1.4 will allow more headroom at the expense of efficiency. But that extra headroom is needed for minimum distortion and splatter, since driving a modulator or linear amplifier (exactly the same thing except the modulator amplifies audio while the linear amplifies rf) right to the saturation point results in more distortion. But watch the modulator plate current and make sure you don't exceed the tube ratings. If so, double up to use a pushpull parallel modulator. However, this may increase the audio driver requirements. >You may want to > consider a modulation reactor even if your XFMR says it can handle the > secondary current. Keeping the current out of the secondary will greatly > improve the low frequency capability of the XFMR. You want regret it. Taking the DC off the secondary greatly reduced the talk-back when I was using a UTC VM-5 modulation transformer. With the DC going through the secondary, the thing sounded like a small speaker inside the transmitter cabinet. With the reactor, it was totally quiet. This will also reduce the distortion from the modulator, since it will reduce the magnetic saturation of the core over the audio cycle. A modulation transformer designed to run DC through the secondary will usually have a gap in the core, filled with paper or some composition material. A transformer designed to be used with a reactor will usually have a core stacked like a power transformer, with no gap. But this is not strictly true. I have seen transformers designed for DC with an extra large core and no gap, while I have seen ones designed for use with a reactor that had a very narrow gap, not much more than .001 inch. Don k4kyv ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body.
