Paul, The modulation transformers in these kind of rigs are a big compromise. They must carry the RF current of the final amp through the secondary winding, and therefore must do so without saturation. In order to do this, these transformers do not interleave the E & I laminations, and instead use the E's together, and I's together like a filter choke with a fish paper gap in between. This reduces winding inductance, and increases the flux saturation point. As a consequence of this, low frequency response suffers as the modulators see a decreasing P-P load impedance as the af frequency drops below 500 hz.
So what can you do? The easiest thing is to lightly load your final, and that alone should improve things. Your modulation percentage should go up, as well as the frequency response will broaden (low end). You can extend the LF response a little more with negative feedback from the modulation transformer secondary back to a low level stage so long as there is not too much phase shift. An alternative is to take the WA1HLR technique and interleave the mod transformer so that there is no gap. Then add a outboard modulation choke (> 30 Henry), and a coupling capacitor of (> 5 uf) in such a way that the final PA plate current goes through the choke, and the modulation transformer only has to deal with audio. This requires a big external modification to the Valiant, so many will not like the idea. On last parting thought is that the Valiant Hi B+ filter capacitor may be too low in value such that as the modulation frequency drops, you increasingly modulate the power supply while decreasingly modulate the final RF amplifier. Think of this as a see-saw. Most ham rigs of this vintage could use more B+ capacitance than stock especially if you extend the audio modulator response below 100 hz. Regards, Jim -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Paul Baldock Sent: Sunday, August 28, 2005 10:09 AM To: amradio@mailman.qth.net Subject: [AMRadio] Johnson Viking Valiant Modulation Transformer I'm renovating a Valiant. I've made the usual audio chain mods to improve the frequency response. However, when I monitor the modulated wave form on a scope I notice that as you take the audio lower in frequency that the modulation gets distorted. Its OK down to about 500Hz, but below that gets progressively worse. The drive to the 6146 modulators is reasonably distortion free. Is this a feature of the original modulation transformer? Is a better transformer available? Thanks Paul - KW7Y ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005 -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005